Reading Slicks.

[oldcorollas]

yah, i'm always late, but i thought i'd back you up with "Famous Pat"

I'll draw pic of angle stuff

[oldcorollas]

these are all simplified (and not tryin to take thread off track )

bump steer should just be a function of rack width and rack height.
if the rack is too wide or too skinny, then you need to fart ass around with the rack to try and get the arcs of movement so that they kinda match (but same offsets?) and give minimum change.
ie, if rack width is correct, the LCA and tie rods will be parallel and same length = no difference in their arcs of movement
http://i301.photobucket.com/albums/n.../bumpsteer.jpg


ackermann, is mostly a function of the forward/rearward position of rack, and is highly affected by the steering arm angles..

top pic below is simple case, all straight, no ackermann.
the bottom case is like the AE86 power steering rack/bits i just looked at.
the steering arms are angled in (most are?) and the rack is forward of the steering arms.
when you turn, the inside wheel moves more (ie on the bottom bit, the dashed purple line is LHS wheel, and dashed blue is RHS)
solid blue lines and dashed blue lines are angle wheels are pointing etc....

so what i'm trying to say is, although there are lots of little things that are interdependent, if the rack width is ok, then you should be able to mostly seperate the adjustments...

edit: of course now if you look from back of rack (re:bottom pic) the LCA's and tie rods appear as different lengths, so then you induce bump steer , since LHS tie rod is effectively shorter, and RHS tie rod is effectively longer
http://i301.photobucket.com/albums/n...ann-angles.jpg

[JustenGT8]

The practically expression of ackermann is the angle from the rear diff to the steering arm. Steering arm is a fixed point so while i got the best ackermann i could, based on disc clearance constraints, it was greater than stock and resulted in a less than ideal scrub radius.

This seems to be the likely cause for over working the inside of the tyre, if indeed that is what is happening.....to be confirmed by tyre pics this arvo i hope.

[OnAll-FOUR]

So you think the inside of the tyre was being over worked rather than under worked?

You definetly had a ridge or rubber dividing the two halves of the tyre. Would that be something you would eppext to see?

In that pic showing the overworked tyres the larger graining still looked rather clean.

[JustenGT8]

Yeah i'm not 100%, hence need to confirm with actual pics. I didn't make it home until after dark so will have to wait until weekend now.

[ed]

ill thrown mine in for discussion whilst were at it...

and fish - if youve got any material worth reading, please fire it my way too... [email protected]


dunlops @ 28psi
not sure what the temps were on this session, but previously, before getting them hotter (still learning the track) we were getting 60deg outside edge and 80deg inside edge

subjectively, the car felt perfectly balanced, no front/rear bias towards under/oversteer, nothing. behaved very well infact. just need to go faster

cheers
ed

FRONT


REAR

[Fish]

ed

i got the carroll smith books and i did have the Fil punn book too.

[Fish]

Ed

See the graining on the outside of the tyre?

thats the stuff you want all over it.

See the blue sheen coming through the tyre thats all its grippy goodness so to speak.

[JustenGT8]

interesting... spring rate in lbs/in should be the same as corner weight in lbs?

350 is too soft. I run 14kg/mm in the front (~700lbs/in) and 8kg/mm in the rear (~450), which is just how the teins come. in an ideal world those front would be a little softer, but I drive a pretty fat bitch

sorry mate, just ran back thru the thread and saw this post.

No, is the short answer Rule of thumb being a spring rate that gives you a a wheel rate in lbs/inch equal to the corner weight in kgs....only a rule of thumb so a good starting point, nothing more.

Having spent the last few days looking into how to get a starting point this seems the best in my view. There are a heap of calcs you can do using roll rates and spring freqs etc etc but all are sensitive to the estimates you input for things you can't measure and at the end of the day they don't take into account other factors which can have a huge influence on the end result. If is was easy i guess race teams wouldn't spend so much time testing

My driving isn't consistent enough yet to be chasing every last 10th thru setup changes but at present there is clearly a significant issue worth trying to resolve.

[Rodger]

Justen,

Just to clarify. You say you have 2.5 negative castor?? I hope you do mean 2.5 positive.

Regards

Rodger

[Fish]

Justen,

do you have an exercise book you take with you to the track? If not it is a wise investment for $2 with a pen.

[JustenGT8]

Justen,

Just to clarify. You say you have 2.5 negative castor?? I hope you do mean 2.5 positive.

Regards

Rodger

2.5 deg neg camber...no idea what castor is, it's fixed from factory and no easy way to change

[rms]

Justen,
When you get pics, LH front AND LH rear please.

At a guess, how much does the nose dive under brakes (strut compression) ?

[JustenGT8]

A tricky one Erol as relative to what? I would say very little dive under brakes, enough to give good bite for the fronts but it stil feels very planted in the rear unless i try some extreme trailbraking.

[rms]

I meant movement at the strut.
Trying to get some idea of sprung mass frequency.

Have run 750lb/in front - 32mm tubular sway bar, and 350lb/in rear - 25mm tubular sway bar, in the first of the 'fastback' Celica/18RG's.
Car was circuit only, but was by no means 'harsh' or 'stiff', coped with ripple strips really well.
Shocks were custom valved for this setup.