AdamR20
|
| posted on 12/10/15 at 11:11 AM |
|
|
Suspension geometry feedback
Morning all,
I've decided to have a go at a (pretty much) scratch build, that will nick various bits from an MX-5 (uprights, diff, half shafts, brakes,
possibly steering rack), then chuck a bike engine in the back and see what happens...
One of my 'gripes' with the other affordable kits I've seen is that the suspension geometry isn't really where I'd want
it to be, so I've had a play with VSusp to see what I can come up with.
My considerations are:
- Track only, not road legal.
- Single seater, approx 55-60% rear weighed.
- Moderate wheel rates - approx 1.7-1.75Hz front and 1.85-1.9Hz rear (plus ARBs) - and decent amounts of travel, around 100mm at each end, front split
65/35 bump / droop and rear 80/20.
- Car must be easy / fun to drive on the limit (which I believe is governed by roll axis, roll centre migration and roll couple height?).
- Have the roll centre heights converging during roll, to give the rear end a compliant feel on exit and mid-corner (having driven a car which does
the opposite, it's a handful!).
- I will probably have two or three mounting positions for the rear lower wishbones to allow some adjustment of roll centre axis inclination after the
build, without affecting chassis rake.
- I love the way a standard MX-5 drives, so have the X axis roll centre migrations in the same direction as one of those, but wasn't able to
match the amounts as closely as I wanted - the rear migrates more than the front - any tips?
- There is quite a lot of camber correction, I am hoping this will be ok given that the car will only be used on track.
- Track with at the rear is constrained by the MX-5 running gear, and chassis width at the front by the size of the steering rack.
So - yeah - any tips / pointers / feedback appreciated: VSusp Link
Cheers!
Adam
[Edited on 12/10/15 by AdamR20]
|
|
|
|
|
phelpsa
|
| posted on 12/10/15 at 07:21 PM |
|
|
80/20 bump/droop on the rear sounds like a recipe for lift off oversteer and braking instability to me! For the average RWD car there's rarely
any reason to droop limit the rear as far as I can see. Whats your reasoning for it?
You've gone into quite a lot of detail without mentioning the most important aspect of suspension design.... what tyres you're planning to
use? What makes a car predictable (and therefore fun to drive) is keeping the tyre within a reasonably linear operating window in terms of load and
camber.
You'll never find a car with the 'perfect' suspension geometry because, in the grand scheme of car design, it doesn't make
that much difference, especially in circuit racing where tracks are smooth and geometric roll/pitch resistances are low. My recommendation would be to
package the rest of the car first, then add your suspension points in where they fit rather than the other way around. You can then look at the tyre
you want to use and how the kinematics work with it, and iron out any major issues. Take a good look at how the suspension locates structurally as
well as kinematically, as this can cause much bigger problems! A lot of designs start off trying to achieve the perfect suspension kinematics, and
then suffer terribly with compliance issues because of poor structural design required to achieve the mounting positions.
|
|
|
bi22le
|
| posted on 12/10/15 at 10:09 PM |
|
|
Chassis stiffness is key from the books that i have on chassis and suspension design.
I think getting the right mix of pickup points and angles should not be to hard. Its the rest of it that is hard work.
As Phelpsa said. Get the rest of the package together first. The mounting point location will be in the general area if your going for normal double
wishbone rear front inboard.
This comes casually from someone who has never designed a chassis but i have read some decent books on it and know that there are a lot of medium
ground to be right.
Track days ARE the best thing since sliced bread, until I get a supercharger that is!
Please read my ring story:
http://www.locostbuilders.co.uk/forum/13/viewthread.php?tid=139152&page=1
Me doing a sub 56sec lap around Brands Indy. I need a geo set up! http://www.youtube.com/watch?v=EHksfvIGB3I
|
|
|
Sam_68
|
| posted on 13/10/15 at 06:52 AM |
|
|
quote:
Originally posted by bi22le
Get the rest of the package together first. The mounting point location will be in the general area if your going for normal double wishbone rear
front inboard.
I'd disagree slightly on this... the job of the chassis - particularly a spaceframe chassis - is to 'join the dots' between
the point loads. You can't do that if you don't know where the dots are going to be!
To take just one example on the design the OP has posted, the big point loads from the rocker front suspension would be completely differently
located if he chose to go with a pushrod or conventional outboard suspension.
For what it's worth, OP, I personally avoid using roll centre migration as a means of trying to manage weight transfer, as I think it
risks falling into the category of 'trying to be too clever by half'. 
Cars don't simply roll around the roll axis - due to differing roll stiffnesses front and rear, they adopt a posture of 'skewed
roll' (ie. a combination of roll and pitch), so it's not easy to predict at the early design stage exactly where your roll centres will
migrate to. And then there's the potential for them to leap off in random directions if the chassis hits a single wheel bump, mid-corner.
Personally, I therefore stick to KISS principles and try to design roll centres that don't move at all, under any variation of
suspension movement, so at least I can be confident of knowing where the tricky little buggers are at! You can then manage weight transfer more
reliably using springs/ARB's (steady state) and dampers (transient).
But maybe that's just me being conservative/cautious... using roll centre migration is a clever idea, if you think you have all the variables
covered to make it work predictably. 
But as Phelpsa says, it's probably not the most critical thing anyway - you won't be getting much effect from the roll centre migration on
a circuit car, because there won't be all that much suspension movement. Circuit cars are easy - as Colin Chapman once said (paraphrasing
slightly), any suspension will work if you don't let it move too much!
And echo his comments about considering tyres first, and the limited rear droop.
|
|
|
phelpsa
|
| posted on 13/10/15 at 07:29 AM |
|
|
quote:
Originally posted by Sam_68
quote:
Originally posted by bi22le
Get the rest of the package together first. The mounting point location will be in the general area if your going for normal double wishbone rear
front inboard.
I'd disagree slightly on this... the job of the chassis - particularly a spaceframe chassis - is to 'join the dots' between
the point loads. You can't do that if you don't know where the dots are going to be!
This is true, but once you start to get a spaceframe together based around your packaging and safety requirements, it will start to become clear where
your structurally efficient load points could be. If there is a good reason not to put them there, then you can gradually move away from them, but the
further you go the heavier it gets!
|
|
|
AdamR20
|
| posted on 14/10/15 at 08:33 AM |
|
|
Thanks for all the feedback, info and discussion gents, I knew the forum wouldn't disappoint! I'll try and reply to each bit...
Rear bump / droop - that is want I guesstimated the split would come out to given the wheel rate that I'm aiming for, taking into account
experience from other similar cars. With hindsight I think that estimated was maybe a touch short on droop, but to be honest as long as I have enough
bump travel I don't mind. Lack of droop at the rear hasn't ever bothered me on circuit, but I won't be limiting it deliberately.
What is lifting off? 
Tyres - I appreciate the tyre does make a huge difference, but I always thought a radial tyre had a pretty narrow range of optimum camber for
cornering purposes (1-1.5deg negative)? I'll be using radials as this project will share the wheels with an MX-5 I have - on which I've
used anything from Chinese road tyres to Formula Renault slicks. Obviously they feel completely different but a pyrometer reveals that it
doesn't seem to make too much difference to how evenly the car uses the tyre. I am sure there would be time to gain from a 'proper'
setup though, especially given the suspension setup (yellow shocks and blue springs - no really, that's all I know about them, haha).
I also gathered that weight transfer (and this tyre loading) was a function of the basic dimensions of the chassis and not really linked to suspension
geometry, roll stiffness or spring rate. Obviously a 'stiffer' car will transfer the load more quickly, but the amount of load transfer
doesn't vary from my experiments with a set of corner weight scales, an inclinometer, a ratchet strap and a suitable anchoring point.
Completely agree that there is no perfect suspension geometry! However, I have found that it does make a huge difference to how the car feels, and
hopefully from studying a few cars I have driven it gives an indication of my preferences... Examples with links to VSusp as follows:
My current Fisher Fury (link) - high roll axis inclination, rear RC barely migrates through bump or roll
in either X or Y direction, whereas the front takes a massive dive under roll (approx 3 degrees at max cornering load) and migrates to the contact
patch. Upon kitting a bump (kerb) with the inside wheel, it migrates further sideways. From the driver seat, this gave me zero confidence to touch any
kerbs as the rear end would suddenly break away, and it would also have a tendency to do the same mid corner and on exit if you weren't very
careful with it. I believe a higher RC transfers a higher percentage of the load through the wishbones rather than the dampers, so the car is more
'snappy' and less compliant. It seems the numbers and the bum dyno add up in this case.
One of my favourite cars to drive is a standard (standard suspension, not lowered or stiffened) Mk1 MX-5
- they are so predictable and easy to pedal at the limit, which is where I want this car to be rather than an out-and-out lap time machine. Here the
roll axis is less steep, the RC migration in roll in the Y direction at both ends is minimal, and the X direction sees the front and rear diverge, by
approximately the same amount. Under bump, again there is little migration. This keeps the car stable and predictable. It also has lovely camber gain
so allows a 'soft' car, giving the driver plenty of time to react and lots of confidence, without mullering the outer edges of the
tyres.
Lowering an MX-5 to the sort of ride height my Race MX-5 is completely screws up the static RC heights
and there's a huge roll axis inclination, but at the sort of body roll angles we run mid-corner (3.5-4deg) the roll centres converge and
it's almost back to standard in terms of roll axis. The front end's RC does migrate a lot in the X direction though, however if you bash a
kerb with the inside wheel during cornering load it comes right back towards the centreline of the car, and the car still feels predictable to
drive.
The next one is my Westfield and this actually ties in quite nicely with some of Sam's points
(thanks a lot for your input by the way, you're one of the chaps I hoped would pass comment!). The wishbones are nice and long and there is very
little RC migration under bump (single or double wheel) or roll in either X or Y directions - but - the camber correction is quite poor, and I found
myself having to run huge amounts of static camber to prevent munching the outside edges of the tyres. I ended up over 3.5deg front and 3deg rear and
needed more if the adjustments on the car would allow. This obviously impacts straight line grip (both braking and throttle) which is why I wanted
something a bit more MX-5-y in terms of camber correction (so I could run less static camber). Using the uprights, diff, half shafts and steering rack
kinda nailed down quite a few of the potential 'dots' fairly quickly, so I was left with limited options. Having noticed the RCs migrating
around when playing with the MX-5 model I decided to pretty much copy how they acted rather than trying to be clever and using this to control body
roll, as I knew it already worked I didn't even consider that Sam but you have just given me some extra stuff to look into I think! Haha.
On a similar subject, I like the car to be pretty compliant as you can probably tell from the wheel rates, in the region of 1.3-1.4deg/g of body roll,
so there is actually reasonable amount of wheel movement going on, which is why I've spent a bit of time researching this. Again it comes back
to there being no perfect solution as phelpsa said, just the best compromise for you preferences and physical limitations. Looking at it again, I can
maybe widen the rear track a little, so the longer wishbones may help keep the RC more static.
Chassis design - I had actually read that it's a good idea to start with rough suspension locations first and then design the chassis around
them, which made sense to me, especially as I 'value' how the car drives - especially in the transitions - so much. I figured I could
always get the chassis to support the pickups sufficiently, even if it involved a little extra weight. Given the budget I'm setting myself, if
it comes in under 450kg, I'll be well chuffed.
I think the next stage is to bash some bits of 1" box (I haven't got the patience / tooling to build it all from round tubes!) around the
little grey bloke up there and see how things work out.
Thanks again for the input so far and future comments more than welcome, it's great to get the grey matter working!
|
|
|
Sam_68
|
| posted on 14/10/15 at 12:58 PM |
|
|
quote:
Originally posted by AdamR20
I also gathered that weight transfer (and this tyre loading) was a function of the basic dimensions of the chassis and not really linked to suspension
geometry, roll stiffness or spring rate. Obviously a 'stiffer' car will transfer the load more quickly, but the amount of load transfer
doesn't vary from my experiments with a set of corner weight scales, an inclinometer, a ratchet strap and a suitable anchoring point.
Total weight transfer is a function of basic chassis dimensions, yes, but how this is apportioned diagonally depends on other things, including
relative front:rear roll stiffness and roll axis.
The latter is the main reason you don't want your geometric roll centres to move around upredictably: if the roll axis shifts, then diagonal
weight transfer shifts with it. The worst case scenario (which can be encountered under some limited circumstances on the TVR Griffith/Chimaera, for
example) is that the roll centre at one end of the car leaps both horizontally and vertically in an instant - giving a shift in the direction of
forces analogous to a lever arrangement going 'over centre' - which at best will demand a clean pair of trousers, and at worst will kill
you.
In very basic terms, the rate of weight transfer is more dependent on the dampers than anything else: you can use the dampers to temporarily
'prop up' one end of the car to alter transient weight transfer.
For your corner weight experiments to give meaningful results, incidentally, you'd have to attach the ratchet strap at the centre of gravity of
the sprung mass ('cos that's the point that the 'real' forces act on), which is usually easier to do on a computer spreadsheet
than on a real car!
|
|
|
phelpsa
|
| posted on 14/10/15 at 07:40 PM |
|
|
quote:
Originally posted by AdamR20
Thanks for all the feedback, info and discussion gents, I knew the forum wouldn't disappoint! I'll try and reply to each bit...
Rear bump / droop - that is want I guesstimated the split would come out to given the wheel rate that I'm aiming for, taking into account
experience from other similar cars. With hindsight I think that estimated was maybe a touch short on droop, but to be honest as long as I have enough
bump travel I don't mind. Lack of droop at the rear hasn't ever bothered me on circuit, but I won't be limiting it deliberately.
What is lifting off?
Tyres - I appreciate the tyre does make a huge difference, but I always thought a radial tyre had a pretty narrow range of optimum camber for
cornering purposes (1-1.5deg negative)? I'll be using radials as this project will share the wheels with an MX-5 I have - on which I've
used anything from Chinese road tyres to Formula Renault slicks. Obviously they feel completely different but a pyrometer reveals that it
doesn't seem to make too much difference to how evenly the car uses the tyre. I am sure there would be time to gain from a 'proper'
setup though, especially given the suspension setup (yellow shocks and blue springs - no really, that's all I know about them, haha).
I also gathered that weight transfer (and this tyre loading) was a function of the basic dimensions of the chassis and not really linked to suspension
geometry, roll stiffness or spring rate. Obviously a 'stiffer' car will transfer the load more quickly, but the amount of load transfer
doesn't vary from my experiments with a set of corner weight scales, an inclinometer, a ratchet strap and a suitable anchoring point.
Completely agree that there is no perfect suspension geometry! However, I have found that it does make a huge difference to how the car feels, and
hopefully from studying a few cars I have driven it gives an indication of my preferences... Examples with links to VSusp as follows:
My current Fisher Fury (link) - high roll axis inclination, rear RC barely migrates through bump or roll
in either X or Y direction, whereas the front takes a massive dive under roll (approx 3 degrees at max cornering load) and migrates to the contact
patch. Upon kitting a bump (kerb) with the inside wheel, it migrates further sideways. From the driver seat, this gave me zero confidence to touch any
kerbs as the rear end would suddenly break away, and it would also have a tendency to do the same mid corner and on exit if you weren't very
careful with it. I believe a higher RC transfers a higher percentage of the load through the wishbones rather than the dampers, so the car is more
'snappy' and less compliant. It seems the numbers and the bum dyno add up in this case.
One of my favourite cars to drive is a standard (standard suspension, not lowered or stiffened) Mk1 MX-5
- they are so predictable and easy to pedal at the limit, which is where I want this car to be rather than an out-and-out lap time machine. Here the
roll axis is less steep, the RC migration in roll in the Y direction at both ends is minimal, and the X direction sees the front and rear diverge, by
approximately the same amount. Under bump, again there is little migration. This keeps the car stable and predictable. It also has lovely camber gain
so allows a 'soft' car, giving the driver plenty of time to react and lots of confidence, without mullering the outer edges of the
tyres.
Lowering an MX-5 to the sort of ride height my Race MX-5 is completely screws up the static RC heights
and there's a huge roll axis inclination, but at the sort of body roll angles we run mid-corner (3.5-4deg) the roll centres converge and
it's almost back to standard in terms of roll axis. The front end's RC does migrate a lot in the X direction though, however if you bash a
kerb with the inside wheel during cornering load it comes right back towards the centreline of the car, and the car still feels predictable to
drive.
The next one is my Westfield and this actually ties in quite nicely with some of Sam's points
(thanks a lot for your input by the way, you're one of the chaps I hoped would pass comment!). The wishbones are nice and long and there is very
little RC migration under bump (single or double wheel) or roll in either X or Y directions - but - the camber correction is quite poor, and I found
myself having to run huge amounts of static camber to prevent munching the outside edges of the tyres. I ended up over 3.5deg front and 3deg rear and
needed more if the adjustments on the car would allow. This obviously impacts straight line grip (both braking and throttle) which is why I wanted
something a bit more MX-5-y in terms of camber correction (so I could run less static camber). Using the uprights, diff, half shafts and steering rack
kinda nailed down quite a few of the potential 'dots' fairly quickly, so I was left with limited options. Having noticed the RCs migrating
around when playing with the MX-5 model I decided to pretty much copy how they acted rather than trying to be clever and using this to control body
roll, as I knew it already worked I didn't even consider that Sam but you have just given me some extra stuff to look into I think! Haha.
On a similar subject, I like the car to be pretty compliant as you can probably tell from the wheel rates, in the region of 1.3-1.4deg/g of body roll,
so there is actually reasonable amount of wheel movement going on, which is why I've spent a bit of time researching this. Again it comes back
to there being no perfect solution as phelpsa said, just the best compromise for you preferences and physical limitations. Looking at it again, I can
maybe widen the rear track a little, so the longer wishbones may help keep the RC more static.
Chassis design - I had actually read that it's a good idea to start with rough suspension locations first and then design the chassis around
them, which made sense to me, especially as I 'value' how the car drives - especially in the transitions - so much. I figured I could
always get the chassis to support the pickups sufficiently, even if it involved a little extra weight. Given the budget I'm setting myself, if
it comes in under 450kg, I'll be well chuffed.
I think the next stage is to bash some bits of 1" box (I haven't got the patience / tooling to build it all from round tubes!) around the
little grey bloke up there and see how things work out.
Thanks again for the input so far and future comments more than welcome, it's great to get the grey matter working!
I still don't understand the justification for the 80/20 damper ratio. I'd be interested to hear of a similar car that has that by design.
I can go into more detail about how i'd spec it if you'd like?
I'm not denying that kinematics CAN have a large effect in isolation, but their effects are often countered or masked in practice. Your design
might work very well in your ideal situation, but add in some compliance, a bit of non-linear damper actuation and a higher than expected CoG and you
still have a car with handling problems. My point is, get a design together that has the best top-level characteristics you can achieve with your
build constraints, in rough order....
1) Good driving position
2) Correct track and wheelbase for your chosen sport
3) Minimal mass
4) Minimal CoG
5) Minimal MoI
6) Minimal compliance
Once you have that design then you can look at how important some of these characteristics you're looking at really are. If your car is a 3rd of
the weight of an MX5, has a half the CoG height and uses the same tyres then you're probably going to struggle to influence the handling
characteristics at all using weight transfer as the tyres will be operating well within their linear region most of the time. Certainly the small
difference that a moving RC makes will be lost.
[Edited on 14-10-15 by phelpsa]
|
|
|
Sam_68
|
| posted on 14/10/15 at 08:41 PM |
|
|
quote:
Originally posted by phelpsaf your car is a 3rd of the weight of an MX5, has a half the CoG height and uses the same tyres then
you're probably going to struggle to influence the handling characteristics at all using weight transfer as the tyres will be operating well
within their linear region most of the time. Certainly the small difference that a moving RC makes will be lost.
Based on both experience and calculation, I'd disagree with this profoundly, but I have neither the time nor the inclination to get involved in
debate at the moment.
I'd argue that management of weight transfer is petty much everything in the handling characteristics of lightweight, non-aero cars. If
it wasn't, we could all give up worrying about spring rates, damping and anti-roll bars, because nothing beyond basic track/wheelbase geometry
and static weight distribution would have the slightest effect.
But I'll bow out of this discussion at this point...
|
|
|
phelpsa
|
| posted on 14/10/15 at 09:58 PM |
|
|
quote:
Originally posted by Sam_68
quote:
Originally posted by phelpsaf your car is a 3rd of the weight of an MX5, has a half the CoG height and uses the same tyres then
you're probably going to struggle to influence the handling characteristics at all using weight transfer as the tyres will be operating well
within their linear region most of the time. Certainly the small difference that a moving RC makes will be lost.
Based on both experience and calculation, I'd disagree with this profoundly, but I have neither the time nor the inclination to get involved in
debate at the moment.
I'd argue that management of weight transfer is petty much everything in the handling characteristics of lightweight, non-aero cars. If
it wasn't, we could all give up worrying about spring rates, damping and anti-roll bars, because nothing beyond basic track/wheelbase geometry
and static weight distribution would have the slightest effect.
But I'll bow out of this discussion at this point...
Come on Sam, play the game! That should have read 'geometric weight transfer', however, for a given tyre, a car with a 3rd of the weight
and half the CoG height will be much less sensitive to total weight transfer as well. My point is more that it CAN be everything, or it CAN be
nothing. There is no one rule fits all, and until you've understood how your tyre characteristics meet with your car characteristics it is
impossible to know how to characterise your suspension.
I've designed, set up and driven cars working with very linear tyre characteristics and it really does bring it back to the raw basics of
track/wheelbase, static weight distribution and MoI. Diff settings and steering geometry have a much greater effect than any suspension parameter.
|
|
|
Sam_68
|
| posted on 15/10/15 at 06:46 AM |
|
|
quote:
Originally posted by phelpsa
Come on Sam, play the game! That should have read 'geometric weight transfer'.... My point is more that it CAN be everything, or it CAN be
nothing.
Well, sorry, but telepathy is not amongst my many talents... :p
The word 'geometric' makes a big difference (though I still disagree to an extent - unpredictable roll centre movement is, even at best,
capable of making a car feel unsettled and 'squirrelly'.
And your previous statement didn't read to me that you were suggesting it 'CAN be everything, or it CAN be nothing'... it seemed to
be suggesting that you believe it to ALWAYS be insignificant.
Certainly, as I implied earlier with the Colin Chapman quote, it's possible to turn a chassis into such a blunt instrument that nothing else
matters on smooth surfaces, but if you choose to endow your suspension with a reasonable amount of compliance and/or drive it on surfaces that
are less than billiard-table smooth, with no aero, then you'll need to deal with a lot more than the raw basic factors that you quote.
But honestly, I'm afraid I don't have time to fully engage in this discussion at present, so if you're happy to continue to
promote the 'go-kart' solution as the way to go, then I'm happy to leave you to it!
|
|
|
ettore bugatti
|
| posted on 16/10/15 at 06:17 PM |
|
|
What's your estimation on the sprung/unsprung weight?
Roughly described: static deflection equals droop travel of the suspension and bump wise I imagine you would like to have as much travel without the
chassis hitting the floor.
If you are designing a car with 40mm ground clearance then 35/20mm bump travel is feasible, but with a soft ride you will be hitting bumpstops all the
time.
Also, I noticed you have a big difference between the front and rear track.
Another subject to dive into is inertia.
A sportscar with 50/50% distribution and a 'heavy' engine in the front and diff in the back has different properties than a 40/60% car
with all the mass centralized.
I reckon that influences the feel of the car more than the suspension geometry.
|
|
|
Sam_68
|
| posted on 16/10/15 at 06:49 PM |
|
|
quote:
Originally posted by ettore bugatti
Also, I noticed you have a big difference between the front and rear track.
This is a very fair point - I must confess that I hadn't even bothered to check your Vsusp link until Ettore mentioned it, but you have a much
wider rear track than front?
Not saying that it's impossible to get that to work, but it's unusual to say the least, and would require a fairly extreme approach to
front:rear roll resistances.
|
|
|
motorcycle_mayhem
|
| posted on 16/10/15 at 07:05 PM |
|
|
450Kg looks entirely feasible, given that what you've drawn a schematic of appears to have more than a slight resemblance to the Riot (except
the central seat position).
|
|
|
AdamR20
|
| posted on 25/10/15 at 04:19 PM |
|
|
Oops, didn't mean to cause a storm!
Sorry for taking ages to reply, been a hectic few weeks and not had time to sit down and compose a reply properly.
Anyhoo - here goes.
The 80:20 bump:droop thing, as I said it was just a guesstimate: Rear bump / droop - that is want I guesstimated the split would come out to given the
wheel rate that I'm aiming for, taking into account experience from other similar cars. With hindsight I think that estimated was maybe a touch
short on droop, but to be honest as long as I have enough bump travel I don't mind. Lack of droop at the rear hasn't ever bothered me on
circuit, but I won't be limiting it deliberately.
I have seen how quick your car is Adam so would love to hear your thoughts if you wouldn't mind - thank you! I do like the car to have a lot
more wheel travel than most though, so maybe RC migration is more of a consideration for me than others, who knows.
Regarding your list of design considerations, I'd have thought you consider each and every one of those during every single decision throughout
the whole design and build of the car? That's what I've been trying to do anyway... Apart from correct track and wheelbase - that is
pretty much fixed as mentioned before: MX-5 rear end fixes the rear track, front can't deviate too much from that, then wheelbase is somewhere
in the 1.55-1.65x track width (so I'm shooting in the middle - MX-5 is 1.59, my Westfield is 1.64).
Interesting you consider diff settings and steering geometry have more effect on the car than any suspension parameter - I really can't get my
head around this! Maybe a bit of an extreme example, but surely having a 3hz wheel rate at one end and 1.5hz at the other completely destroys the
handling of the car, and no amount of diff tweaking or steering fiddling will compensate? Extreme example, but I think you know what I mean.
Ettore - I am hoping for around 420kg sprung and 110kg unsprung mass (450kg for the car, 80kg for me, 25kg per wheel unsprung at the front and 30kg at
the rear). Thanks for the info about static deflection, I had intended to try and work that out but what you said makes perfect sense
I have updated the suspension 'design' now and changed track widths quite a lot. I had intended to get the front as narrow as
'possible' (I know standard track Westfields are about 4" narrower at the front than the rear) to aid with frontal area, but ten
decided to widen it to help with geometry and after what Sam mentioned. Also worked it around an Escort rack (found one cheap) instead of MX-5, so I
can move the lower front wishbone pickups inboard a fair amount.
Agreed that the position of the components within the chassis changes the feel of the car quite a lot - but I guess the only way to say for sure if
it's a bigger effect than geometry would be to build two cars the same and try them out! I am comfortable with a lot of weight at the rear (wife
has an Elise which is 62% rear on my scales), but the Fury I had didn't feel anything like this, so it leads me to believe it was the RC heights
and migration.
Thinking about it - I should probably set up a VSUSP for the Elise in that case...
Here's the latest one, but still will probably change! http://tinyurl.com/phefzjt
|
|
|
ettore bugatti
|
| posted on 25/10/15 at 07:23 PM |
|
|
I dont have much too add at this point, others might.
Probably, it is worth to plot the interesting characteristics of the cars you've put in V-susp in a table/ graph for comparision/ discussion
material.
This might be worth a read, it is about the Sylva Mojo suspension:
http://www.neileverett.madasafish.com/mods/mods_springs_dampers.htm
|
|
|
Sam_68
|
| posted on 25/10/15 at 10:14 PM |
|
|
quote:
Originally posted by AdamR20
Regarding your list of design considerations, I'd have thought you consider each and every one of those during every single decision throughout
the whole design and build of the car?
Yes, indeed.
Design is always an iterative process, which is why I was too ambivalent to be bothered taking issue with Phelpsa's earlier suggestion that you
should draft out the spaceframe before knowing whether the major load points will be. For what it's worth, I think that your way is perfectly
correct, whereas Phelpsa's will be likely to end up with an unnecessary amount of abortive iteration in the design, but it's not a
big enough deal to be worth making an issue over... so long as it's not me who's paying the bills on the design time!
But the iterative factor does mean that you have to be careful not to suffer unintentional 'drift' on early, key factors as you perhaps
lose focus on them as the design progresses. So you're quite right in saying that you need to maintain consideration of those basic factors with
every decision made.
quote:
Originally posted by AdamR20
Interesting you consider diff settings and steering geometry have more effect on the car than any suspension parameter - I really can't get my
head around this!
It's not so difficult to appreciate, and I agree with Phelpsa on this one, when you consider that what it ultimately all boils down to is loads
and slip angles on the tyres. Steering geometry and diff settings certainly can have just as much, or more influence on relative front:rear
slip angles than diagonal weight transfer.
This is particularly the case on very wide, low, stiffly sprung cars. With more suspension compliance, the emphasis shifts progressively toward
management of weight transfer through the sprung mass, however (not to say that steering geometry and diff action don't maintain a big
influence, even then).
The other biggie on some cars (not Sevens!!) is aerodynamic downforce.
I'm guessing from his previous comments (apologies and correct me if I'm wrong...) that Phelpsa's experience is biased toward
wings-and-slicks type circuit race cars, with very little suspension compliance of any sort - essential to maintain a level aero platform; fast, but
not especially pleasant, progressive or fun to drive, and not particularly conducive to mechanical grip if you don't have the invisible hand of
aerodynamics pushing you down onto the track.
The suspension frequencies you're suggesting are much lower than you'd design for a 'proper' wings-and-slicks race car,
but again I think you're probably on the right track (pardon the pun), given that easy/fun handling on the limit is your stated priority, rather
than ultimate lap times, necessarily.
|
|
|
|
