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]
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.
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.
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.
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!
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!
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.
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!
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.
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...
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.
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.
quote:
Originally posted by ettore bugatti
Also, I noticed you have a big difference between the front and rear track.
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).
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
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
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?
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!