Geometry vs Sheer mechanical width

[jon]

I'll try and outline things quickly so as not to bore.

 

My mr2 currently wears 225/45 and 205/45 boots on 16x8jj and 16x7jj volks.

 

Before my last trackday i had a tony special but due to certian issues i left without the ideal camber setup for the tyres. I ended up doing the day on -1.6 rear and -2.6 front.

 

As we might have guessed, the front was stella, the car was FAST (faster than anything else i found on the day) but i was forever chasing the rear in the high speed stuff as the car tried to twist around the front.

 

On discussion with Tony the conversion went something like (i'm sure tony will correct me)

Tony: No problem, I expected that. We'll just reduce the front camber, reducing the grip at the front and giving you a better balance.

Me: Reduced grip for balance? Never!

 

So, on to the point. I'm thinking of throwing on 245 wide rear boots to address this and see what happens. I always though Toyota were quite conservative with their tyre sized steps considering the power and weight balance of the stock mr2 turbos (270bhp with 56r 44f).

 

My mind is telling me ultimately 245/215 will be ideal but it will be interesting to see what people think.

[Tony]

Your car suffers from very bad over-steer, hence why we need to reduce the front Camber, then invite neutral or slight over-steer.. the over is better for the rwd... but you are the driver you tell me once tested?

 

The tyre combination should be 245/40 rear and 215/45 front.. if i am wrong I'm sure Dan will advise.

 

So what will this gain..... in truth very little. The mechanical track is 1470 front 1450 rear. With the wider rear tyres the mechanical track is the same, whereas the pneumatice track is +40mm

 

Geometrically the rear does not have a scrub radius only a contact patch.. Depending on the wheel off-set the only calculation needed is 'Rolling resistance' and 'Lateral resistance' both relate to toe positions... Although some my argue pneumatic distortion due to the Camber would become an issue, i personally disagree since the positions are not aggressive enough.

 

Wider tyres -ve weight transfer and roll centre need actual track testing... Even the Cray Super Computer would have trouble calculating that in advance.... It's a 'Drivers decision.

[jon]

Lets take this in (english) bits.

 

You car suffers from very bad over-steer, hence why we need to reduce the front Camber, then invite neutral or slight over-steer.

 

Ego reduce frontend grip to invite balance. The purist in me doesn't like that.

 

The tyre combination should be 245/40 rear and 215/45 front.. if i am wrong I'm sure Dan will advise.

 

Why? Forget the rolling radius differences at the moment, how did you decide 20mm should be thegap with a car balanced at circa 55/45. The Boxster, which is 50/50 has a much larger (over double) 'step'.

 

So what will this gain..... in truth very little. The mechanical track is 1470 front 1450 rear. With the wider rear tyres the mechanical track is the same, whereas the pneumatice track is +40mm

 

Geometrically the rear does not have a scrub radius only a contact patch.. Depending on the wheel off-set the only calculation needed is 'Rolling resistance' and 'Lateral resistance' both relate to toe positions... Although some my argue pneumatic distortion due to the Camber would become an issue, i personally disagree since the positions are not aggressive enough.

 

I try and keep up but i'm lost.

 

Wider tyres -ve weight transfer and roll centre need actual track testing... Even the Cray Super Computer would have trouble calculating that in advance.... It's a 'Drivers decision.

 

At least we agree on something Unless you object i'm going to try 245 wide rear and use the fine tune to adjust the camber accordingly. Then track test.

 

Science and theory is one thing, feeling the car squirm on track is firmly another and my gut feel is a tiny bit more mechanical rear and at race speeds the car will be ok.

[Tony]

Lets take this in (english) bits.

 

You car suffers from very bad over-steer, hence why we need to reduce the front Camber, then invite neutral or slight over-steer.

 

Ego reduce frontend grip to invite balance. The purist in me doesn't like that.

 

The tyre combination should be 245/40 rear and 215/45 front.. if i am wrong I'm sure Dan will advise.

 

Why? Forget the rolling radius differences at the moment, how did you decide 20mm should be thegap with a car balanced at circa 55/45. The Boxster, which is 50/50 has a much larger (over double) 'step'.

 

So what will this gain..... in truth very little. The mechanical track is 1470 front 1450 rear. With the wider rear tyres the mechanical track is the same, whereas the pneumatic track is +40mm

 

Geometrically the rear does not have a scrub radius only a contact patch.. Depending on the wheel off-set the only calculation needed is 'Rolling resistance' and 'Lateral resistance' both relate to toe positions... Although some my argue pneumatic distortion due to the Camber would become an issue, i personally disagree since the positions are not aggressive enough.

 

I try and keep up but i'm lost.

 

Wider tyres -ve weight transfer and roll centre need actual track testing... Even the Cray Super Computer would have trouble calculating that in advance.... It's a 'Drivers decision.

 

At least we agree on something Unless you object i'm going to try 245 wide rear and use the fine tune to adjust the camber accordingly. Then track test.

 

Science and theory is one thing, feeling the car squirm on track is firmly another and my gut feel is a tiny bit more mechanical rear and at race speeds the car will be ok.

Quote 1: Why the front is to tight.... Why do you feel nervous about losing that over-steer?

Quote 2: The 'gap' is the track not the weight ratio....Anyway 45/55 would be better. MR2 not a Boxster, different cars indeed!

Quote 3: The 245mm is fine by me... but understand if we re-define the front camber positions with the new 245's on the car then the result will become 'blind'......... Was the performance due to the tyres or the Geometry?.... Gut feeling is get the Geo correctly placed... Deduce from that what you will, but lets at least complete one stage!