Different sizes of wheels and tyres
Discussion
Trying to avoid a thread hijack:
Whilst I agree with everything that you've said I haven't seen a reasoned arguement for a clear performance benefit of 18" wheels on a T-chassis and it doesn't appear clear to me.
As I see it:-
We know that assuming that you keep the rolling radius the same then larger wheels will: Downside: - cost more, have more expensive tyres, perform worse in the wet, perform worse on bumpy roads, be more susceptible to tram lining, increase the weight of the car, increase the unsprung weight, produce a harsher ride, increase the rotating unsprung weight (second moment of inertia) and be more liable to scuffing on kerbs when parking. On the plus side they should: - prevent the tyre from distorting thus giving greater cornering grip in the dry and on smooth roads, be more direct giving greater steering feel and control, have less tyre wall flex (thus reducing both heat and fatigue and be better for long high speed journeys) and they should look good. Perhaps most of all they allow you to use top quality (UHP) rubber.
Larger wheels will effectively increase the spring rates and fitting lower section tyres with less sidewall flex can make the car more sensitive to suspension geometry changes.
We also know that if a car was set up for one wheel size then by increasing the wheel diameter, keeping the same rim and tyre widths and the same rolling radius then the matematical advantages are not as much as the disadvantages, even after minimising them by adjusting the suspension set-up; as long as the car's weight and cornering speed are not enough to mean that the tyre rolls onto its sidewall causing sidewall scrub during cornering.
The real world isn't always as clear though. For the Griff/Chim cars it appears to be simple: if you fit 16" or 17" wheels all round then this allows you to fit bigger front brakes, higher performance tyres such as GSD3s, and rims usually more suited to the tyre widths;this appears to have been shown by quite a few to outweigh the disadvantages.
For the T-chassis cars it's not so simple though. The tyre of choice for 18" is usually GSD3s and that is available for 16" wheels; the 16" rim widths appear to be fine so that just leaves the brakes and the tyres rolling onto the sidewalls causing sidewall scrub during cornering. Are these real problems for the T cars? Can T cars make use of brakes which are too big to fit into 16" wheels? I stil like the look of the 18s though.
Any thoughts?
richb said:
Fact is that bigger rims allow for bigger brakes and as long as the suspension is tuned to match the tyre a lower profile is better. Suspension can be tuned with springs, dampers and of course tyre pressures, but to say 16" is better on the road is an over simplification. Anyway as has been said TVR altered the std suspension on the Tuscan after a few years to suit the 18" wheels that became standard. And... IMO Tuscans on 16" wheels look silly Rich...
Whilst I agree with everything that you've said I haven't seen a reasoned arguement for a clear performance benefit of 18" wheels on a T-chassis and it doesn't appear clear to me.
As I see it:-
We know that assuming that you keep the rolling radius the same then larger wheels will: Downside: - cost more, have more expensive tyres, perform worse in the wet, perform worse on bumpy roads, be more susceptible to tram lining, increase the weight of the car, increase the unsprung weight, produce a harsher ride, increase the rotating unsprung weight (second moment of inertia) and be more liable to scuffing on kerbs when parking. On the plus side they should: - prevent the tyre from distorting thus giving greater cornering grip in the dry and on smooth roads, be more direct giving greater steering feel and control, have less tyre wall flex (thus reducing both heat and fatigue and be better for long high speed journeys) and they should look good. Perhaps most of all they allow you to use top quality (UHP) rubber.
Larger wheels will effectively increase the spring rates and fitting lower section tyres with less sidewall flex can make the car more sensitive to suspension geometry changes.
We also know that if a car was set up for one wheel size then by increasing the wheel diameter, keeping the same rim and tyre widths and the same rolling radius then the matematical advantages are not as much as the disadvantages, even after minimising them by adjusting the suspension set-up; as long as the car's weight and cornering speed are not enough to mean that the tyre rolls onto its sidewall causing sidewall scrub during cornering.
The real world isn't always as clear though. For the Griff/Chim cars it appears to be simple: if you fit 16" or 17" wheels all round then this allows you to fit bigger front brakes, higher performance tyres such as GSD3s, and rims usually more suited to the tyre widths;this appears to have been shown by quite a few to outweigh the disadvantages.
For the T-chassis cars it's not so simple though. The tyre of choice for 18" is usually GSD3s and that is available for 16" wheels; the 16" rim widths appear to be fine so that just leaves the brakes and the tyres rolling onto the sidewalls causing sidewall scrub during cornering. Are these real problems for the T cars? Can T cars make use of brakes which are too big to fit into 16" wheels? I stil like the look of the 18s though.
Any thoughts?
If you eliminate the aesthetic value and consider only the drivability factors you'll end up with the right answer. Then it's a simple case of deciding how much drivability you are prepared to give up in order to improve the appearance of your car.
Oh and the statement that lower profile 'prevent the tyre from distorting thus giving greater cornering grip in the dry and on smooth roads'. There are advantages to tyre wall flex when cornering too. This flex gives a lot of feedback and enables progressive oversteer. Without it there is more ultimate grip but at a price, when it lets go there's no chance of catching it.
Oh and the statement that lower profile 'prevent the tyre from distorting thus giving greater cornering grip in the dry and on smooth roads'. There are advantages to tyre wall flex when cornering too. This flex gives a lot of feedback and enables progressive oversteer. Without it there is more ultimate grip but at a price, when it lets go there's no chance of catching it.
When I changed from 16" to 17" rims on my Cerbera, I deliberately went for a 45 profile tyre all round, as opposed to the more usual 40 profile. I've also gone for slightly narrower tyres than is the norm.
Normal Cerbera on 17" is 235/40 front, 255/40 rear. I've gone for 225/45 all round (the same rolling radius and size as the speed 6 cars that had 225/50x16" all round).
The reason I did this is for the drivability and progressive grip that was stated on the first post. Also, I live out in the sticks, so low profile tyres wouldn't give a very compliant ride.
Seems to have worked well so far.
Normal Cerbera on 17" is 235/40 front, 255/40 rear. I've gone for 225/45 all round (the same rolling radius and size as the speed 6 cars that had 225/50x16" all round).
The reason I did this is for the drivability and progressive grip that was stated on the first post. Also, I live out in the sticks, so low profile tyres wouldn't give a very compliant ride.
Seems to have worked well so far.
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