Track day wheels - the bigger the better?

bigger tyre diameter = bigger contact patch

I was told this by a chap who used to work for Bridgestone F1. Also from one of the senior motorsport advisors at BMTR in B'Ham (and they supply British F3). And Ian agrees also (and he and the family C have done a bit). Now it dont make sense to me as no matter how big the diameter of a circle, its tangent by definition is at an infinately small point but I have heard this from several 'knowing' sources....

 

Tyres don't stay perfectly round either so the % they squish at the bottom (if i can use that word) should increase as the tyre gets bigger if it has the same properties.....

 

more rubber on the road?

Keep in mind guys, that the whole question of this thread is about fitting bigger wheels. The outer diameter / rolling circumference of the tyre stays pretty much the same, because you use increasingly lower profile tyres, the bigger the wheel diameter gets, to keep much the same rolling circumference, otherwise you'd end up with horrendous great steam rollers with no wheelarch clearance at all and gearing all gone to pot. So, the tangent of the tyre dia on the road is pretty much the same; no matter whether you've got 185.60-14s or 215/40-16s, the RC is still about 1760 mm.

I too have heard quite a few experts talking about putting 'more rubber on the road' but it's not quite as simple as that, is it... (?)

 

Iv'e heard a few people on here say that their speedo's don't read correctly at higher speeds because of the wheel/tyre they use.

 

I am talking about bigger rolling circumference - increasing this is possible to a degree on most cars as is sorting out the issues that come with it

I have recently gone up from 15" (21.5" diameter) to 16" (23.3" diametrer) slicks and also wider - I have no doubt there is more grip - of course this is on a car with sufficient wheelarch clearance (see sig pic) and also I have changed the front suspension geometry to accomodate

but you gotta be keen
the regs have to allow it

and possibly its outside the scope of a 'trackday' car. But also people spend 1000's on trackday cars so go figure. maybe it is relevant

 

& with a slightly bigger circumference should come a little more contact patch just weighing up the pro's & con's like Rob say's.

 

There seem to be two different discussions here.

1. Bigger wheels, but same rolling radius of the tyres.

2. Increasing the rolling radius of the tyres.

I was assuming the original question was about discussion 1, i.e. not changing the rolling radius, but increasing the wheel (rim) size.
In this case (assuming the weight doesn't change when going to bigger wheels/rims), is it better to run 15s than 17s (or 14s and 18s)?
I thought that bigger wheels and smaller side walls would be better, as the tyre would flex less, both in corners and over bumps. This would mean that suspension setting changes would have a bigger effect and (hopefully) be easier to feel.

 

How much tyre wall flex can you do without?

 

It all boils down to geometry surely, where the roll centre lies determins the action of the car in the corner, with other factors such as unsprung weight and tyre contact patch.

On a trackday car with standard geometry (i.e no ball joint extenders etc) going any degree larger on total circumference than the standard wheel raise the roll centre, to accomodate the larger wheels effect on the ride height, the initial solution would be to lower the car.. Most people do this with no consideration for geometry, for example to all important level of the wishbone in relaion to its pivot point.

Providing you have the correct geometry, space to accomodate the wheel in the arch, and the power to justify it.. I.e a touring car, then larger wheels can be better, however on your run of the mill mk2 track car going much bigger than the standard rolling circuference wont gain you anything?

 

the big wheels on super tourers need gearboxes with higher [dimensionally] drive shafts to get everything to line up. they do it for brake clearance, for a track day surely cost is the biggest factor, if so 15"s seem to be very cheap and the reeves boys post some good lap times on them