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Guillaume · 01-13-2008, 11:28 PM

Bigger diameter = bigger polar inertia. Even a lighter 16" wheel might perform worse than a heavier 15". Keep in mind that the polar inertia is proportional to (diameter)^2.
In this case, the BBK upgrade will also increase the polar inertia and the unsprung mass: all bad for straight line performance and handling.
The side wall strength? Yeah that's important when hard cornering, but does it counterbalance the rest? Not quite sure...

Just my $0.02...

matt mead · 01-14-2008, 12:07 AM

Bad_Dog Wrote:Finally, (probably) the overall diameter of 16" wheels will be a little bigger than 14" (or 15") trading some power for higher top speed.

thats not lose of power, thats change in gearing... gearing maybe worse but cornor speed should increase dramticily when compared to 15s... resulting in faster lap times Smile

improvedae86 · 01-14-2008, 03:28 AM

There is no way of overcoming if you have a larger diameter you are going to have more rim material from the center. But if you where to compare the same overall diameter from a huge sidewall to a low profile the tyre weight will offset this difference in rims,if with the latest rims there is any at all.Also rims designed for low profiles have less tyre and have to be designed for high cornering force so therefore need to be stronger adding to the weight.

Also i am refering to newer rims ie: ray's ce28's etc vs say a old works equip etc

**NoHachi** · 01-14-2008, 09:14 AM

matt mead Wrote:thats not lose of power, thats change in gearing... gearing maybe worse but cornor speed should increase dramticily when compared to 15s... resulting in faster lap times

I'm not seeing the self evident increase in cornering speeds..why? Lower sidewalls are not necessarily better when using (semi) slicks. I would be more carefull about making blanket statements like these.

improvedae86 · 01-14-2008, 10:15 AM

NoHachi Wrote:
matt mead Wrote:thats not lose of power, thats change in gearing... gearing maybe worse but cornor speed should increase dramticily when compared to 15s... resulting in faster lap times

I'm not seeing the self evident increase in cornering speeds..why? Lower sidewalls are not necessarily better when using (semi) slicks. I would be more carefull about making blanket statements like these.

You right its very vehicle dependant , depending on the vehicle power / weight / chassis design / aerodynamics etc etc there can be a point which larger profile tyres are faster due to there better heat dissipation and load torque transfering absorption ability.

But maybe we are way off topic from a ae86 , its light so overheating small area low profile tyres shouldn't be a issue unless chassis set up produces it. Traction effort / transferring the torque shouldn't be a issue to a limit as most cars just have small engines , the four link also is great for straight line traction if the correct angles are set. And aero the car is a upside down wing in shape so downforce is out of the question.

So any area decrease in side wall will only increase stiffness , resulting in everything from laterial stability to less distortion under heavy loads like braking

From memory Kai power , KMS , Pro sprits etc have all found 17" faster on the track but 17" tyres where instantly banned from N2 racing.

matt mead · 01-14-2008, 10:17 AM

NoHachi Wrote:
matt mead Wrote:thats not lose of power, thats change in gearing... gearing maybe worse but cornor speed should increase dramticily when compared to 15s... resulting in faster lap times

I'm not seeing the self evident increase in cornering speeds..why? Lower sidewalls are not necessarily better when using (semi) slicks. I would be more carefull about making blanket statements like these.

larger contact patch Wink

saitrix · 01-14-2008, 12:16 PM

matt mead Wrote:
NoHachi Wrote:
matt mead Wrote:thats not lose of power, thats change in gearing... gearing maybe worse but cornor speed should increase dramticily when compared to 15s... resulting in faster lap times

I'm not seeing the self evident increase in cornering speeds..why? Lower sidewalls are not necessarily better when using (semi) slicks. I would be more carefull about making blanket statements like these.

larger contact patch

All well and good having a larger contact patch but by doing that you are reducing the kg/m^3 of them too. So if you use the exact same tyres but in the two sizes then the larger tyre will have a smaller grip per m^3 of the contact patch.

In the dry it might be favourable to use the wider tyres, but you have to weigh up where you are spreading the cars weight too much on the tyres. Not quite as simple as larger contact patch = more grip. Especially in the wet I would say you would be much better to run thinner tyres, also in the snow you will gain an even bigger benefit.

I personally would use a high grip, but not too wide, probably roughly stock. Real shame tyre companies do not release the coefficient of friction for their tyres, otherwise you could work all this out easily beforehand.

improvedae86 · 01-14-2008, 01:02 PM

saitrix Wrote:
matt mead Wrote:
NoHachi Wrote:
matt mead Wrote:thats not lose of power, thats change in gearing... gearing maybe worse but cornor speed should increase dramticily when compared to 15s... resulting in faster lap times

I'm not seeing the self evident increase in cornering speeds..why? Lower sidewalls are not necessarily better when using (semi) slicks. I would be more carefull about making blanket statements like these.

larger contact patch

All well and good having a larger contact patch but by doing that you are reducing the kg/m^3 of them too. So if you use the exact same tyres but in the two sizes then the larger tyre will have a smaller grip per m^3 of the contact patch.

In the dry it might be favourable to use the wider tyres, but you have to weigh up where you are spreading the cars weight too much on the tyres. Not quite as simple as larger contact patch = more grip. Especially in the wet I would say you would be much better to run thinner tyres, also in the snow you will gain an even bigger benefit.

I personally would use a high grip, but not too wide, probably roughly stock. Real shame tyre companies do not release the coefficient of friction for their tyres, otherwise you could work all this out easily beforehand.

Larger contact patch can also not apply to wets if there channels are wider than a standard road tyre the area is the same . Compounds of wet tyres also = more grip in wet conditions.

But so much of all this is dependant on the vehicle.

Weight of the vehicle on the tyres or loading comes down to all the suspension geometry so will never be just a set rule of loading. There is never going to be a perfect road surface or perfect geometry for that road. Especially with the old dated mac strut or live axle . And what would the loading of a thin tyre be ? it can only generate low amounts of mechanical grip so its loading is less than a larger tyre with bigger contact area to a point.

Contact or friction area being larger will regardless of vehicle for sure have a limit that beyond has negative effects. But how many light modern cars have 16" 17" 7'or 8' wide tyres factory now ? is it really to big for a ae86.

Lots of tyre companys release all the information on the coefficient of friction , pressure , temp etc . I have Toyos R888's and they gave me all the information when purchasing them . If you really needing information on the tyre that badly there are lots of companys that do testing , mostly the best are in france.

I think for snow or mud nobody is going to need the increase in width but wouldn't a bigger diameter still equal more drive and braking traction ?

peter · 01-14-2008, 06:56 PM

Great amount of information from you guys.
I just don’t know the result. Would you install 16 inch wheels or not on weekend and track car?
When I installed 16 inch wheels on my Integra, I killed a bit of handling dynamics and car felt slower in acceleration and breaking.
I would like to avoid of same with Ae86 with even smaller engine capacity.
Probably the 15 inch wheels are just optimal for bigger breaks and good handling, I guess.

**NoHachi** · 01-14-2008, 10:04 PM

Like I said..to many blanket statements, too many generalisations and some misinformation added in. Tires(which is what we are really dealing with) are difficult stuff..even to trained mechanical engineers. They do not lend themselves well to this kind of pseudo-scientific internet discussions. I'll try and address a couple things I noticed:

Quote:So any area decrease in side wall will only increase stiffness , resulting in everything from laterial stability to less distortion under heavy loads like braking

From memory Kai power , KMS , Pro sprits etc have all found 17" faster on the track but 17" tyres where instantly banned from N2 racing.

I know some racing teams that experienced exactly the opposite.. The interesting question is WHY 17s were faster in some cases and WHY they were banned at all. Were they faster because of the bigger brakes that could fit under them? Superior traction? etc etc.

Quote:larger contact patch Wink

Sorry but you are very wrong. With given tire pressures and vehicle weights, contact patch size is exactly equal for ALL imaginable tires. Think about it. If tire pressure is xx and vehicle weight = yy then contact patch size is always the same and equal to yy/xx. This is one of the most repeated internet fallicies you find. The only way to change contact patch size is to change tire pressures. (which is what they do for example in the desert).

Quote:All well and good having a larger contact patch but by doing that you are reducing the kg/m^3 of them too. So if you use the exact same tyres but in the two sizes then the larger tyre will have a smaller grip per m^3 of the contact patch.

Again this is not correct people. pressure/cm2 is again exactly equal between tires. What does change is the shape of the contact patch. Narrow tires have longer, less wide contact patch that allows rubber particles to stay in contact with asphalt particles longer and allows for more time to squeeze the water out. That is why they are better in the wet.

Quote:I personally would use a high grip, but not too wide, probably roughly stock. Real shame tyre companies do not release the coefficient of friction for their tyres, otherwise you could work all this out easily beforehand.

Not correct. Wider tires actually can produce faster laptimes. For example, they reduce the friction experienced by the car due to turning. I wont go into details as I dont fully understand the elastic and dynamic behaviour of tires myself. Milliken&Milliken has a great introductory section that goes into this. I suggest you read it if you are really interested.
Less friction also means less force required at the steering wheel and less information arriving at the driver. Its all a trade-off..

Quote:Lots of tyre companys release all the information on the coefficient of friction , pressure , temp etc . I have Toyos R888's and they gave me all the information when purchasing them . If you really needing information on the tyre that badly there are lots of companys that do testing , mostly the best are in france.

Almost NO tire company does this kind of testing and releases the results. Whatever information you got with the R888s was most definately a very rough approximation, as true tire/force diagrams are not readable for 99.9% of the engineering community..nevermind the enthousiast.