Wheels ...again

[F908 Tim]

Read the recent posts with interest on the size of wheels/tyres.If and when I get the money they are a mustjudging from the pictures posted.
But...answer me this...
If a driven wheel covers a certain distance each revolution,ie. its tyres circumference,then won't enlarging the wheel and tyre to a larger circumference increase the distance covered? Given that it must,then if the engine is capable of rotating the wheel at the same speed wouldn't the car automatically travel faster? It would travel a greater distance in the same amount of time wouldn't it?...thats faster!
Or is the problem that the engine could not turn a bigger wheeland tyre at the same speed when operating at the same revs,so the theory falls down!
OK,Its Friday afternoon and its a head scratcher, but someone could explain to me if the bigger wheels propel the car faster couldn't they?

[greenv8s]

Increasing the rolling circumference of the wheel has the same effect as using higher gearing: you'll get more mph for a given rpm. This won't necessarily increase the vehicles max speed, and typically it will reduce the top speed slightly because the engine won't have enough torque to reach the rpm for maximum power, with the new gearing.

[gefopsman]

greenv8s said:

Increasing the rolling circumference of the wheel has the same effect as using higher gearing: you'll get more mph for a given rpm.

I found that accereration seems better due to using more of the torque in the early gears.

Top speed seems around the same, its hard to tell as the wind makes so much difference at top speed,but fuel economy will be slightly better.

Yes, I did say fuel economy!!!!

[19560]

Note also that assuming that you keep the roling radius the same then larger wheels will: Downside - cost more, have more expensive tyres, perform worse in the wet, perform worse on bumpy roads, 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, produce less heat and be better for long high speed journeys, and effectively increase the spring rates being an easy way to fine tune your springing rates from summer to winter if you swap back to the smaller wheels for winter and they should look good.

and

ultimasimon said:

GreenV8S said: Fitting lower section tyres with less sidewall flex can make the car more sensitive to suspension geometry changes. Lowering the car can mess up the handling by throwing the static geometry out, and giving you too much camber correction and bump steer. If you're doing it for looks then fair enough, but be aware that it has the potential to make a right mess of the car's handling characteristics. And don't forget it reduces traction too. When power is applied to the wheel it causes the sidewalls to flex helping cushion the force which would otherwise end up in wheel spin - lower profile tyres reduce this cushioning effect as the sidewall dimensions are halved, and cause the tyres to jolt suddenly, breaking traction. All of my best runs down the pod were confirmed using this theory and backed up by the top fuelers. This is why dragsters use Hoosier tyres with huge sidewalls and run low air pressure in them, for more soft contact area and saggier sidewalls, thus, better traction Hence why big rims are bling on public roads. Defo good for track days with slicks and big off brakes, and big wheels also halp improve air-flow to the disks, but they suck for traction away from the line.

[kentishS2]

In theory, changing from 15" to 17" for example will not make any difference to gearing or top speed if you select the correct size of tyre to maintain the same rolling circumference. Mine are the same overall as when they were 15" standard wheels and there is no difference in acceleraton, gearing, economy or top speed.