Rear suspension bump steer

[David Briggs]

This is a wonderful discussion! Who can resist? There is a great road near my house that has a long straightaway that happens to be the only passing area for about 9 miles of twistys, at the end of the staight is a dip that causes the car to unload just as you are braking hard for a sweeping right hander.
The first time I had my CAV out(after extensive shakedown and engine break in) I blasted by 5 ordinary citizens at about warp 6, pulled back into the right lane and started braking for the turn just as I hit the dip. The rear suspension unloaded and when the car came back down it was all over the road! Must have been very entertaining for the masses following me, as I am sure they would like to see me get my comeuppance!.
Needless to say my interest in bump steer began to peak.
I found several issues with the stock CAV rear set up. I saw Bob Childresse's excellent build pages on his CAV and contacted Ian(Canuck 40) and purchased one of his lower control arm kits(spherical bearing on the inner) toe adjustment possible without shims and some bushings that allowed the upright to be pushed further back in the wheel opening. I found that at full droop the sway bar links were binding on the lower control arms and had to relieve the clevis' to allow more movement, I removed the spring spacers from the shocks(I did not want to redesign the entire car), tilted the uprights back at the top and headed for my favorite real world bump steer test site.
The car was transformed and is now much more predictable and nuetral handling.
I too am a closet engineer but sometimes you can just look at something and know if it will work or not, but the best thing(for me) is to experiment a little and drive it.
My goal with this car is to have it brake and handle like my Euro Swift Formula Ford, ride better than my SLK 350, and be scary fast and utterly reliable. (beside the obvious looks and sound of the thing).
I hope this helps somebody with this subject.
Dave

 

[Russ Noble]

Chris, you're right it's a comparative thing. The standard TR7 handling sets a very low base! I'm sure your RF in standard form would be much better than my TR7V8 in race trim!

For comparison, I had 6" rims all round and was mixing it with a gaggle of Porsches with 7" fronts, 9" rears. The handling was "hectic" to say the least. One of the P car drivers complained I was holding them up through the twisty bits but they couldn't nail me on the straights. However he wasn't very receptive when I suggested we swap cars for one race!

It'll be interesting to see how your suspension figures compare to Ross's when you post them.

Cheers

 

[Chris Coote]

Chris,
This is purely a question.You state you have been progressively making "adjustments" but have not elaborated.These one would suggest be anything from tyre pressure-shock absorber setting-different tyre brand maybe as you run 17s -coil rate change -a bit of castor adjustment one might assume.As a "standard" RF built street car (and note the word standard as per RF offered suspension) it does the job.Lets call it the "poverty pack" and as such does what it is supposed to.Stops and goes round corners at legal speed limits.To me I built the XR8 version (to o/seas readers the HiPo version) and as such is pretty tragic.Robert Logan said to me to be careful as it "will bite you very quickly if pushed hard". The 1st time I thought crap driver-2nd time thought dooooh-3rd time this aint right-4th time this aint fun anymore.All a learning curve but I could hammer my old 302 Boss which had a few suspension mods a lot harder.Obviously there was a problem and my 1st thoughts were rear sway bar until this whole suspension thing was put on the forum.Looking at it it started to make sense.Another discovery was the car actually "bump steers".Rack height is wrong.
We are currently working on this with a threaded sleeve and bush scenario.Stay tuned,
My eyesite aint that good these days but I could swear that SPF GT40 had a similar set up to what Jim Cowden has come up with so why not try it.I am sure if it did not work for you Jim would happily refund.Lets put it another way "would you fit an sports aftermarket suspension to your family hack over the standard factory offering?"knowing full well all the additional benefits.
I know a former Australian Formula Ford champion who happens to own a "standard" RF who has turned his car around a few times,said it is all wrong,looked at my set up and said "thats next on the agenda".I know the roads are pretty good in Vic compared to NSW.Cheers and all IMHO.

 

[ChrisL]

Chris,
Here are some conclusions I've made in my investigations, and I am not at all claiming to have any qualifications in this area at all, but I feel I can analyze things well enough to satisfy my own curiosity.

The "all wrong" comment is both true and false. All wrong, yes, by modern Formula Ford theories and for that matter all modern open wheelers. They use the reverse characteristics of the earlier race cars. Specifically, modern open wheelers are designed to be unstable under braking and stable through corners (see my next post), whereas the early race cars are designed for stability into corners and instability out. I guess time has shown that the modern way is better, but that does not make the old way bad by any means. The handling of my RF consequently is good at track speeds, as well as road speeds.

I'll briefly gloss over what I have done to the front suspension in this post and then post another larger one for the more complex rear.

Firstly, I have found that the steering is very sharp at the wheels. However, this did not translate to sharpness at the steering wheel. The reason for this was average quality rose joints at the suspension arms and the shockers. There was enough play there to allow the front wheels to minutely steer on their own and this was then amplified by the sharpness of the steering. I found I was constantly compensating for the self-steer and needed to concentrate to keep it going straight. Once I removed ALL play by both upgrading the rose joints AND eliminating play between the retaining bolt and the rose-joints, I noticed an immediate improvement.
Secondly, (for high speeds) I ensured that there was minimum air getting under the car by removing grill-mounted number plates and angling the small nose-to-radiator panel upwards, and finally by ensuring that the floor of the car was slightly higher at the back.
Thirdly, since the front has so much more grip than the rear, I stiffened the front shock absorbers to 3 notches from full and the rear to 7 notches. This didn't improve the handling through the corner but it did make it more predictable to the initial approach.

That’s basically it for the front. No massive camber or castor change and no spring or shock replacements.

 

[ChrisL]

My rear suspension findings.

I think my approach is slightly different to Ross’s so the figures should also be different in the final wash. My starting point however was to listen very closely to everything people had to say (Ross and Will have put in some serious thought into their solution) but then try and work with what I have. As well as listening to others, I’ve also done some general researched on the rear dynamics of existing production mid-engined cars and some open wheelers. Interestingly, I found quite a lot of info on the Toyota MR2 which is a car I’ve previously owned so I’m familiar with its characteristics. A common thread though that seems to exist amongst all mid-engined cars in that they all seem to run about 0.5deg static toe at the back. The Honda NSX however to name one, has a geometry that will near enough hold that value through the whole of the suspension travel range. The MR2 on the other hand, dynamically increases the toe-in under compression and consequently reduced the toe-in under rebound. This results in the car being very stable through the corner (to the point of understeer) but very flakey under hard braking, swapping ends quite readily.

Given however that I am stuck with a geometry on the GT40 that has the opposite dynamics to the MR2 (decreases toe-in under compression and increases toe-in under rebound) I decided to work with it instead of against it.

My approach was to firstly minimize the amount of dynamic toe. Using Ross’s findings I adjusted rear castor as much as the standard suspension would allow. Secondly, I removed the shock absorbers and raised the suspension to what would be full compression and adjusted for 0deg toe. When I then lowered the suspension to the static position, I was ecstatic to find a toe-in value of 0.6deg. I then adjusted for 0deg static caster and reassembled the car. This setup ensured that I would never get toe-out under any circumstances so that made me feel I had a good starting point.

The remaining dynamic toe however did not necesarily concern me because I figured that, ideally, a car with this type of dynamic toe should exhibit the following rear-end characteristics…

Brake-and-turn.

Will be extremely stable under hard braking since the rear wheels would toe in as the rear lifted. This would allow the driver to brake very late while approaching a corner and continue braking well into that corner (and hopefully even promote a little understeer while braking). At mid corner, the driver would complete the braking maneuver and begin to accelerate out of the corner. If the car is traveling fast enough (and the transaxle has an LSD) then the tendency would be for the car to push (understeer) further. However, at this point, the car would squat, the rear suspension would compress, so reducing the amount of toe-in to the loaded wheel. The harder the car accelerates the less toe-in results and the more the car will turn out of the corner. Balance and steer would then be achievable through the throttle.

High speed through a sweeper.

As the car approaches the corner in a straight line, both rear wheels are evenly loaded so each toe-in value will negate the other. As the car begins to move through the corner, the inner wheel will begin to unload and so will contribute less in terms of rear-wheel steer. Conversely the outer wheel will contribute more toward rear-wheel steer as it begins to load up, so would cause an imbalance between left and right. This however is negated by the fact that the loaded wheel (the outer) is now reducing its toe-in under compression so balance is maintained. This effect is maintained even if the road is irregular (bumpy). When the loaded wheel encounters a bump, the forces between road and tyre increase and the friction values therefore also attempt to increase. If there was no further change, then the loaded wheel (given that it is toeing in) will either tend to kick the car inwards if the friction max was not met, or kick outward if the friction levels were exceeded. However, since the suspension dynamically reduces toe under compression, the effect of the increased forces are also negated, so balance is maintained. Note that this bump effect is also valid while traveling in a straight line.

Oversteer stability.

Simply put, if a driver has pushed the car beyond its limits into a corner to the point at which the back loses traction, in theory the suspension will not compress much further as by definition, the rear end has reached its max lateral force. If the driver could at that point raise the rear of the car, either by lifting off the throttle or in some cases even applying the brake, then the loaded wheel (the outer) will toe-in further and oppose the rear-end slide. Backing off during oversteer is a natural response for an inexperienced driver, so this characteristic should be good.


Well that was the theory. The car was on the track at Sandown with this setup a few weeks ago but unfortunately I developed rear brake problems after a few short laps so my testing was minimal. The signs however were all good. The car was very stable under hard braking and at the long sweeper at the end of the back straight. I pushed the car into oversteer through the esses and managed to recover cleanly by backing off the throttle. The scrub on the rear tyres also seemed to be very even. I didn’t have my temp gauge with me though so I can’t confirm that the tyres were actually loading evenly.

Note that all my measurements are approximate since they were all done with strings and rulers so as I said previously, I’ll confirm my findings and get accurate reading at a later stage.

 

[Ross Nicol]

Chris
I can't go along with a lot of this. Just a couple of points I have a problem with. When you near a corner at the end of a straight and throw on the brakes the car's weight transfers to the front, the rear goes light and if brakes are biased too much to the rear they will lock up the rear wheels. In an extreme case you will spin. At this point you are not turning the car either so toe effects at the rear are not an issue. Now as you turn in the car's weight transfers to the outside wheels and toe on that highly loaded rear is extremely important. If you have any toe out on that wheel it will steer outwards which is the opposite to the steering your putting on the front.It is necessary to set enough rear static toe in for the previous situation to never occur, and from what you say you have done this.Lightly loaded inside wheels which are generally in rebound (in a corner) play a much lower part in the grip and handling stakes, to a point you wouldn't care about the toe settings on them. It is good to hear the owners of road cars getting into this to get a safer set up, as toe out steering of the rear is an unstable condition and a road car driven hard into a corner could experience it, unless the tyres lost their grip first.
Ross

 

[Russ Noble]

Chris
I can't go along with a lot of this. Just a couple of points I have a problem with. When you near a corner at the end of a straight and throw on the brakes the car's weight transfers to the front, the rear goes light and if brakes are biased too much to the rear they will lock up the rear wheels. In an extreme case you will spin.
Ross

If you turn in on the brakes as is often done in racing and have too much adjustment on the back you will get tail happy or spin LONG before the rears lock up! If the rears are locking you've got miles too much on the back. Under racing conditions you should wind front brakes on until they just start to lock at maximum straight line braking then back them off slightly. That way you are getting maximum braking effort and can go deeper into the corner under brakes. IMHO of course and every racer has different preferences.

If you are running too much rear brake and start screwing more onto the front you will find that the harder you brake the more weight transfer to the front you can get and hence more grip at the front meaning you can run even more front brake. Of course you can't get this same weight transfer in the wet which is why you have to back off the front brake adjustment on a wet or slippery track. This is why road cars generally are set up with too much rear brake so you don't get the fronts locking so easily in the wet. With dry bias a race car just won't slow down in the wet, the front wheels will lock and one is left with the impression of accelerating to one's impending doom!

Cheers

 

[Chris Coote]

Guys,
4 years ago the GT40 forum thread was a wealth of information building more so than today.The original majority now have built our cars and are moving on as well as developing them.We seem to be deviating from the (our) original concept as owner builders as our own cars are personally developed.It is to each their own and their own modifications.For instance my car does not go round corners "as is" but Chris L can find a way.I change mine and it goes faster around corners faster again and post what I have done.I guess at the end of the day we all strive to improve on ourselves and our car (GT40).
The point I am making is that I expressed a view regarding rear suspension on a RF car that to me was a concern and a possible rectification .
I have also noticed a lot of the early posters do not post anymore and this is not because they have sold their cars but a bit "over it".Most of us are not Engineers but ordinary folk and we should be talking ordinary folk and if one looks back 2-4 years on this forum that was our langauge.

 

[Russ Noble]

Chris, I see what you are getting at, but building a GT40 particularly when you might not have done anything like that before is a learning experience. Some people who build them like to learn the whys and wherefors so they can make better decisions. This naturally becomes technical, but whenever I have asked a question it has been explained to me in simple language, if I still don't understand or miss the point and say so, I am given another even simpler answer until I grasp the concept.

I have learnt heaps in the last few years since I found this forum, thanks to those who have chosen to enlighten me. It is probably natural that anyone with a work in progress is going to be more active on the forum than someone who has completed their build and probably moved on to some other challenging and time consuming occupation.

The forum content is determined by those using it at the time. Possibly everything from four years ago has already been thrashed to death and is available via the search button anyway so it is natural that threads should now be expanding and treading fresh, maybe more technical, ground. A bonus I would have thought? Those who wish to get more technical are able to, those who have no interest in such things need simply ignore them.

Cheers

 

[Jac Mac]

Chris, I tend to agree with Russ on this which is a scary thought in itself. I find that in trying to answer questions of a tech nature I have to try and get a mental picture of the level of understanding the forum members asking questions at the time have on the subject. I would hope that on this Forum if a member cannot understand due to tech terminology that they would ask for clarification in more simple terms or PM to that effect rather than say nothing and remain uninformed. I have only been on here for about 11 months and have seen some leave, but also a few 'old' faces return in that time. I think thats the nature of the beast. Really there have not been many "Flame" jobs compared with some other sites that I can think of.
With suspension development there are so many variables that while a fix may work on car "A", car "B" may require a totally different adjustment. At least the Forum will give you many suggestions to try.

Cheers
Jac Mac

 

[roaldin]

These frame grabs are intended to demonstrate the problems of bump steer and positive camber (outside tyre with regard to the road) on roll on a standard RF (as delivered to me anyway). Admittedly they aren't the clearest shots but I think they show the problems to some degree.
The road is heavily cambered although it's not all that apparent in the pic's.

After this session it was apparent that more camber was needed - the tyre was only worn on the outside 1/3, in fact the "tread" was almost gone on the outside while the dags were still attached inside.
Bump steer was quite extreme, something like 18-20 mm (at tyre) across full bump to full droop. I don't think my GT-R has that range of movement in it's computer driven and designed-in rear steering. I don't know whether that was typical or just my car.
It turned out that the car also had a bent sway bar which made things a bit more difficult on the day.

Tim.

 

[ChrisL]

Tim, (and everyone else) do you have the upper swing arm bolted to the top position or lower position on the chassis? The lower position would give you more dynamic camber which is what we are after but I was told by RF that the lower position shouldn't be used. Has anyone actually tried using the lower point in track conditions? It will lower the roll centre and in theory increase the amount of roll but what is the actual (as opposed to theoretical) overall effect on the handling?

... yeah I know, took a long time to reply but it only just dawned on me

 

[roaldin]

In the standard position. We looked at changing it but decided not to. To be honest I got such a negative response from Ross H when we discussed it that I didn't even bother plotting it.
Maybe it's worth another look but as you say the roll-centre would likely be very low.

The rear camber on my car seems to work very well with little counter-productive effect, from Ross N's posts it seems his similar settings are on the money also.

On the subject of my car: theoretically things are much improved now. The rear bump steer is now negligible. This was achieved with a 4.75 degree "castor" setting on the rear, slightly different to Ross N's setup but close enough for gov't work.
The rear end is certainly a bit of a sod to set up and get working but I have high hopes now. I have some events coming up with another car so I'm still not concentrating on the GT but I hope to get it to the track soon for testing.

I've neglected something, so time to make good -
A big thanks to Jim Cowden for supplying his beautifully made and fantastically useful suspension (and other) components to me at short notice and at reasonable cost.
This saved me hours and dollars developing things and I doubt we could have done a better job.
This once again demonstrates the usefullness of this forum and the quality of its contributors.

Tim.

 

[Ross Nicol]

Guys It was pointed out to Ross H by an early builder Geoff T that this lower hole was causing way too much camber gain and subsequently the new hole for the top link inner was raised. I think the lower hole has been removed from later chassis but I'm not sure. It's interesting that Ross H while driving one of the cars at Calder put it into the wall up the back straight.The extreme camber gain could well have been the reason and I never believed the BS that Robert L claimed, caused this accident. Personally I'd leave the inner mount in the top hole and alter the static camber for more corner grip, but how much will depend on your springs and the rest of your setup. Sounds like you've made major changes Tim, let me know how it goes.
Ross

 

[flatchat(Chris)]

Reading through this thread with interest and IMHO (of course) these "things" were manufactured to some guide lines as a reasonable compromise for road use, at worst would give some accelerated tyre wear and maybe some funny handling responses in emergency situations,-- and 110 KPH is this countries (Aus.)public road max. speed limit.
"But when we go racing" (we need to build a race car!) be confident that the chassis is up to the task, as in a competitive mode it becomes quite stressed which can add to the symptoms of bad wheel alignment and suspension settings. Because, there are so many adjustments available, you'll be chasing your tail for years until you tick the first box.
End of rant -- good luck chaps

 

[roaldin]

"Handles like a go-kart"
"Yeah, built in flex"

Tim.

 

[Ross Nicol]

According to the Westfield driver behind me at Calder I was lifting an inside front wheel.I couldnt feel this from inside the car but there was a bit of hopping going (on at the rear) whilst applying power after the apex.A bit more work to do, but oh the grip is just so good now the slicks are working. I have to admit up to the historic race meeting at Sandown in Nov I thought the car was a dud in the handling dept. Very much changed my mind now. Flatchat I'm glad you mentioned tyre wear as it is a factor for road cars and too much negative camber will wear the inside of the rear tires prematurely and to an unroadworthy point long before the outside wear is noticed.The fully adjustable suspension on these cars does create a minefield of possible setups doesn't it.
Ross

 

According to the Westfield driver behind me at Calder I was lifting an inside front wheel.

Like this Ross? Very interesting topic. This is my Elise on Kumho V70A r-spec tyres.
I have not had the GT40 on the track yet, but taking plenty of notes... Jim C's suspension will be my first modification, along with a quicker rack.
If the GT40 gets ANYWHERE NEAR the Elise for handling, i will be very happy (if not somewhat surprised).
Keep the ideas & feed back coming.
I also look forward to seeing Iian Pretty's car hit the track. It is one awesome piece of gear!

 

[Ross Nicol]

Yes just like that Tim. Is your Elise still with Rover motor or Blown Honda?
I think you'll find the cars will always feel different the Elise (dainty lady) and GT40 (the Brute).You can't beat the low rev torque of a V8 for excitement as opposed to a 4 cylinder motor that develops it's power with stratosphere revs. However I do like my Europa.The light weight Westfield had a pretty fast Beams 3SG Toyota in it, and he was all over me coming in to the corners but my 2nd gear coming out just blew him away and he had no answer. I was intoxicated by the 40s torque the first day I drove the car on the track at Winton.I remember being very excited even to a point I got totally soaked, from not having the wheel well cover on and yes it was pouring with rain. I look forward to Iain's debut with his 40 too, we've had a few interesting talks on the phone.
Ross

 

Hi Ross, the Elise now has supercharged Honda K20A. My car is now log booked, and i ran the Eatsern Creeek 6-Hour last november. That was FUN.
The Elise has 280hp (only running 5psi boost) and weighs 700kg. It is a potent package. It ran 12.0 on the quarter at Eastern Creek drags! But still friendly enough for my wife to drive around on the road.