What causes heavy steering at racing speeds

[Trevor Booth]

Now that got everybodys attention 11º is correct. (-7.5- +.5-3=11)

For the advocates of 10º (alt 167 pressed slowly)

The KPI is 7.5º with a positive camber of +0.5º

Increase KPI by -0.5º (ie 8.0º) the static camber is then zero add three more degrees of neg camber the KPI is 11º

Now given the KPI 7.5º bench measured by Ross N with effectively no camber.

Installed KPI = 10.5º

Ross N put the car on a wheel aligner. Then you have a known basis to work from. Whilst on the aligner note what adjustment does what.
say, 2 turns in on the top rear wisbone rod end = change in castor = change in camber = change in toe in. do the same for camber change/toe in change. You can then make in the pits adjustments with some degree of accuracy. It is also a good idea to square the car up with 0 camber and 0 toe in and then set castor camber toe in / or out as the case may be.
Have you checked the wheelbase Right and left ?

Scrub radius- project a line to the floor thru upper and lower ball joints. Drop a line down to the floor from the hub bolt face. ( hold a straight edge across the bolt face , not a vertical line) measure the distance (A) between the two points. Then measure wheel offset and add to (A)
Car must be sitting at ride height

Tim, working on doing an RF plot

Russ N , no I was not being that cryptic but I was confusing the issue with the +5º static )

 

[Malcolm]

I understand now (with Jac Mac's help ) that altering the inboard rose joint spacers on the top wishbone will seriously alter the camber so caster adjustment will be by screwing rose joints in and out of the wishbone in future.


Malcolm I've never had any brake pad knock back problem thank god.
Does the GTD use Cortina uprights as in the RF? and could the spindle flex be due to the smaller spindle shaft at 1.06" as Jac Mac described?

Is that first part is it what you meant to say about castor adjustment?

Yes, bog standard GTDs use the UK cortina upright. Can't say whether that is the same as RF as not seen one up close. Spindle flex certainly not helped by small shaft and small bearing. Does it matter what makes the pads get knocked back? If you got it you need to solve it. If you don't you can keep going with current set up.

 

[Ross Nicol]

Thanks for those Ideas Trevor, we did set the wheelbase at 95" left and right when the rear suspension was modified.I do have data on rose joint turns versus camber change and I will record more when I do the caster.
I had a feeling the scrub radius would be done like that.
Malcolm if you read Jac Macs post#28 his explanation is better than the way I tried to.
Ross

 

[roaldin]

ALT 248 = °
ALT 167 = º

A difference but only of a small degree.


Tim.

 

[Jac Mac]

Im busy looking for the 248 & 167 keys on my keyboard, I must have the budget version ,it only goes up to 9!!!!!!!!

Enough of that crap--- Ross , another area of deflection you may still have in your car is the Nolathane inner bush's of the lower wishbones ( assuming they are still fitted & you have not done any tricks like hide a spherical bush inside them )

Another thing that may help others come to terms with what we are discussing here; I tend to look at this problem as follows--

When this car is on the straightaway I regard the front tyre as only being say 5" wide ( the inner half of the tread width ) and the effective scrub radius of this 5" footprint is probably around 0" to 1".

Now move to the cornering situation -The car now has approx 2 or 3 deg of body roll, perhaps 5 +deg of steering lock, and the lower wishbone with lateral cornering loads has deflected inboard /rearward.
Lets assume that the static camber was 3 deg negative.
The changes of body roll- steering lock- deflection @ say 3-1-1 degrees each will have moved our static camber from 3 deg neg to a dynamic 2 deg pos. Now the outer 5" of the tyre width is doing the majority of work and our scrub radius will be in the order of 6" , hence the greater steering effort reqd.

Ross, I guess what Im really trying to say is; Do whatever you can to shift your wheel/tyre inboard in relation to the upright/stub axle- Compensate for this inboard shift with new longer top & bottom wishbones with rod ends at inner pivots along with any tie rod extensions that it may require.

Jac Mac

 

[Russ Noble]

Jac Mac,

Don't forget that with camber compensating suspension which we have here the top link (mainly) will have pulled on more negative camber, possibly the order of a couple of degrees so the tyre is probably flat on the road. Same effect as you hypothesise, just not so pronounced. BTW Ross has confirmed he has set camber to -4 degrees (Damn can't get the ALT 248 thing to work!) Presumably this is the figure that keeps it working across all the tyre.

I think he's got to get the car set up symetrically before exploring other options.

Cheers

 

[Jim Cowden]

Jac mac quote
Ross, I guess what Im really trying to say is; Do whatever you can to shift your wheel/tyre inboard in relation to the upright/stub axle- Compensate for this inboard shift with new longer top & bottom wishbones with rod ends at inner pivots along with any tie rod extensions that it may require.


That is the conclusion I came to on my RF and have done exactly as you suggest

Jim

 

[Pete McCluskey.]

Ron, I think you should make this thread sticky, it is turning into the definitive how to set up your front end thread. Very interesting thanks guys.

 

[Jac Mac]

Russ,
I agree that Ross needs to set the car up symetrically, but feel it would be better to address the lack of adjustment, nolathane bush problem and the scrub radius issue prior to this. Not much point in a symetrical setup if its still going to be limited by these when you find you need to further adjust for the track. The other thing that concerns me is the chance that the chassis pickup points may not be the same from side to side going by input from other owners, just another variable you have to keep in mind when making changes.

Jim C,
I for one will be interested to see how you car performs with your mods.

Now if I can only keep Russ from fitting that BOC Locker unit to his 930 all will be well.

Jac Mac

 

[Ross Nicol]

Thought I'd better have a look this morning and sure enough I'm under discussion. Sorry guys I keep assuming you know my car.You suspect correctly Jac Mac there are spherical bearings inboard on the lower wishbones at the front. I have no rubber and no Nolathane anywhere on my car.
Ross

 

[Jac Mac]

Im slowly getting a mental pic of it Ross, perhaps I should work for Jim C @ Mental Performance. Whats it feel like to have your pride and joy so closely scruitinised ? Also wondered if there are any skid pads like the BMW Driver training outfit etc that you may be able to use in your area.

Jac Mac

 

[Ross Nicol]

Yes I think Jim C must have been a muso in a former life with a business called Mental performance ( great name Jim ). Sounds like I should measure my scrub radius as a starting point and my wheel offset.I can tell you I have 9" 3 piece rims with a 4" outer rim and a 5" inner rim with the rear hub face of approx 1" from the rims join line.Jac Mac is this like airing your dirty laundry in public. I'm not phased though if we can collectively fix my problem and others can benefit, I'm all for it.
Ross

 

[Russ Noble]

Hey Ross. Get off my thread.boomsmile I started this so I could get a head start on you for SFOS! epper:

Now looks like you could pick up maybe 5 secs a lap due to input from the Forum and my EX Pit Crew Chief.

Seriously though, I've picked up quite a bit from this discussion and if it helps get your car sorted, so much the better. You're going to be a hard nut to crack in Feb!

Cheers

 

[Jac Mac]

Now looks like you could pick up maybe 5 secs a lap due to input from the Forum and my EX Pit Crew Chief.



Cheers

Yippee, Thats how you get the Sack & a Pay Rise at the same Time!!!!!!!!!

Jac Mac

 

[roaldin]

After reading through my one bourbon and Coke too many comments last night I was feeling I should apologise to Russ but reading further I think I fit in ok.

Russ, If you can't do the Alt number thing you can always cut (ctrl ins) and paste (shift ins) from someone else's message.
Here they are to choose from º ° .

Trevor, I for one look forward to your results.

Pete, I am still considering power-steering and will let you know if I get anywhere.
I have heard that there is a guy in Western Australia (maybe) who has fitted power steer to an RF, does anyone know anything further?

Has anyone played around with rack position? Apart from changes to bump steer I am led to believe that some improvement in effort and feel may be gained by moving the rack backwards or forwards. Before I try it it'd be great to hear from someone who's done it or has modelled it.
Which owner of an RF lives closest to Trevor?

Ross, those are some big front tyres!

Great thread! This is what this forum is all about.

Tim.

 

[Pete McCluskey.]

yoº °u bewdy I can do it..º °epper: epper: epper:..Thanks Tim.

 

[Jim Cowden]

Tim

If I had a choise I would put the rack behind the wheels.
When I made a new uprights I clawed back 20mm from the wheel flange to the ball joint position.

BUT in doing that were the ideal steering arm position would be someone has put a brake disk there know.

If the rack was behind the wheels this would give me perfect ackerman as I would have no clearance problems with the disk.
My steering arm is a seperate part so I can play but I can feel a compromise coming on.

Jac mac
thanks for the kind word but I think I would learn more if I worked for you.

Jim

 

[Iain]

I have a RF under construction that has power steering installed, not running yet but all things being equal it should work.
We have a rack from BMW which is one of the few "front" steer power asisted racks available, we have an Hydro / electric pump as used in Carrera Cup Porsche ( Identical unit can be found in a Holden Astra !)
We have coupled this with a fairly short steering arm, moved it back in the chassis so it lines up with the pick up point on the hub and hopefully it will all work. The car is at the workshop in Wetherill Park (Just behind Eastern Creek) very happy for locals to visit. Very happy for overseas vistors too but it seems a little excessive.
Iain

 

[Ross Nicol]

°°° Hey Pete,Russ JacMac you hold down the Alt key and then hit 2,4,8 in sequential order, the ° pops up when you let the Alt key go ° see! it's easy.
Ross

 

[Jac Mac]

°°° Uh Hah,
Jim C- If your 'must' have ackerman shift your steering arm and rack up to the height of the top ball joint in a similar manner to the RCR cars, as the top ball joint is further inboard you will be able to create some ackerman without fouling the brake disc. I can see that the RF frame has a few obstacles in the way of such a mod, but as other 'bas*****' are always telling me, I am sure you will be able to overcome them.°º

Jac Mac