tilly819
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posted on 13/4/11 at 10:09 PM |
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How much Mechanical Trail
Hi all
Bit of background first.
Car details
weight 600kg
split 40F 60R
Kpi 0 deg
Scrub radius 26.65mm
King pin Offset 0mm
tire width 215mm
Front tire radius 313mm
Now then i am in the process of deciding how much caster to incorporate and this will determine the trail since im not using any king pin offset or
(pin lead)
Obviously the tire radius is quite large so i do not need much caster to create the trail, and dont want to much caster to prevent weather cocking as
someone kindly pointed out to me in another post. however i need some self centering, essentially enough to pass IVA
so the question is how much mechanical trail??
cheers tilly
F20C Haynes roadster 440 BHP/Tonne www.youtube.com/handmadeextreme
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blakep82
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posted on 13/4/11 at 10:50 PM |
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have you read this book?
click
i still reckon you need KPI! kpi helps lift the side of the car when you turn the wheel (ie lifts the right side when you turn right) and the weight
helps self centering. well, kpi and castor, but i don't think castor itself will. reckon you're onto plums without kpi, thats all
i'm saying on it.
________________________
IVA manual link http://www.businesslink.gov.uk/bdotg/action/detail?type=RESOURCES&itemId=1081997083
don't write OT on a new thread title, you're creating the topic, everything you write is very much ON topic!
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tilly819
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posted on 13/4/11 at 11:34 PM |
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i understand you comment about kpi however the way i see it, kpi's conns outweigh its pros's, i dont want this to come across wrong as it
is very easy to sound rude when typing but that is not my intention i assure you. i would like to point out that the lifting of the car due to its KPI
is not a desirable characteristic.
the trail should be able to provide adequate self centering, think of a caster wheel on a trolly it has no KPI and no caster however it uses a King
pin Lead to create mechanical trail which allows it to self center, i now this is a rather extreem example but it shows the concept.
cheers tilly
F20C Haynes roadster 440 BHP/Tonne www.youtube.com/handmadeextreme
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britishtrident
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posted on 14/4/11 at 05:58 AM |
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Castor & KPI are like Morcambe & Wise
KPI is the best source of steering self-centering.
The trail will create weather cocking even if the actual castor angle is low, how much will depend on the moment of th lateral force on the tyre
contact about the virtual king pin.
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Doctor Derek Doctors
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posted on 14/4/11 at 07:09 AM |
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Not meaning to sound rude but you will need some KPI, there is a reason that virtually every single road and race car ever designed has it.....
Also in general Mechanical trail is usually left as a product of the caster angle and wheel size, there are very few cars were the wheel hub centre is
not on the centreline (between the pivots) of the upright.
One thing (amongst many others) that I have learnt when at uni' studying motorsport design and out in the real world designing race cars is that
there are many things that seem like a good idea on paper or after reading a book (no KPI, Roll centres under ground, solid axles) but in real life
they make horrible handling cars that don't win races or get sold to the public.
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NOTE:This user is registered as a LocostBuilders trader and may offer commercial services to other users
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tilly819
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posted on 14/4/11 at 08:32 AM |
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quote: Originally posted by Doctor Derek Doctors
you will need some KPI, there is a reason that virtually every single road and race car ever designed has it.....
Also in general Mechanical trail is usually left as a product of the caster angle and wheel size, there are very few cars were the wheel hub centre is
not on the centreline (between the pivots) of the upright.
could you possibly expand on your first point Ie What is the reason?? since the only positive i can see to KPI is that it helps with self centering
but this is at the expence of some jacking when the wheel is steered.
I agree that mechanical trail is ushaly a result of caster and in my case it will be since as i said abive i will not be using any pin lead.
thanks
tilly
F20C Haynes roadster 440 BHP/Tonne www.youtube.com/handmadeextreme
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v8kid
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posted on 14/4/11 at 08:36 AM |
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DDD
Sorry to pick on you but you were the last contributor! If everything remains the same we will never progress. I guess that's OK if you only
want to be one of the crowd but it does stifle things a bit don't you think?
Sure a lot of progressive ideas have turned out to be poo but by analysing the reasons we progress just as much as the ideas that work straight out of
the box.
So why is no kpi such a bad idea? Come to that why is "the wheel hub centre not on the centreline (between the pivots) of the upright" a
bad idea. Seems to me that Tilly is balancing pro's and cons to come up with a design and is being open about it. If there is a logical
engineering reason why not I think we have a duty to spell it out!
For my part I don't know of one apart from it's jolly difficult to package all the stuff into the wheel and get zero kpi - I know
I've tried and failed! I've seen Tilleys drawings and it seems to be possible due to a clever twist. But what is this overarching reason
not to have zero kpi?
Cheers!
You'd be surprised how quickly the sales people at B&Q try and assist you after ignoring you for the past 15 minutes when you try and start a
chainsaw
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v8kid
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posted on 14/4/11 at 08:43 AM |
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Check out this link to a forum with quite a respected contributor on the subject
http://www.eng-tips.com/viewthread.cfm?qid=113181&page=8
Cheers!
You'd be surprised how quickly the sales people at B&Q try and assist you after ignoring you for the past 15 minutes when you try and start a
chainsaw
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britishtrident
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posted on 14/4/11 at 10:43 AM |
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KPI causing the front of the car to rise when the steering is turned a bit of a red herring because (ignoring effects of wheel width) castor
causes the nose of the to fall as the steering is turned.
The relationship between camber castor and kpi and the weight/download carried by the front wheels needs careful consideration. A major factor
involved that has to be considered are the tyres.
Using the Cortina Mk3+ as an example the tyres were much heavier loaded than on any car we are likely to design so nearly all of the steering
self-centering and stability came from the self-aligning torque generated by the deflection caused in the radial tyres.
Getting a near zero scrub radius with wide wheels and a low KPI is very difficult because of wishbone clearance and steering lock considerations.
Interestingly , production cars (Audi) with strut suspension can actually have negative scrub. Back in in the 1970s and 1980s racing saloon cars
were built with a ridiculous ammounts of scrub radius because racers were using very wide wheels on suspensions (for example Capri) that were
designed before tyre widths increased.
An increased scrub radius is not desirable but not a major problem in tarmac use.
Incidentally the only cars I can think of that had the stub axle axis offset from the king pin centreline to give mechanical trail were the Hillman
Imp and the rear engined Fiats ----- both types very prone to weather cocking.
[Edited on 14/4/11 by britishtrident]
[I] “ What use our work, Bennet, if we cannot care for those we love? .”
― From BBC TV/Amazon's Ripper Street.
[/I]
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tilly819
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posted on 14/4/11 at 01:29 PM |
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Right i was hoping that this wasnt going to turn into a you need KPI you dont need KPI but it has.
This discution has happen a couple of times already and since knowone has been able to give a definitive "you need KPI because" or
"Zero KPI is a definate No NO because " then i see no reason why my choise to have no king pin inclination should be recived so badly..
I have given the basic geometery in my first post and what i was hoping for was some responces to establish some aproxermations of a suitable caster
angle and resulting trail, now i have made a staring estimate of 5.5deg caster which results in 30mm mechanical trail. this seems reasonable to me
since i am awere of someone who has build a similar weight car the the same aproximate weight distribution with about 25mm of trail and there steering
is very light and has some self centering
thanks
tilly
F20C Haynes roadster 440 BHP/Tonne www.youtube.com/handmadeextreme
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Doctor Derek Doctors
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posted on 14/4/11 at 01:33 PM |
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quote: Originally posted by v8kid
DDD
Sorry to pick on you but you were the last contributor! If everything remains the same we will never progress. I guess that's OK if you only
want to be one of the crowd but it does stifle things a bit don't you think?
Sure a lot of progressive ideas have turned out to be poo but by analysing the reasons we progress just as much as the ideas that work straight out of
the box.
So why is no kpi such a bad idea? Come to that why is "the wheel hub centre not on the centreline (between the pivots) of the upright" a
bad idea. Seems to me that Tilly is balancing pro's and cons to come up with a design and is being open about it. If there is a logical
engineering reason why not I think we have a duty to spell it out!
For my part I don't know of one apart from it's jolly difficult to package all the stuff into the wheel and get zero kpi - I know
I've tried and failed! I've seen Tilleys drawings and it seems to be possible due to a clever twist. But what is this overarching reason
not to have zero kpi?
Cheers!
Because it just doesn't work. I know that doesn't sound right but 50+ years of motorsport developement have always led to the same
conclusion. I could spell it all out here but there are plenty of massive books on the subject as well as 24 F1 cars that demonstrate it better than I
can in my short lunch brake....
....but think about this, a shopping trolley self centres the wheels as it goes forwards (from the froward motion), but there is no resistance to side
loading except from the steering and as you increase the mechanical trail the lever ratio of this load is increased meaning it is harder to counteract
this load. This load in corners will come from the centripetal acceleration (centrifugal force to the lamen) of the car rotating around the corner,
thus the more mechanical trail you have the harder is will be to steer around a corner and it won't just be trying to self centre it will be
trying to follow the load path thus steering in the opposite direction! It will also be a nightmare to resist side loading from side wind and bumps.
With with KPI on the other hand the it self centres to the chassis via gravity and the centripetal acceleration acts directly on the centre of hub so
it is not trying to steer the car, the increased resistance you feel is from the increased load on the outside wheel working harder to centre the
wheel because of KPI.
To try and sum up in a sentence Mechanical trail self centres to the direction of the load acting on it (cornering force, wind, bumps etc) but KPI
self centres to the cars chassis so it will always try to go straight, whereas a shopping trolley will try to go where other outside factors are
pushing them
When I was a wide eyed design student we read the books and weighed up the pro's and cons and decided that we knew better and had new ideas....
and for every idea there was always an example of why it didn't work. For instance placing the IRC under the ground seems like a great way to
counteract the CoG and stop the car leaning in a corner, in theory it works, in reality it has been tried to death and it always causes odd handling
characteristics.
As for lack of progress? I'm sure Adrian Newey or Ross Brawn would love to be told how they have made no progress.... oh wait.
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NOTE:This user is registered as a LocostBuilders trader and may offer commercial services to other users
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v8kid
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posted on 14/4/11 at 01:36 PM |
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Interesting I thought the Hillman Imp has front swinging axles with a true king pin - basically a beam axle cut in half. Blessed if I can see how they
get any spindle offset with this setup you don't have any pics do you I'm very interested?
Also can't find any reference to Imps "weather cocking" what do you mean by it - as I remember the handling was OK unless you got a
bit overenthusiastic!
Cheers1
You'd be surprised how quickly the sales people at B&Q try and assist you after ignoring you for the past 15 minutes when you try and start a
chainsaw
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tilly819
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posted on 14/4/11 at 01:45 PM |
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quote: Originally posted by v8kid
Also can't find any reference to Imps "weather cocking" what do you mean by it - as I remember the handling was OK unless you got a
bit overenthusiastic!
Cheers1
Weather cocking is a tendency to turn into wind when exposed to a sidewind /crosswind
F20C Haynes roadster 440 BHP/Tonne www.youtube.com/handmadeextreme
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tilly819
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posted on 14/4/11 at 02:06 PM |
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quote: Originally posted by Doctor Derek Doctors
quote: Originally posted by v8kid
DDD
Sorry to pick on you but you were the last contributor! If everything remains the same we will never progress. I guess that's OK if you only
want to be one of the crowd but it does stifle things a bit don't you think?
Sure a lot of progressive ideas have turned out to be poo but by analysing the reasons we progress just as much as the ideas that work straight out of
the box.
So why is no kpi such a bad idea? Come to that why is "the wheel hub centre not on the centreline (between the pivots) of the upright" a
bad idea. Seems to me that Tilly is balancing pro's and cons to come up with a design and is being open about it. If there is a logical
engineering reason why not I think we have a duty to spell it out!
For my part I don't know of one apart from it's jolly difficult to package all the stuff into the wheel and get zero kpi - I know
I've tried and failed! I've seen Tilleys drawings and it seems to be possible due to a clever twist. But what is this overarching reason
not to have zero kpi?
Cheers!
Because it just doesn't work. I know that doesn't sound right but 50+ years of motorsport developement have always led to the same
conclusion. I could spell it all out here but there are plenty of massive books on the subject as well as 24 F1 cars that demonstrate it better than I
can in my short lunch brake....
....but think about this, a shopping trolley self centres the wheels as it goes forwards (from the froward motion), but there is no resistance to side
loading except from the steering and as you increase the mechanical trail the lever ratio of this load is increased meaning it is harder to counteract
this load. This load in corners will come from the centripetal acceleration (centrifugal force to the lamen) of the car rotating around the corner,
thus the more mechanical trail you have the harder is will be to steer around a corner and it won't just be trying to self centre it will be
trying to follow the load path thus steering in the opposite direction! It will also be a nightmare to resist side loading from side wind and bumps.
With with KPI on the other hand the it self centres to the chassis via gravity and the centripetal acceleration acts directly on the centre of hub so
it is not trying to steer the car, the increased resistance you feel is from the increased load on the outside wheel working harder to centre the
wheel because of KPI.
To try and sum up in a sentence Mechanical trail self centres to the direction of the load acting on it (cornering force, wind, bumps etc) but KPI
self centres to the cars chassis so it will always try to go straight, whereas a shopping trolley will try to go where other outside factors are
pushing them
When I was a wide eyed design student we read the books and weighed up the pro's and cons and decided that we knew better and had new ideas....
and for every idea there was always an example of why it didn't work. For instance placing the IRC under the ground seems like a great way to
counteract the CoG and stop the car leaning in a corner, in theory it works, in reality it has been tried to death and it always causes odd handling
characteristics.
As for lack of progress? I'm sure Adrian Newey or Ross Brawn would love to be told how they have made no progress.... oh wait.
Thanks thats a very good answer with some good technical input thank you.
I would like to respond by saying that in my oppiniun in a very lightweight road car which will not be experiancing massive lateral acceleration that
i am willing so sactifice some steering weight for the beinfits of the camber change due to caster rarther than KPI. my philosophy has always been to
keep the tyre vertical for the longest duration possible.
In a 600Kg car with a 40F 60R weight distribution exposed to a 1G lateral acceleration with a small amout of mechanical trail (aprox 30mm) the
steering effort should be perfectly managable
tilly
F20C Haynes roadster 440 BHP/Tonne www.youtube.com/handmadeextreme
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Doctor Derek Doctors
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posted on 14/4/11 at 02:10 PM |
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quote: Originally posted by tilly819
quote: Originally posted by v8kid
Also can't find any reference to Imps "weather cocking" what do you mean by it - as I remember the handling was OK unless you got a
bit overenthusiastic!
Cheers1
Weather cocking is a tendency to turn into wind when exposed to a sidewind /crosswind
Which supports what I was saying about mechanical trail trying to self centre to the outside forces applied and not to the chassis direction
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NOTE:This user is registered as a LocostBuilders trader and may offer commercial services to other users
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blakep82
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posted on 14/4/11 at 02:14 PM |
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quote: Originally posted by tilly819
This discution has happen a couple of times already and since knowone has been able to give a definitive "you need KPI because" or
"Zero KPI is a definate No NO because " then i see no reason why my choise to have no king pin inclination should be recived so badly..
sorry, you've received plenty of reasons why having no kpi is such a bad idea, i've even linked to a whole chapter in a book by des hamill
about it and you choose to ignore it lol.
trail works on shopping trolleys, becasue, as mentioned above, the wheels go in the direction the trolley is being pushed in by the person pushing,
this isn't self centering. if you push a trolley sideways, at no point will it try to go forwards. if you push it round a corner and let go, it
will continue to go round a corner, and not straighten up.
trail is on trolleys an castor wheels to make sure that when you push, the wheels go the way you want them. imagine trying to push a trolley with no
trail, and the wheels are pointing across you. it won't go anywhere.
what are these pros and cons of kpi you've identified? i saw you'd mentioned somewhere that it affects the camber of the wheel and thats
bad. its not bad at all! if you have absolute zero body roll, then ok, but thats hardly ever going to happen unless all your suspension is solidly
mounted with no shocks or springs. once you get a bit of body roll, you need a little bit of camber change to make sure the wheels stay flat on the
ground. with no change to camber, the car will be trying to balance on the inside of one wheel and the outside of the other. you want maximum contact
with the road for most grip, so you need a small change toward positive camber to correct this. if you don't you'll put additional stress
on the ball joints
KPI and castor together is what gives you self centering! you need some built in, without any kpi at all, you'd need something like 15 degrees
castor, which i'm not completely sure is practical.
do the top and bottom ball joints of the uprights make a perfect line to the centre of the tires? thats essentially what you're aiming for.
still, if you deside to continue without kpi, good luck lol. i'd love to be proved wrong, but i don't think i will be... think the
handling will be great, when the cars parked in the garage
[Edited on 14/4/11 by blakep82]
________________________
IVA manual link http://www.businesslink.gov.uk/bdotg/action/detail?type=RESOURCES&itemId=1081997083
don't write OT on a new thread title, you're creating the topic, everything you write is very much ON topic!
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Doctor Derek Doctors
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posted on 14/4/11 at 02:18 PM |
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quote: Originally posted by tilly819
quote: Originally posted by Doctor Derek Doctors
quote: Originally posted by v8kid
DDD
Sorry to pick on you but you were the last contributor! If everything remains the same we will never progress. I guess that's OK if you only
want to be one of the crowd but it does stifle things a bit don't you think?
Sure a lot of progressive ideas have turned out to be poo but by analysing the reasons we progress just as much as the ideas that work straight out of
the box.
So why is no kpi such a bad idea? Come to that why is "the wheel hub centre not on the centreline (between the pivots) of the upright" a
bad idea. Seems to me that Tilly is balancing pro's and cons to come up with a design and is being open about it. If there is a logical
engineering reason why not I think we have a duty to spell it out!
For my part I don't know of one apart from it's jolly difficult to package all the stuff into the wheel and get zero kpi - I know
I've tried and failed! I've seen Tilleys drawings and it seems to be possible due to a clever twist. But what is this overarching reason
not to have zero kpi?
Cheers!
Because it just doesn't work. I know that doesn't sound right but 50+ years of motorsport developement have always led to the same
conclusion. I could spell it all out here but there are plenty of massive books on the subject as well as 24 F1 cars that demonstrate it better than I
can in my short lunch brake....
....but think about this, a shopping trolley self centres the wheels as it goes forwards (from the froward motion), but there is no resistance to side
loading except from the steering and as you increase the mechanical trail the lever ratio of this load is increased meaning it is harder to counteract
this load. This load in corners will come from the centripetal acceleration (centrifugal force to the lamen) of the car rotating around the corner,
thus the more mechanical trail you have the harder is will be to steer around a corner and it won't just be trying to self centre it will be
trying to follow the load path thus steering in the opposite direction! It will also be a nightmare to resist side loading from side wind and bumps.
With with KPI on the other hand the it self centres to the chassis via gravity and the centripetal acceleration acts directly on the centre of hub so
it is not trying to steer the car, the increased resistance you feel is from the increased load on the outside wheel working harder to centre the
wheel because of KPI.
To try and sum up in a sentence Mechanical trail self centres to the direction of the load acting on it (cornering force, wind, bumps etc) but KPI
self centres to the cars chassis so it will always try to go straight, whereas a shopping trolley will try to go where other outside factors are
pushing them
When I was a wide eyed design student we read the books and weighed up the pro's and cons and decided that we knew better and had new ideas....
and for every idea there was always an example of why it didn't work. For instance placing the IRC under the ground seems like a great way to
counteract the CoG and stop the car leaning in a corner, in theory it works, in reality it has been tried to death and it always causes odd handling
characteristics.
As for lack of progress? I'm sure Adrian Newey or Ross Brawn would love to be told how they have made no progress.... oh wait.
Thanks thats a very good answer with some good technical input thank you.
I would like to respond by saying that in my oppiniun in a very lightweight road car which will not be experiancing massive lateral acceleration that
i am willing so sactifice some steering weight for the beinfits of the camber change due to caster rarther than KPI. my philosophy has always been to
keep the tyre vertical for the longest duration possible.
In a 600Kg car with a 40F 60R weight distribution exposed to a 1G lateral acceleration with a small amout of mechanical trail (aprox 30mm) the
steering effort should be perfectly managable
tilly
You seemed to have missed the point though that Mechanical trail does not give self centreing to the chassis, it gives self centreing to the outside
forces applied. You will spend your entire time fighting to keep the thing in a straight line as every change in the road and the wind pushes the
front of the car around with the mechanical trail helping it out and working against you. In a very light car this effect will be even worse as the
caster won't be helping as much.
You can build it if you like but I bet you end up changing back as everyone else who has designed cars is of the opposite opinion.
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NOTE:This user is registered as a LocostBuilders trader and may offer commercial services to other users
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v8kid
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posted on 14/4/11 at 02:33 PM |
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quote: Originally posted by Doctor Derek Doctors
To try and sum up in a sentence Mechanical trail self centres to the direction of the load acting on it (cornering force, wind, bumps etc) but KPI
self centres to the cars chassis so it will always try to go straight, whereas a shopping trolley will try to go where other outside factors are
pushing them
As for lack of progress? I'm sure Adrian Newey or Ross Brawn would love to be told how they have made no progress.... oh wait.
Nice summary thanks. Easy to see the argument about KPI when you put it that way and I can see the point now.
Can you do the same for "there are very few cars were the wheel hub centre is not on the centreline (between the pivots) of the upright"
please? Certainly thats true but are there could be considerations other than handling ones that cause it. Why is it a no no after all it seems to
comply with your points already made.
quote: Originally posted by Doctor Derek Doctors
As for lack of progress? I'm sure Adrian Newey or Ross Brawn would love to be told how they have made no progress.... oh wait.
I've lost the plot a bit here that was the very point I was trying to make - unless we try something different there will be no progress -
anyhow no matter
Cheers!
You'd be surprised how quickly the sales people at B&Q try and assist you after ignoring you for the past 15 minutes when you try and start a
chainsaw
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blakep82
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posted on 14/4/11 at 02:36 PM |
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quote: Originally posted by v8kid
quote: Originally posted by Doctor Derek Doctors
As for lack of progress? I'm sure Adrian Newey or Ross Brawn would love to be told how they have made no progress.... oh wait.
I've lost the plot a bit here that was the very point I was trying to make - unless we try something different there will be no progress -
anyhow no matter
Cheers!
i think he means these are designers at the top of their game, and if 0 KPI was a good idea, they would be doing it but they don't. doing it now
would be doing something different, but not progress
________________________
IVA manual link http://www.businesslink.gov.uk/bdotg/action/detail?type=RESOURCES&itemId=1081997083
don't write OT on a new thread title, you're creating the topic, everything you write is very much ON topic!
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Doctor Derek Doctors
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posted on 14/4/11 at 02:45 PM |
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quote: Originally posted by v8kid
quote: Originally posted by Doctor Derek Doctors
To try and sum up in a sentence Mechanical trail self centres to the direction of the load acting on it (cornering force, wind, bumps etc) but KPI
self centres to the cars chassis so it will always try to go straight, whereas a shopping trolley will try to go where other outside factors are
pushing them
As for lack of progress? I'm sure Adrian Newey or Ross Brawn would love to be told how they have made no progress.... oh wait.
Nice summary thanks. Easy to see the argument about KPI when you put it that way and I can see the point now.
Can you do the same for "there are very few cars were the wheel hub centre is not on the centreline (between the pivots) of the upright"
please? Certainly thats true but are there could be considerations other than handling ones that cause it. Why is it a no no after all it seems to
comply with your points already made.
quote: Originally posted by Doctor Derek Doctors
As for lack of progress? I'm sure Adrian Newey or Ross Brawn would love to be told how they have made no progress.... oh wait.
I've lost the plot a bit here that was the very point I was trying to make - unless we try something different there will be no progress -
anyhow no matter
Cheers!
Well the point about Brawn (hero) and Newey (legend) are that they are always trying to do something new but they still use double wishbone suspension
with KPI, Caster and Camber like most other race cars, just alot more refined and optimised..... because it is the best way. But yes sorry it may have
come accross as a little rude.
As for the hub off centreline, this I know less about but it seems from experience (and design) that if you use a calculated amount of Caster and KPI
the mechanical trail that leaving the hub on the centreline gives is just about right for handling, grip and feel, I think its just one of those lucky
coincidences. Especially when you consider that its no harder to make an upright with the hub off centre that know one does it.
Suspension design is bloody hard, really really hard in fact and the best place to start is by looking at what works and start with that.
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NOTE:This user is registered as a LocostBuilders trader and may offer commercial services to other users
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v8kid
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posted on 14/4/11 at 02:53 PM |
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Ta - Good series of posts here it has clarified my thoughts quite a bit.
Cheers!
You'd be surprised how quickly the sales people at B&Q try and assist you after ignoring you for the past 15 minutes when you try and start a
chainsaw
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britishtrident
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posted on 14/4/11 at 03:15 PM |
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With a light front end you need a fair ammount of caster and kpi if you dial in caster alone you end up with a silly ammount of negative camber at
full lock.
If you look at the Lotus Elise forums you will find that although the Elise comes with a fair ammount of both caster and kpi as standard owners are
adjusting the suspension to give more kpi and caster to get sharper turn in.
As to Hillman Imp front ends the swivel hub casting was arranged so the king pin passed about 25mmmm ahead of the stub axle centre, this had the
advantage of allowing room for the cable drive for the speedo to pass through the stub axle.
Imps used a lot of both caster and KPI, istr the caster was about 11 degrees and on a racer the kpi about 7-9 degrees depending on camber, this
combination would result in too much negative camber on hairpins so quite often the leading wishbone pivot point was lowered 25mm giving anti-dive
and reducing caster.
Imp handling when lowered was nothing short of fantastic BUT the driving experience in motorway cross winds more like tacking in a sailing boat
than driving most owners always carried something heavy in thre friont boot to ballast the front and move the cg forward of the centre of
pressure.
[Edited on 14/4/11 by britishtrident]
[I] “ What use our work, Bennet, if we cannot care for those we love? .”
― From BBC TV/Amazon's Ripper Street.
[/I]
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tilly819
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posted on 14/4/11 at 03:58 PM |
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OK
thank you all for your input it has been invaluable as i am no expert.
Just to clear a few points up:
I can see the value of some KPI to prevent the effects on the steering from crosswinds and road defects.
A large amount of negative camber on the outside wheel under full lock dosnt bother me at all since under normal driving conditions this scinario will
never happen, only when doing low speed opperation such as parking will it become relevelnt.
I am not trying to achive centerpoint steering and would like a small amount of scrub radius.
I have indeed read the book a link was provided to along with many others ( the problem i find is that information in one book contradicts another and
finding out which is correct/best can be difficult)
I am coming round to your way of thinking that a small amount of KPI should be included to prevent the weather cocking and aid self centering. the
explanation by DDD was very good. I am still however reluctant to include a large KPI angle due to its adverse camber effects on the outside wheel, I
am thinking after some very quick maths that an angle of about 3deg would not sacrifice to much outer wheel control, how does this sound?? this would
also bring my scrub radius down to the 10mm range
thanks again
tilly
F20C Haynes roadster 440 BHP/Tonne www.youtube.com/handmadeextreme
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v8kid
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posted on 14/4/11 at 04:06 PM |
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Sounds like compromise is the order of the day! Would that take your caster up from 7 to around 9 then? Back to the original question then trail will
be around 45 -50mm without stub axle lead.
[Edited on 14-4-11 by v8kid]
You'd be surprised how quickly the sales people at B&Q try and assist you after ignoring you for the past 15 minutes when you try and start a
chainsaw
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tilly819
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posted on 14/4/11 at 04:11 PM |
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I hate compromise i like to acchive perfetion with evrything but this is one of those things that it just dosnt seem to to posible there has to be a
compromise like you say, i had actualy considered bringing the caster down to 5.5 with 0 kpi but with 3 kpi i think 6-7 caster might work il have to
have a play with it late when i get home
tilly
compromise GRRRRR why carnt it be perfect lol
F20C Haynes roadster 440 BHP/Tonne www.youtube.com/handmadeextreme
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