Mythbuster: continuing project thread of my 1303 '75

[evilC]

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

[Wally]

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.

[aartjan]

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?

[Wally]

Originally Posted by aartjan

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?

I like how you say this!
All good points I think AJ, some of them I didn't even think about before.

[evilC]

Originally Posted by Wally

.................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.

Wally, You may be right about the wheel change. principally the larger the rim diameter the heavier the wheel/tyre combination is for the same o/a diameter. This would be of significance with the unsprung weight that will affect the damper performance and bearing in mind the sprung/unsprung weight ratio on the front of a strut bug. You could also look at a compression strut installation that Rob and i have talked about in the past as that will immediately turn the anti-roll bar into sprung mass rather than unsprung with a significant effect on the ratio.

It may be fashionable to fit large diameter rims with low profile tyres but the same overall diameter with a taller tyre is lighter (tyre wall rubber is lighter than aluminium wheel.

If you set the suspension travel to standard then a 17" tyre wheel combo will lower the front by 5-35mm depending on individual tyres that has to be beneficial whilst still maintaining the geometry.

I was going to suggest a crossply racing/road legal tyre but I could find any 17" ones just 15". You could revert to 15" and use crossplys to give that extra wall stiffness and keep the tread flatter but only if your brakes will accommodate the 15" wheel. you certainly have enough power to make them work.

[volkdent]

Speaking strictly from a weight/diameter perspective, I was surprised that, with all other things being equal or better, once I went from a 17" OEM Cup II to the heavier 18" Cup II repro my 1/4 mile time slowed appreciably and consistantly. I just didn't expect it to make THAT big a difference, but it was almost half a second.

Jason

[Wally]

Originally Posted by evilC

If you set the suspension travel to standard then a 17" tyre wheel combo will lower the front by 5-35mm depending on individual tyres that has to be beneficial whilst still maintaining the geometry.

Very much agree with your whole post Clive! You just said it so much better
I will change the 7,5x18 with 225/40 fronts for 7,0x17 with 205/40. That alone gives a 20mm drop in ride height wih equal suspension geometry.

Mark, I do agree I shouldn't change multiple things at the same time so the proper effect of each can be judged better, however...there are only so many hours in the day and so much time I can spend on wrenching and driving in a season (unfortunately).
I don't agree with your saying that 18 or 19" is better for circuit driving with one car as an example. Remmele may have changed more things at the same time also...or just learnt the track better. You may be referencing to the gyroscopic effect a larger and heavier wheel has? That is indeed beneficial for straight line stability ONLY, but works counter w hen you want to change direction, i.e. turning in for a corner. With competition use I was referencing to me taking part in the TimeAttack series in the Netherlands. I even won my class last year (on points, but still) if you didn't caught up with that yet :-) http://www.timeattack.nl/nl/news/ite...iciteerd_.html .
You are absolutely right that I don't push it extremely hard in corners at the track and that is part of my poor driving abailities to push it on the edge of slip but part of me doesn't want to explore that fine line too closely. That doesn;t help my track times on circuits, but I am having fun nonetheless, I assure you!
Jasons' experience is what I was expecting and speaks volumes for what a heavier wheel does imo. Thank you for sharing that Jason!

In my opinion, wheel size just needs to be as small as to just fit your brakes and width adjustable to power you run. My front 350mm disks limit that to 17"which suits me fine tbh.

The wheel I chose are about 1,7 kg lighter for the fronts and 1,2 kg lighter for the rears. I expect about 1 kg per tire savings as well, so this should give a pretty big gain i reducing unsprung weight. The bummer is, I bought them with winter tires and have to get new summer/semi's now for them as well, so that will probably have to wait a while for budget to recover.

I'll try to make a pic coming weekend with the new wheels on the car

[Wally]

Picked up a set of 9Jx17 / 7Jx17 with 205/50/17 M+S on the front.
These front wheels weighted 3,7 kg less per wheel then my current front wheels (!!).