Whats the answer, who knows?

[Deleted member 113659]

Ant, I understood the original post, and I haven't a clue what the answer is, he then asked why his speedo and sat nav were giving different readings with the new wheels, which what my answer referred to.

Hehe. You must be tired. The sat nav/speedo post was me. My fault completely looking back my post was not even relivent.

I was just trying to explain that wheel/tyre size obviously does alter speed.

Ant.

 

[Scooby_Doo]

oops !

 

[Peter103]

Thanks for all input, though mostly conjecture,none definitive. Would it help if we made the overall dia. Of wheel and tyer 12 inches greater than the other vehicle? Are there any Professors of Physics out there?

 

[bolide]

Ah, I think it is definitive. The wheel / tyre combination with the larger overall radius will give a greater top speed

This assumes that the vehicle has the power to pull the higher speed. This will be dependent on power output and gearing

It's not rocket science, it's… automotive science

If you remember your O Level physics, v=u + at, v^2=u^2 + 2as, etc

Nick Froome

 

[markjay]

The transmission gear ratio and the differential final drive ratio determine the overall ratio between the engine crankshaft speed and road wheel speed.

They do this by using different size cog wheels.

Bigger road wheels on the driven axle are very simply equal to driving the car in higher gear.

Everything that applies to driving in higher gear - lower acceleration, higher speed, better mpg - will apply to bigger wheels.

And vice versa for smaller wheels.

 

[markjay]

Top gear used to be 1:1, but nowadays the top gear on most cars is overdrive.

Diff final drive ratio are typically 4:1, meaning that when in top gear (1:1) the road wheels turn at one fourth of the crankshaft speed.

 

[Peter103]

Yes , very good Nick, and well explained Markjay, I was thinking you Mark would have the answer. But, are you sure? Only kidding, that'll do for me.

 

[Codger49]

I don't know if larger diameter wheels give better mpg if diff and gearbox ratios remain the same.

My W209 CLK280 moves up a tax band if you use 18" wheels instead of 17" wheels. I think this is because the government CO2 measurements increase from 220 to 227.

I always thought this higher CO2 output reflected increased fuel consumption. I.e. the engine is working a bit harder to drive the larger wheels through the higher overall gearing. I would also guess the diff and gearbox ratios are the same for both tyre size options.

I would be happy to be corrected though.

 

[Peter103]

I always thought this higher CO2 output reflected increased fuel consumption. I.e. the engine is working a bit harder to drive the larger wheels through the higher overall gearing. I would also guess the diff and gearbox ratios are the same for both tyre size options.

I would be happy to be corrected though.[/QUOTE]

Does this put the cat amongst the pidgeons.
What does Markjay think of this?

 

[Scooby_Doo]

I know that bigger wheels push up the Co2 but it can't be because of the actual size of the wheel as the tyre profile reduces to ensure that the overall diameter of the wheel/tyre combination remains the same. It could be that the larger wheels normally have wider tyres which increases the rolling resistance. From memory some car companies (Volvo I think) promoted low rolling resistance tyres as an aid to better mpg & emissions.

 

[Bellow]

Ah, I think it is definitive. The wheel / tyre combination with the larger overall radius will give a greater top speed

This assumes that the vehicle has the power to pull the higher speed. This will be dependent on power output and gearing

As above, only faster if the engine can pull the additional 'gearing' - which is unlikely as it will be geared as high (probably higher) as it can pull as it already is. Vmax occurs at the engine's maximum power. If the gearing prevents the engine attaining its rpm at which Pmax occurs then not faster - possibly slower.
Only competition vehicles are likely to be geared low such that they can be beneficially geared higher and the choice of gearing is dependent on the circuit (or stages) they are running on.

The transmission gear ratio and the differential final drive ratio determine the overall ratio between the engine crankshaft speed and road wheel speed.

They do this by using different size cog wheels.

Bigger road wheels on the driven axle are very simply equal to driving the car in higher gear.

Everything that applies to driving in higher gear - lower acceleration, higher speed, better mpg - will apply to bigger wheels.

And vice versa for smaller wheels.

Over geared (as per slapping an additional 6'' on the wheel diameter) will only aid economy if top gear can be utilised ie, without constantly changing down to lower ratios to maintain progress.

 

[Bellow]

I know that bigger wheels push up the Co2 but it can't be because of the actual size of the wheel as the tyre profile reduces to ensure that the overall diameter of the wheel/tyre combination remains the same. It could be that the larger wheels normally have wider tyres which increases the rolling resistance. From memory some car companies (Volvo I think) promoted low rolling resistance tyres as an aid to better mpg & emissions.

Correct - as other factors come into play, not least the additional weight (hence rotational inertia) of larger diameters. Four heavier flywheels soak up power just to get them turning.

 

[markjay]

The weight of the wheel and the friction with the road surface all come into play, as are other potential factors (air resistance, etc).

But for the purpose of answering the OP's question, and to properly understand the mechanism behind it, we should first answer the question assuming that all other parameters including the weight and friction are the same for both the larger-diameter and the smaller-diameter wheels. And in this case the effect of larger wheels will be quite simply similar to that of a higher gear ratio.

 

[190]

There is no way to answer the question definitively about top speed without knowing the overall gearing for each of the cars with respect to the maximum power revs.

If all things were equal the car that requires the least number of gear changes would accelerate fastest. However all things are not equal. Even if both cars were geared the same then the car with the larger wheels would have slower acceleration due to the increase in weight and rotational inertia. Wheels are the worst possible part of a car to add weight as there is double whammy effect of 1. the cars overall weight goes up to the detriment of acceleration and braking and 2. there is an additional detriment to acceleration and braking due to the increased rotational inertia that has to be overcome.

The same goes to any part of the engine and drive train that rotates. So for example if you lighten the flywheel the vehicle will accelerate faster. It's quite a noticeable effect on a motorcycle. Not so much on a car due to the higher overall weight but the theory still holds.

 

[HB]

I have 2 C320 cdis one with 17" wheels and the with 18" wheels. The 17" car accelerates quicker than the 18" car and is faster overall.
I can only put it down to wheels.

 

[Peter103]

Well Harry, having read all of the above, I wouldn't bet on it.

 

[Meldrew2]

...BHP measured in 5 figures!

Like my old Hillman Imp.

35.002

 

[bolide]

Bottom line: the diameter of the wheels is immaterial for top speed. Only the rolling radius of the tyre has any effect

Interestingly enough, larger diameter wheels tend to end up being heavier and gyroscopic forces mean that the bigger wheels can make the car accelerate more slowly

But top speed is limited by drag, power & gearing, not by weight, so is not affected

Nick Froome

 

[markjay]

I have 2 C320 cdis one with 17" wheels and the with 18" wheels. The 17" car accelerates quicker than the 18" car and is faster overall.
I can only put it down to wheels.

The question is not what is the diameter of the wheel rims... but the overall diameter of the wheel with the tyre fitted.

If the diameter of the wheel+tyre is the same for both 17" and 18" cars, then the acceleration differences will be due to other factors, e.g. the weight of the wheel, or the engine / gearbox etc, but not the diameter (i.e. the external size) of the wheel+tyre as such.

 

[Meldrew2]

If the wheels have a diameter 6 inches more than the others, the vehicle might well have a greater ride height, which implications to aerodynamic drag.

If the ride height is the same, the wheel arches may well have been modified. Again a possibility of worse aerodynamics.

Larger wheels also increase the weight of the vehicle and increase inertia, reducing acceleration but not affecting top speed.