Suspension design/car design and outcomes

[NeilRoshier]

I have read much in the press (tech and popular) regarding the handling of certain designs of cars and the amount of work to get the said cars to where they are. Some of this has been specifically related to the engine position within the car such that front/mid engine placement would seem to be reckoned to be more stable (particularly with lift off cornering/braking) than mid/rear engines.
There would seem to be a number of reported overall design elements that affect this, such as:
1. c/g (vertical and horizontal)
2. Polar moments
3. Tyre contact patches
4. Camber change in roll (not too sure on this one)
5. Toe changes in roll (ditto)
6. Others? (sorry I do have a list but do not want to take up too much forum space with possible ramblings)
After discussing these things with fellow interested persons, the consensus was that mid/front was fundamentally easier to construct/tune/setup and that the mid/rear was best left to the professionals. Is there a fundamental advantage in the front/mid RWD setup as far as predictable setup and is there any fundamental disadvantages is such a design (I have noted that the vast majority of 'GT' and 'sports' cars are mid/rear eng)?

 

[GregLocock]

I'd like to see your full list.

To me the most obvious example of this was driving a Lotus Excel (based on the Eclat) compared with an Esprit. Developed by the same people at the same time, using the same engine, weighed roughly the same. The front engine Excel was a very nice car to drive quickly, and, at least when it had passive suspension, tended to stay on the road.

The mid-engined Esprit was a nutter's car. Driving quickly in it was a wracking experience. Obviously the limits were higher, which complicated things.

I suppose the cars that established the evil tendencies of rear engine in folklore were the Corvair, of which I only know what is in Milliken, and the 911.

The popular press seemed to think that the high polar moment of inertia of the 911 was the cause of all the trouble. This is ridiculous. A (Ford Australia) Falcon wagon has a much higher PMI than an early 911, and is very happy sideways even in my unskillful hands.

I think you'll have to be careful to separate out the effects due to the higher limits of the typical mid engine car and those due to the fundamental suspension properties.

Cheers

Greg Locock

 

[NormPeterson]

I'd think that a slightly larger PMOI would normally be associated with a front/mid design than a mid-rear layout of equal weight since the final drive bits are separated from the engine by a significant distance (the tranny could be attached to either). That would slow down the yaw response a little bit, particularly in lift throttle cornering situations. You'd sense the extra "time cushion" in which to take corrective action as better predictability.

Packaging a front/mid arrangement might be more difficult, particularly with the smaller chassis and a V-type engine vs footroom, though cooling issues would be simpler to solve.

Norm

 

[hpfreak]

In my mind, the major factor would be c.g location. The front engine placement typically results in front weight bias. This is a more stable condition b/c the heaviest loaded tires give up first. If they are in the front, you get understeer, if it's in the back you get oversteer.

When you have a rear weight bias, combining go/whoah forces and the higher cornering forces in the rear is a big request from the tires and lift off is the worst case scenario. If you have a front weight bias, the higher cornering forces are in the front and the rear can tolerate more go/whoah forces before sliding.

 

[NormPeterson]

I'd like to see that full list myself. If nothing else, it'll help control the assumptions I might make.

Re: cg location: it's entirely possible to construct a front/mid car that's more than 50% rear-heavy. Things like some replica Cobras and Sevens and the Panoz endurance racer come to mind.

I'd sidestepped item #1 on Neil's list partly because of the above and partly because you can crutch the effects of weight distribution by tinkering with such other parameters as the wheel/tire package, inflation pressures, camber & toe curves, specifically developed tires, etc. But once you've done that to maximize steady-state lateral acceleration you've pretty much defined what the effect on yaw response will be (either turn-in or incipient spin), which is where PMOI comes in - the little one will always result in greater yaw response (nice for the former at an auto-x, not so nice for the latter). To close the loop to the topic starter, I guess that this level of effort is getting somewhat away from most amateur efforts and into the realm of people who do it for a living.

Norm

 

[GregLocock]

Generally you adjust the height of the rack or the outer joint to change the centre of the curve, in jounce terms.

You change the curvature of the curve by changing the length of the tie rod. With an appropriate choice of tie rod length it can be straight, or toe in on jounce, or toe out on jounce, compared with design.

By using the combination of relative height and tie rod length you can end up with characteristics that run the full gamut of toe out at rebound to toe in at rebound, and the same at jounce.

It is worth adding that compliance effects are very important in the toe curve, in practice. The steering 'loop' should have a total stifness of at least 5000 if not 10000 N/mm at each end of the steering arm. Since in a production car you are also trying to use bushes with rates of the order of 1000 N/mm in various directions it goes without saying that the forces need to be carefully considered, and the 3d geometry is crucial. If you are using spherical joints everywhere then this is perhaps less important.

Getting this (and similar curves) right for a new front suspension geometry typically takes two months of argy bargey between the suspension design dude and the draughtsman. Then we do it all again when we build the protos.



Cheers

Greg Locock

 

[NeilRoshier]

I am sorry to say that I think that I gave the full list to my brother-in-law along with a number of books and the 2 clubman chassis I was given (I'm too tall to fit them!). However I do have some notes here and will compile them and post them tomorrow night. Thanks to everyone for the responses so far.
Neil

 

[NormPeterson]

Don't forget that toe changes also occur at the rear wheels. In the case of a stick axle rear, you have rear axle roll steer, which is essentially the same thing. And it does affect handling - as one illustration as to the effect, setting a little static rear toe out is one quick and dirty way to get a FWD car to 'turn in' better on an autocross course. Kind of a band-aid, and not really an adjustment of the curve shape (just a point on a different curve in the same family of toe/steer curves), though it demonstrates the effect toe has on handling. The NASCAR and other circle track folks have used this [axle steer] as a chassis tuning method, albeit asymmetrically, to favor left turns.

I'll go a little further and say that getting the rear toe steer correct is arguably more important than the front, since you have only indirect control over rear steer (via chassis roll and its associated time lag with respect to steering input).

Norm

 

[NeilRoshier]

Ok, some more brief notes. Please understand that these are not necessarily my opinion but things that have been read in the 'press' and I have no firm idea of their validity/value (hence my queries on this forum!) though I would appreciate peoples opinion.
6. contact patch size relative to C/G
7. driveline torque reaction at limit of adhesion relative to vehicle mass (supposedly transverse engine torque reaction in chassis of mid/rear is less likely to 'upset' the car approaching limits of adhesion when compared to front/mid which supposedly has greater rotating masses reacting longitudinally....(really not sure of this one)
8. Driver awareness of attitude of car, with rearward (front/mid)driver having an advantage
9. Placement of driver mass better in mid/rear car, closer to midline and C/G thus less variability when drivers change weights etc.
10. Better packageing of fuel mass in mid/rear cars realative to C/G (I don't agree with this one as given a clean sheet of paper lots of solutions are possible)
11. Front/mid car has greater contact patch area at front (assuming contact patch relative to C/G with front weight bias) and thus better breaking...would seem feasable in straight line but only so much adhesion so not sure about in turns?

There are a few more but I will have to go and get the notes, but I would appreciate any comments.

Greg I am interested in your comment that the typical mid engine cars had/have higher limits. Do you think that it is due to the intended/designed performace of the cars or does the mid engine car have a specific advantage in layout?

Norm thanks for you replies. I agree re the packaging of the 'V' engines in a front/mid car vs the mid/rear. There also seems to be a weight penality with the front/mid engine location due to the increased driveline mass and the chassis mass. This was demonstrated recently with a friends clubman (lotus 7) car weighing 500kg with a toyota 4AGE and his lotus 23 replica weighing 390kg with the same engine. There was substantially less steel in the chassis and the driveline mass was much less. Interestingly apparantly the chassis performed at similar numbers when tested.
Packaging my 203cm/120kg frame in a car is also a factor in the decision, but essentially will mean a longer wheelbase...I digress.
Also what to do with all that heat infront of the passenger compartment is also an issue for me.

My concern with the front/mid vs mid/rear is that if I proceed with a front/mid car that I will concede too much performance compared to a mid/rear layout, conversely if I go mid/rear I will be biting off more than I can chew and the layout is intrinically less easy to make successful (for an amature like me in a 'hobby' car.

 

[GregLocock]

"Greg I am interested in your comment that the typical mid engine cars had/have higher limits. Do you think that it is due to the intended/designed performace of the cars or does the mid engine car have a specific advantage in layout?
"

I think that you would only bother with mid for a handling oriented car so they are prepared to make other sacrifices as well to preserve the handling.

"conversely if I go mid/rear I will be biting off more than I can chew and the layout is intrinically less easy to make successful (for an amature like me in a 'hobby' car."

I don't really agree with that. Anyway, if it handles badly you'll learn a lot sorting it out! Evil grin.

I don't see why a mid should be a bad car, for your purposes. I do agree that a rear engine is tempting fate, but a mid engine seems to me to be eminently do-able.



Cheers

Greg Locock

 

[NeilRoshier]

Dick I have seen a couple of 4WD clubman cars in the UK, DAX do a kit for one called the 'Rush', held the 0-100-0 world record there for awhile. I generally like clubman cars, but when at Phillip Island GP track my wife commented that they were going awful fast and close to the concrete barriers and just at that moment a 911 speared off the main straight (standing water on the track) into the tyres. I am 'encouraged' to have as safe a car as possible and clubman cars are just not that great in that area.
I have been doing scale drawings with my frame as the central element....its just placing all that weight off to one side of the car, gordon murray had a good idea with the F1's central pos'n.
Greg thanks for your thoughts. I will relook at the mid/rear engine location. In truth finances and transaxle choice will probably detirmin which way I go as much as anything else. I have been looking at the nissan VQ30DET engines due to power/size/weight being very good (did have a quick flash of the falcon T6 and auto in a TVR type car...bit heavy though).
You have mentioned in that past that the Front eng RWD cars seem to be more controlable when grip is lost, could you expand on why this is the case?

 

[GregLocock]

One reason I'll push you towards a mid engine car using a FWD transaxle is the Ginetta G15 (?)- an old kit car in the UK. Very nice to drive, using a Hillman Imp engine, brought up to perhaps 45 or 50 hp.

I haven't got a good answer to your question. Part of it is driver position, I think. I am investigating the effect of polar moment of ienrtia at the moment, but I really don't think it is much to do with it.




Cheers

Greg Locock

 

[NeilRoshier]

Thanks for you honest response Greg. I have seen a couple of Ginetta G15's (always wanted a G12 myself...just too damn tall!...also liked the G4, but Rochdale Olympic was the eventual choice) and I thought that they were rear eng ala Imp?
They certainly do well on the hills.
I have spent a couple of frustrating hours on the phone trying to find FWD transaxles for 3lt+ V6 engines. Very little choice in manuals, basically mitsubishi or toyota. I have found that Renault 21 turbo transaxles are cheap enough...if you can find one in reasonable condition. Given plans for 160-200kW at the start the transmission strength is an issue.
Do you have a 'favorite' car for driving in your memory?

 

[GregLocock]

G15 might well be rear engine, can't say I remember looking. I know I did look, as I remember trueing the carb castings once to stop a leak.

High torque manual transaxles will be a problem... always have been.






Cheers

Greg Locock