tilly819
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| posted on 11/3/11 at 10:17 AM |
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Front wheel trail
Hi all
Iv just been considering the caster angle of the front upright i am designing and have got a little stuck...
I have a tyre radius of 312mm (215-45 R17) a KPI of 8 deg and a scrub of 10mm from the center of the tyre contact patch.
my inital thought was to apply equal caster to KPI to try and cancel out the worst of the camber change effects of having the 8deg KPI however after a
bit of thought and a bit of trig i think this may give too much trail due to the radius of the tyre. (results in a second)
Now i want a reasonable caster to allow for the fact that the car is mid engine 40F 60R % weight distribution at a design weight of 600Kg but i also
dont want the steering to be too heavy.
Caster to Trail Results:
8deg caster = 43mm Trail
6deg caster = 32mm Trail
4deg caster = 21mm Trail
as you can see the trail is moving at a factor of about 5.5mm per degree of caster
the question is how much trail / caster do i want???
thanks Tilly
F20C Haynes roadster 440 BHP/Tonne www.youtube.com/handmadeextreme
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v8kid
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| posted on 11/3/11 at 12:00 PM |
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I'm going through the same exercise and considered:-
1. On a lightweight car the effects of trail and scrub will be less critical than on the heavy saloons most data is available for
2. Cortina's ( which were lighter than modern saloons but heavier than locosts) had around 32mm trail combined with 75mm scrub.
3. There is merit in minimising kpi and maximising caster exemplified in the SAE designs
4. Bicycle forks give a clue to the answer with trail from large caster minimised by offsetting the axle in front of the steering axis
My problem is how much can I advance the front axle? A 9 degree caster gives a 42mm trail on my front wheel. I can reduce this to 15mm by advancing
the axle 27mm but that makes the upright very heavy cos it has to be wider.
Alternatively I can advance the axle 8mm without adding weight to the upright leaving a trail of 34mm.as a compromise this sounds good to me as with a
kpi of 3 deg and a caster of 9deg I will be adding neg camber on turning the wheel which takes pressure off the front SLA suspension design.
Dunno how it works in practice as I am currently destructively testing uprights to see how thin I can make the material - in this respect i have just
bent the 20tonne hydraulic press so it is fair to say my upright is a bit over-designed!
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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tilly819
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| posted on 11/3/11 at 03:45 PM |
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quote:
Originally posted by v8kid
I'm going through the same exercise and considered:-
1. On a lightweight car the effects of trail and scrub will be less critical than on the heavy saloons most data is available for
2. Cortina's ( which were lighter than modern saloons but heavier than locosts) had around 32mm trail combined with 75mm scrub.
3. There is merit in minimising kpi and maximising caster exemplified in the SAE designs
4. Bicycle forks give a clue to the answer with trail from large caster minimised by offsetting the axle in front of the steering axis
My problem is how much can I advance the front axle? A 9 degree caster gives a 42mm trail on my front wheel. I can reduce this to 15mm by advancing
the axle 27mm but that makes the upright very heavy cos it has to be wider.
Alternatively I can advance the axle 8mm without adding weight to the upright leaving a trail of 34mm.as a compromise this sounds good to me as with a
kpi of 3 deg and a caster of 9deg I will be adding neg camber on turning the wheel which takes pressure off the front SLA suspension design.
Dunno how it works in practice as I am currently destructively testing uprights to see how thin I can make the material - in this respect i have just
bent the 20tonne hydraulic press so it is fair to say my upright is a bit over-designed!
Cheers!
1) I agree with you that the effects of scrub and trial should have less effect on a lighter car. (given the same values they should be proportionate
to the vehical weight)
2) Thanks for the info on the cortina i was not aware of its data.
3) i agree a small KPI is desireable
4) the bike fork consurns me slightly, my first thought is that in moving the stub axle off the steering axis line that it would be intruducing a
second scrub radius i may have this round my ears however please correct me if this is wrong.
I think your idea of only moving your axle 8mm and maintaining the same weight is a good one, afterall every little helps in the weight saving game,
and a compromise has to be made somewere.
with you upright you say you have 3deg KPI a possible trail of 34mm lets say and a caster of 9 deg may i ask what size tyre you are using? and what
your scrub radius is?
tilly
F20C Haynes roadster 440 BHP/Tonne www.youtube.com/handmadeextreme
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designer
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| posted on 11/3/11 at 03:57 PM |
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Second column, five down.
http://www.shedworks.eu/tips.html
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v8kid
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| posted on 11/3/11 at 04:33 PM |
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190/535/13 Dunlops on the front - 535mm dia believe it or not!
scrub radius was 72mm.
Thanks for the dimensions Designer its much appreciated.
I looked at these earlier ( I think it was on your site it looks familiar!) when I was trying to make sense of all the different info out there and
for my purposes, but not everyone else's purposes, disregarded them. I want scrub because it gives feedback to the driver on what the wheels are
doing. I'm no Lewis Hamilton and I need lots of feedback! Also those cars are designed with noise, vibration and harshness very much in mind.
Not qualities that first come to mind on a Losost!
Also kpi on a road car is more a packaging issue and is an unwanted side effect of using struts whuich are used due to cost or chassis construction
considerations - we are using SLA suspension.
I'm still not sure I've got it anywhere right though I suppose there is only one way to find out 
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 11/3/11 at 04:35 PM |
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Sorry about point 4 - Dunno! Don't think so but not sure - works on bicycles!
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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Bare
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| posted on 13/3/11 at 04:15 PM |
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Dunno.
If this helps: the Venerable Triumph upright used in *many* lotii Including serious racers of some success :-) has a 9 degree KPI as part of
it's casting and Lotus' Castor setup on those ranged from 3 degrees on Mid engined chassis to 5 degrees on front engined. Why are
you all over the map on this?
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v8kid
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| posted on 13/3/11 at 05:58 PM |
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That was on hard xplies - different kettle of fish now
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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Bare
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| posted on 14/3/11 at 03:21 AM |
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Nonsense!
Radials effect Toe in / alignment setup.
Tough to make progress when buried under old wives' tales.
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v8kid
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| posted on 14/3/11 at 07:18 AM |
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Interesting arguement can you expand on it for us lesser mortals?
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 14/3/11 at 09:24 AM |
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Sorry bare for my ignorance but im not sure i understand ither of your posts if you could enlighten me in simple mans terms i would greatly apreacate
it
many thanks
tilly
F20C Haynes roadster 440 BHP/Tonne www.youtube.com/handmadeextreme
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