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  #1006  
Old October 24th 2011, 09:53
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I actually did already indeed Rich, just like you described, but think I got the wrong points connected...
With your answer I am a little confused even more what KPI and scrub radius is. MotoIQ had a great article about it the other day, using tie rod(?) and the lower ball joint of our Mc Pherson suspension system. Or is that just the upper and lower suspension point in the picture? Thought the upper one was the tie rod of the steering in the schematic?


However, I took the line through the upper Mc Pherson strut connection with the body and the lower ball joint. Thought afterwards I did it wrong and it needed be through the tie rod and lower ball joint. Maybe I did it correct after all?
Anyways, that gave me a scrub distance of +18mm. Still not sure now if that was correctly measured or not...
Mike specified a distance between 3/4" and 3". With 18mm I am right at the lower bandwidth of that.. a bit more might help. I have a little room in the wheel arch, but I think 17" might be better for track driving anyways, mostly because of wheelweight you feel the negative effect of it most on the front. 7x17" is half an inch less wide and with a few mm more spacer, might just give that added scrub we're after?

Tire centerline above is right against the rim contact path, which is somewhat confusing too. It doesn't seem to be the exact tire centerline..?

I'll post some of the pics I took of that tonight.
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Last edited by Wally; October 24th 2011 at 11:00.
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  #1007  
Old October 24th 2011, 12:38
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I think I found a way to alter KPI on my car: I can change camber with the extra large excenter bolt on the inner side of the control arm, but also still with the lower strut bolt. If I arrange the strut such that it sits most vertical (less camber with the lower strut bolt), and counter this with making the TCA longer with the inner excenter bolt, I will have effectively reduced KPA and increased the scrub whilst keeping the same camber.
That should yield a little better steering self-centering and straightline stability
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  #1008  
Old October 24th 2011, 14:56
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Wally: you did it correctly for a strut front end, however that image you just posted is how to measure on a double wishbnoe front end (ie the tie rod/steering arm is not shown).
one thing to note also, just in case: you need to draw the line through the centres of the actual ball joints, not the centre at the base as this can throw the measurement out quite a bit (probably just how you have the bar leaning in the picture).

From what I've read -ve scrub gives more of a stabilising effect with a bit less feedback. The problem with going too close to zero is that it can flip between +ve and -ve depending on caster and the dynamic effects of the suspension giving very unstable feeling...
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  #1009  
Old October 24th 2011, 15:26
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Thanks Rich; yeah, the double wishbone suspension must have confused me
The rod was held through the imaginary center of the lower ball-joint, but my wide angle lens doesn't picture everything -in line- so to say.

The lower pic was taken with the front high, afterwards for the actual measurement, I have taken the nose to the cars normal height of course.

So, its either much more positive or very negative scrub for better steering stability??
I though that 'some' positive scrub was the goal?
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  #1010  
Old October 24th 2011, 16:09
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The 'norm' for RWD is +ve scrub, I'd guess 10-30mm or so? Too much gives to much kick back through the steering. I equate it to +ve giving an 'oversteer-like' kick back through and -ve as an understeer kick through the car during disturbances (ie safe). Like many of these things, it's finding a set-up that works for you as there are so many variables!
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  #1011  
Old October 26th 2011, 09:14
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I hear you Rich; I run the 21mm spacers up front as well, but more because I couldn't fit the high ET wheels; this was before I ran the slimmer AVO struts. Now the spacers help to have full steering without hitting the inner fenders...

My theory is that the 944 spindles are bad for scrub radius. I think their tie rod ends are located further outwards then the originel VW 2-bolt spindles, which curve more towards the inner side of the car? Not sure though. Whats your take on my theory?
Wally, you are right that the tie rod ends are further out and this is because the steering arm is straighter i.e. more parallel than the 1303 (2 bolt one) I took off. This obviously affects the ackermann but I don't see the corrolation with the scrub radius? The bottom pivot of the 944 strut/stub axle also appears further outboard as my fabricated TCAs, although they match the 1303 pressed steel ones, had to use all the camber adjustment at the base of the strut and the inner bush to get a maximum -1.5* camber. If that is the case then it should move the point at which the kingpin angle i.e. the line through the top mount and the bottom ball joint strikes the ground plane than the VW stub axle.
I found that with 30mm spacers on the front, to fill the arches, the steering became very heavy (+ve scrub radius) and I now run without spacers for easier steering. I also now have inner wheel arch scrubbing with the 6" teledials with 195/65 tyres so I am going to try fitting those tyres to the 7" teledials that will increase the scrub radius but it might provide the compromise between the rubbing and heavy steering as well as stiffening the tyre sidewall.

Clive
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  #1012  
Old October 26th 2011, 12:31
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Originally Posted by evilC View Post
Wally, you are right that the tie rod ends are further out and this is because the steering arm is straighter i.e. more parallel than the 1303 (2 bolt one) I took off. This obviously affects the ackermann but I don't see the corrolation with the scrub radius?
Nah, that was my faulty take on what KPI angle was.. Your right that it doesn't matter for scrub, but will for ackermann principle (if I understand that correctly). Maybe the straighter steering arms on the 944 spindles are a more modern take on suspension geometry?

Anyways, do you think the stance of my front wheel is ok in the turn, or can it have some more negative camber in the corner under stress?
It just may be so that the tca presses the (topline) semi-urethane joint enough inbound that it reduces camber too much? If so, Lee's rose joint solution for the tca may help out?
In the picture, my front camber was -2 degrees in rest!

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  #1013  
Old October 26th 2011, 14:45
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The best way to check if you are running the right amount of camber is to measure the tyre temps across the width after a typical hard drive, if it's constant then you have it all spot on and maximising the grip available.
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  #1014  
Old October 26th 2011, 15:28
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Good idea, I'll start looking for one of those IR thermometers.
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  #1015  
Old October 27th 2011, 12:10
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absolutely.... (as I suggested previously )

The quickest way to find out how your tyres are interacting with the road. Measure inner/middle/outer tread temps for each tyre after a hard few laps. Having a mate handy to write down the numbers as you call them out helps.
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  #1016  
Old October 27th 2011, 13:41
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Good idea, I'll start looking for one of those IR thermometers.
I got this one: Deal Extreme thermometer
(it's smaller then it looks, it's about 10 cm long)


It's range is from -50 to 260 degrees celsius, and works very good.
And...it's very cheap.

You have to be patient though, delivery takes 1 to 2 weeks.
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  #1017  
Old October 28th 2011, 04:03
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i have http://www.sealey.co.uk/PLPageBuilde...iredresults=16 for doing tyre temps and its also proved to be a great diagnostic tool. checking header temps quickly tells you which cylinder is not running right.
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  #1018  
Old November 4th 2011, 09:42
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Hi Wally, In answer to your question, the front does look as though it's running about +1* in the turn in roll but the tyre doesn't look too stressed to suggest that anything is moving around too much. I agree with the lads in that you need to take tyre temps to understand what the suspension is doing.

I would be a little concerned to dial too much -ve camber in to increase the verticality of the wheel in long fast turns as that may give the front too much grip. Understeer would be preferable in the long fast turns and my preference is neutral to slight oversteer in the tighter ones.

As regards the 944 steering arms I suspect that the angle, which is more parallel (lets say tending to 0*) is there to correct the position of the steering rack and the angle of the tie rods. Without the accurate geometry of the 944 I can only speculate. Similarly, I haven't modelled the front end of the bug with the steering box arrangement to check the ackermann, all I can say is that the steering gets heavy beyond a 1/2 turn on the steering wheel and I find that its appears to dig in on the outer wheel with no scrubbing. That may be as a result of the lower roll centre although I will experiment with the original roll centre soon.

Clive
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  #1019  
Old November 4th 2011, 15:45
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Thanks for that Clive! I agree and won't dial in more neg. camber. Lee's new arms and the heim joints in them may however limit the amount of movement inbound the arm makes when hard cornering and so preventing further positive camber change.

I am also considering changing back to a 17"wheel as I feel the 18" are just a little too heavy for competition. I may regret it but I just need to see what it feels like again.
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  #1020  
Old November 4th 2011, 18:47
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If I remember correctly you still wanted to increase caster for more straight line stability. Maybe this is an opportunity to also profit from the inherit increase in dynamic camber related to steering angle? So just that little more bite in the short corners, while maintaining the same amount of (safe) understeer in the long high-speed sweeps?

Besides advantages in mass, smaller wheels might also give the advantage to use them in combination with a tire (shape) that is less sensitive to positive camber angles; if that is a point to consider?
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