Rear suspension bump steer

[Jim Cowden]

Clayton

Hi
Nice to here from you.
When I read Timoz post the first person I thought of was you.

Whats next , seriously I would like to just put it together.
These are the time consuming stages.

Fixing these things are worth it thats why you are sorting yours.
Plus Im learning.

Cris
I was going to rotate my bracket before I checked it out.
Rotating the bracket will only relieve the twist off the bush due to the required caster.
It wont change the amount required.
The thing I noticed that had a big effect was the lower arm (A frame).
Forget the rear part of the arm string a line from the inner bush to the front rose joint on the upright.
In theory it will keep getting longer until it is in line with the inner bush.
Pulling on the caster is also causing this effect to reduce as the arm rolls up at the front and brings the front rose joint closer to the centre bush and also lifting the t/link pickup points.
Ross has obviously found 7deg is enough to flatten the t/links to knock out one lot of arcing.

Jim

 

[Trevor Booth]

Chris L
I respectfully suggest you re-check your calcs, If the upper trailing arm is not affecting toe in what is it doing to the geometry.

Clayton,
The forward end Point D1 is behind the upright casting and a long way from B2. Check out the 4 element sketch below. Point B2 is swinging about the same radius as D1 but it should be a different radius to D1.

Russ,
Geometric interference "winds" up the system and then can suddenly release.
ie. in the 4 element. B and D are rotating about line A-C but at same radii. B is tending to rotate about D (in the plane of the wishbone) this winds up the system and usually stresses the trailink link A-B until it deflects. The trailing link will resist deflection up to a point (and still be straight) all of a sudden it will deflect. It is this sudden and instantaneous deflection that causes a like change in the geometry and un predictable handling.

In the event you have a straight clevis with a reversed lower A arm, slacken all the bolts in the upright and watch what happens. In the event you have offset clevis, point B will adopt the true position on A-D

Lowering the rear roll centre may not necessarily promote more understeer, it depends on its relationship with the front roll centre.

I cant conclusively say that lowering the front inner point on a parrallel link sytem would induce toe in, would need to do the maths but I suspect it may well be cancelled out,,just a thought. You really cant look at any element in isolation, it is a system all working together (hopefully ))

 

[Clayton]

Hi Jim

Yeah when do you ya stop changing things ?? Not saying there’s anything wrong with the RF, just making changes to suit my own taste. And hopefully learn something along the way.

Hi Chris

Haven’t been spending much time on it lately, the Mk2 Escort has been consuming the rest !

Hi Trevor

Out of shear luck I’ve got an offset clevis on the bottom link so the trailing arm would clear the chassis uprights and is close to a 3 element as per your sketch.
The top clevis is straight, did not think about it until you brought it up.
By moving the bottom inner pivot forward and no change to the other plains, would that have a huge effect on the amount of caster required to reduce roll steer ??
Thanks

Clayton

 

[Russ Noble]

Trevor, regarding binding, deflection etc. Are you referring to a bushed or semi bushed system? In the case of a heim jointed system there will be no binding? Or is this a mathematical/geometrical thing rather than physical.

Given an unchanged front roll centre, wherever it is, won't lowering the rear roll centre promote more understeer (or less oversteer which is the same thing) regardless?

Thanks for the input.

Cheers,

 

[Trevor Booth]

Russ,
A bushed system, sorry I did not make that clear. Also you should not run caster on a bushed system unless the bushes at C and C lower are very compliant. With any bushed system the pivot axis of A and C should be on the line A-C and the pivot axis of D parallel to the line A-C, similarly with the lower pivots.

Lowering the rear roll centre will transfer more weight to the front in corners, this may not be what you want. You need to read what the tyres are telling you.

Hi Clayton,
Should not make a lot of difference, the amount of caster is more governed by the plan view relationship of points A.

I am currently building a 3D model with variable geometry for one "corner" of a car. I will then be able to run plots of the effect of moving any point in any direction. This will make it easier for members to understand what does what in non Engineer speak. Give me a week or so.

I could explain it like this !!

A crane, includes a luffing jib having individual telescoping jib members, and a locking device configured for linkage of jib members and movable between an engagement position in which the jib members are locked together and a release position in which a first one of the jib members can be moved in and out relative to a second one of the jib members. The locking device is supported in the first jib member and received in a receptacle of the second jib member. A shifter, movable in an axial direction, is provided to shift the locking device via a coupling device between the engagement and release positions, with the coupling device extending perpendicular to the axis and connected to the shifter. The coupling device has a coupling member movable in a radial direction to effect an interlocking engagement with the locking device, which is movable in a same axis as the coupling member, wherein the locking device forms together with the coupling member a coupling between the shifter and the first jib member, and wherein in a transition phase between release and engagement positions, a projecting portion of the coupling member enters in interlocking engagement with the locking device.

))

 

[Clayton]

OK, thanks Trevor

 

[Russ Noble]

Ah, Trevor.

Bushes!

That explains it all. I was beginning to think I was a bit dim witted! You obviously normally think in terms of bushes, I think of heim joints.

Binding bushes combined with the standard link setup was one if the reasons my TR7V8 used to handle like a pig. Nevertheless I always had good results in the wet. Probably with the lower forces generated in the wet the crap suspension couldn't upset things as much!

I must say this thread has improved my knowledge in some of the finer points of rear suspension setup.

Thanks

 

[Keith]

I could explain it like this !!

A crane, includes a luffing jib having individual telescoping jib members, and a locking device configured for linkage of jib members and movable between an engagement position in which the jib members are locked together and a release position in which a first one of the jib members can be moved in and out relative to a second one of the jib members. The locking device is supported in the first jib member and received in a receptacle of the second jib member. A shifter, movable in an axial direction, is provided to shift the locking device via a coupling device between the engagement and release positions, with the coupling device extending perpendicular to the axis and connected to the shifter. The coupling device has a coupling member movable in a radial direction to effect an interlocking engagement with the locking device, which is movable in a same axis as the coupling member, wherein the locking device forms together with the coupling member a coupling between the shifter and the first jib member, and wherein in a transition phase between release and engagement positions, a projecting portion of the coupling member enters in interlocking engagement with the locking device.

))

'kin 'ell! That's it, I'm outa here.....

 

[Jim Cowden]

Clayton

Come on Clayton
Did you get it or are you just saying that.

 

[Trevor Booth]

Russ,
No I dont think in bushes, The standard RF is bushes and it was to the standard setup to which I was referring.

Clayton,
When are you going to get your car finished, I am on my second one.

FWIW
Earlier in this thread someone excused themselves for possibly asking a dumb question. There is no such thing as a dumb question.

 

[ChrisL]

Chris L
I respectfully suggest you re-check your calcs, If the upper trailing arm is not affecting toe in what is it doing to the geometry.

Trevor,
I guess I worded my comment badly. What I meant was that I was in error in thinking that the upper t/arm contributes to additional toe, where if anything it reduces the toe caused by the bottom t/arm. But I chose to ignore the effects of the upper t/arm because my figures are very approximate anyway. I guess I'm just trying in my own mind to understand how the current geometry can create such large amounts of dynamic toe, and how +ve castor can reduce it. I still don't think I have quite grasped it. I guess I'll just have to jack the rear of the car this weekend and do the practical stuff.

 

[Clayton]

Hi Jim

Nar, I was just responding to Trevor regarding my question.
I thought that stuff down the bottom was in Japanese : )

Hi Trevor
I’m trying to finish it believe me, but between finishing off the house and racing an Escort, well you know the drum.
So spill the beans, is it your car ??

 

[Jac Mac]

'kin 'ell! That's it, I'm outa here.....

Dont blame you, with that description I thought I was reading a patent application for a swingers party at a canine convention!

Jac Mac

 

[Keith]

Check out http://www.bell-performance.co.uk/

Go to "Articles" and "Suspension 1,2 & 3" in which Derek describes the setting up of his Lola MKIIIB suspension together with diagrams and calculations. I think it is one of the clearest articles written on the subject (if I understand it anyone can! ) The Lola suspension seems to be a dead ringer for the GT40 as well. Hope this helps someone.....

Jac.......spot on!

 

[Ross Nicol]

I have just read the remainder of this thread after leaving it to work on my motor and I would like to thank Trevor for his input after I revved him up ( sorry about that Trev) it's great to have engineer input even though some find it hard to understand. I don't believe any explanation is wasted as sometimes a little info is needed just to support your theory and close the final loop.The important thing to remember, everyone has something to offer and information from people like Trevor and Jacmac is great, which is why an information vehicle like this forum which allows us access to their experience is such a great thing.
Ross

 

[Ron Earp]

Ah, Trevor.

Binding bushes combined with the standard link setup was one if the reasons my TR7V8 used to handle like a pig. Nevertheless I always had good results in the wet. Probably with the lower forces generated in the wet the crap suspension couldn't upset things as much!

I missed this post eariler. We had the same problem, but we've done the TR8 up with spherical bearings all around, everywhere possible, and it has helped dramatically and showed some laptime improvement straightaway. Was not a cheap conversion though, not at all.

Ron

 

[ChrisL]

Ron,
I might be telling you something you already know. Anti-dive blocks (tucked over the front sway bar) transforms the TR7/8 from a pig into a very refined handler. Talking from experience having campaigned one for a year at club sprints a few years ago. The modification is so common that the blocks are available off the shelf.

 

[Russ Noble]

Ron,
I might be telling you something you already know. Anti-dive blocks (tucked over the front sway bar) transforms the TR7/8 from a pig into a very refined handler. Talking from experience having campaigned one for a year at club sprints a few years ago. The modification is so common that the blocks are available off the shelf.

Yeah, I had those blocks, not sure about your definition of a "very refined handler" though. The standard basic rear end setup is all wrong for serious competition work. I actually converted mine to a rose jointed 5 link config. But then the GT40 opportunity came along and I haven't had it on the track since. However I was fully expecting those mods to make it a bit less unrefined and gain a couple of seconds a lap.

Rose jointing all the links as Ron has done will allieviate the bush binding problems but the high rear roll centre will still need to be worked around.

Cheers

 

[Chris Coote]

Having experienced 1st hand the horrors of a ill handling RFGT40 both on the road and track I contacted Jim Cowden with a view to developing a "bolt in" replacement scenario that would give us all the castor we wanted (7deg) and allow some additional fine tuning as well.Still to get to the wheel aligners the attached photo hopefully shows the replacement pieces and the available adjustments.I had the parts silver ceramic coated to give that chrome moly look.
Once aligned and tested will advise on the mods but any improvement over what it was I will be elated.Cheers and thanks again Jim.

 

[ChrisL]

I must be driving a different RF40 to everyone else I can’t agree that the handling is that bad at all. In fact I would go as far as to say that it is very good as a road car. I’ve been progressively tweaking the handling of my car at each race meeting and each time I have found another small improvement to the point where I can now say that it promotes confidence. Admittedly it would be nice to have more grip at the rear (I guess that’s why the Le Mans cars had such big rear tyres) but you just need to allow for that when searching for the best setup. The rear end on my car now feels well planted and I can drive the car through long sweepers at high G’s and have it sit rock solid even when hitting a solid bump. And when I do promote over steer, I can back off slightly and the rear will fall nicely back in line. The car is very responsive to adjustments though so in theory it would be possible to dial horrible handling into it, but as has been mentioned before (I think in this thread) if you get it right then dynamic toe is not necessarily a bad thing.

PS. I am planning to document and publish all that I’ve done but needed to first get my settings verified on a professional alignment machine (which won’t be for a few weeks yet). If anyone is interested, you can contact me and I can send my unverified settings.


PPS
Russ, I take your point about my usage of the word "refined" but I guess its a relative term. The TR7 V8 that I drove handled well enough that I could steer with the brake or throttle which is a vast improvement on the standard handling.