Is there a 'minimum' weight?

[Alastor]

typo Alastor ? lighter is LESS grip. Given all other things equal.
Drop the weight and the tyre contact patch shrinks.

I see where you are coming from but maybe I am missing something. If you reduce the weight, all else being equal, the contact patch would have be reduce a proportional amount but the load per unit area of tire would remain the same right?

So if that is true doesn't grip remain the same since it is dependent on load per unit area of tire (generally speaking)?

I think what I was referring to in my original post was 'tire load sensitivity' where the effective coefficient of friction increases with reduced tire load. But I don't know if that would still apply at very light loads or very light loads/unit area.

[wwgkd]

With a lighter car can't you also run lighter tire pressures and thus maintain the same contact patch, though? Also I'm assuming any superlight car would have tire tech as a part of the overall marvelous engineering feat.

[charged]

With a lighter car can't you also run lighter tire pressures and thus maintain the same contact patch, though? Also I'm assuming any superlight car would have tire tech as a part of the overall marvelous engineering feat.

Too true a mate who runs a clubman with 600kg/470rkwk dropped his pressures on his slicks to 13psi from 21 psi and picked up 3 secsonds at our local track from a 1.14 to a 1.11. It sort of suits this discussion with a very healthy power to weight ratio of over 1hp/kg

[Matra et Alpine]

There's a limit on how to maintain tyre contact if you lighten things.
Tyre pressure isn't always one of them
Within the confines of the carcass of the tyre it's VERY non-linear. Check out the Tyre Bible, has been one of the best resourcse in the interweb for about 10 years Car Bibles : The Wheel and Tyre Bible Page 4 of 4
if you try to make the tyres TOO wide/large then all that happens is you lose mechanical grip in the centre of the contact patch as the lower pressure ends up making the tyre wall and banding take most of the forces, leaving the tyre to distort at the contact patch.
And we were very specific, ALL ELSE the same then lighter isn't always better
We've already covered (I think) the benefits of lighter when DESIGNING from scratch and how there are limits but mainly driven by the need of the vehicle owner.
Alastor, if you go throug the link above I think it will set the grey mater thinking along the right lines on what happens at the tyre/road contact patch.
Too often we imagine, like in our physics classes at school, that coefficient of friction is the only contribting factor. Tyre tread, compliance, material all alter this significantly.

But, wen we continue on this I think we each need to make it clearl if we are talkign about the SAME configuration made lighter or a new design for lightness. Otherwise we're going to end up with crossed concepts and mess ALL our brains up

EDIT: Charged ? Are you discussin the car in the pic ? See it's running on Avon slicks. THe normal doesn't apply with a racing tyre carcass design. Especially big fat ones. THe cross ply on these re DESIGNED to keep the centres in contact with low pressures. I'd say your mate had in error put far too much pressure in them thinking he was on road tyres ?? Soft tyres also build up heat faster/more consistently and so you reach peak earlier, but if too low will be overworking the edges of the tread causing them to ball and possibly letting the centre of the contact patch drop in temperature and thus lose grip. Radial or crossply slicks ????

[charged]

Yeah Matra crossplies, we told him for 6 months to lower pressure and work his way up..... but he knew better.... the photo was taken at Sandown and he had massive dramas that weekend due to bad preparation, plus a over rev to 10500rpm on down change. He took our advice as criticism and jealousy and has cut all ties and has since not finished a event and last time at the local track was 3 secs of his PB with his new setup.

On the tyre bible link thanks, tyre pressures are critical and very dependant on sidewall height and flex, at the last state meet I started my pressure at the usual 26psi cold on my new 205/60/13 that I used to run on my old185/60/13 , they were shocking after lap 2 they were like soap, next race started at 23 psi cold and dropped a second straight away and a much more consistent tyre during the 10 lap race.

[Alastor]

There's a limit on how to maintain tyre contact if you lighten things.
Tyre pressure isn't always one of them
Within the confines of the carcass of the tyre it's VERY non-linear. Check out the Tyre Bible, has been one of the best resourcse in the interweb for about 10 years Car Bibles : The Wheel and Tyre Bible Page 4 of 4
if you try to make the tyres TOO wide/large then all that happens is you lose mechanical grip in the centre of the contact patch as the lower pressure ends up making the tyre wall and banding take most of the forces, leaving the tyre to distort at the contact patch.
And we were very specific, ALL ELSE the same then lighter isn't always better

I guess this was kind of opened, what remains the same, without futher clarification I could assume the contact patch stayed the same ;-)

I was assuming everything was the same except the contact patch was proportionally smaller, but this obviously different than you interpenetration so now it make sense we arrived at different conclusions.

Alastor, if you go throug the link above I think it will set the grey mater thinking along the right lines on what happens at the tyre/road contact patch.

I am aware of the website, I even contributed some 5 years back, althought it has changed over the years.

Too often we imagine, like in our physics classes at school, that coefficient of friction is the only contribting factor. Tyre tread, compliance, material all alter this significantly.

I don’t think it is the only factor but it certainly is significant. As I stated before, generally speaking, the less load per unit area of tire the higher the coefficient of friction. So lighter = more grip. There are other aspects of tire performance, but I was speaking generally about only rubber behavior.

But, wen we continue on this I think we each need to make it clearl if we are talkign about the SAME configuration made lighter or a new design for lightness. Otherwise we're going to end up with crossed concepts and mess ALL our brains up

Agreed because I think this has already happened.

In my opinion there is no reason to think that a tire designed for a 2500 kg vehicle would work well for 25 kg vehicle. I would expect that if the technology exists to make an extremely light vehicle the technology is there to make the tire with a reasonable contact patch.

Also it seems to me that a lighter vehicle could use a more complaint suspension which should improve mechanical grip, but I would think there would be a limit there too.

[Alastor]

On the tyre bible link thanks, tyre pressures are critical and very dependant on sidewall height and flex, at the last state meet I started my pressure at the usual 26psi cold on my new 205/60/13 that I used to run on my old185/60/13 , they were shocking after lap 2 they were like soap, next race started at 23 psi cold and dropped a second straight away and a much more consistent tyre during the 10 lap race.

I am assuming that is your car in your avatar. What is it?

[charged]

I am assuming that is your car in your avatar. What is it?

1974 Toyota Corolla built to IP 3J specs , it runs a 210hp 1600cc engine,close ratio box, weighs about 800kg with me in it.

a vid of it at Bathurst this year
[ame=http://www.youtube.com/watch?v=ZYKmUvTfYvg]YouTube - FOSC BATHURST 2010 IP RACE 3 PART 1[/ame]

[Alastor]

1974 Toyota Corolla built to IP 3J specs , it runs a 210hp 1600cc engine,close ratio box, weighs about 800kg with me in it.[/url]

I thought it looked like a Corolla but wasn't sure. I just started to get interested in the first gen Celicas. Don't know much about Corollas but I am assuming 1600cc is the original displacement? So are you running a built up original motor or something newer? What does it take to get 210 HP from a 1.6L, is it turbocharged or naturally aspirated, EFI or carbureted, etc?

Nice video too, thanks.

[Buen]

Isn't it a bummer when you think it's a simple question and hope to get a simple answer

Actually, all this discussion was the excact answer i was hoping to get.

I had a rought idea about the different answers people would contribute with, but it joys me to see so many different view-points and oppinions.

[Matra et Alpine]

Glad to be of service .... you know we've only answered 10% of the question ??

[wwgkd]

Glad to be of service .... you know we've only answered 10% of the question ??

Hehe, this. But it was fun.

[cargirl1990]

Bentley and Rolls Royce defy the laws of physics by making extremely heavy cars go insanely fast. They also come in many body styles which suit the needs of the wealthy soccer mommy so that she can go pick up her little spoiled brats from their Timbits soccer game.

[RacingManiac]

Fast in a straight line is just power, traction and aero. Once you factor in repeated accel/decel, and cornering, inertia dominates and Bentley, Rolls fails miserably....

More weight = more grip but more intertia. Gotsta find the sweet spot, see.

hence downforce......free weight, more grip, no inertia....

[Matra et Alpine]

hence downforce......free weight, more grip, no inertia....

makes it sound like inertia magically disappears
It's all still there and shows up in the tyre wear on a car that can handle.

Ye cannae ignore the laws o' physics. cap'n