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The Effect Of Wheels


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I came across this article that I thought was interesting - and demonstrated the effect that wheel size had both on acceleration and range.

Whilst it's common for 'performance' models to have larger diameter wheels - in this test al least they had a detrimental effect. Its a shame they didn't go on to test the handling too. 

Nevertheless it does make you wonder is that the trend for ultra low profile tyres is not only leading to more 'written-off' wheels as they are less capable of surviving pot-holes, but is actually reducing the performance and increasing running costs. 

https://www.teslarati.com/tesla-model-y-wheel-size-0-60-acceleration/

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I'd say it's 100% lead by styling rather than engineering. Low unsprung mass is a holy grail of performance cars in their purist forms but the company need to sell cars so make sure they look pretty first more often than not and once a trend is started (eg wow, are those 22" wheels?), others follow. 

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Also, the smaller wheels mean the torque of the motors (which decreases with speed) will have been lower with smaller wheels than the same car could have produced on stock wheels for the same speed (once it got past the speed where torque is still virtually 100%).  If you base it on the graph, then torque drops (once past that 100% point) and drops at such a rate that you still get less power at higher speeds that at slower speeds. So, the Tesla must have been operating within/close to its 100% range.

https://www.carthrottle.com/post/how-do-electric-vehicles-produce-instant-torque/

 

This thing for bigger rims was driven by the fact that higher sidewalls start to let go of the tarmac a lot sooner than lower profile tyres when forced sideways. The trouble is that people take that to mean that lower profile is better, when what really happens id the lower profile maintains it's grip for longer but when it lets go, it lets go in a big way, compared to higher side-walled ones that are more progressive and still maintain a bit of lateral grip even when sliding. So, people thought lower profile = better, and to keep the same rolling diameter they fitted bigger wheels, now we are at a stage where we seem 4x4 with low profile tyres and wheels that look like they are based on a penny farthing front wheel.

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Like @Bibs said, low unsprung mass is desirable as there is less inertia that needs to be dampened when the wheels move up and down over bumps in the road.

Larger side walls will flex when put under lateral forces (corners) and make the handling feel a bit woolly and imprecise. They will also flex a bit under acceleration and deceleration. Low profile tyres with small side walls only flex a bit making the handling feel more precise and "planted". Larger side walls can also be used as part of the suspension as they do in F1 cars.

On the video clip with different size and weight wheels they were mainly seeing the difference that the flywheel effect can have on acceleration and possibly gearing (and the torque that @andydclements mentioned). When the wheels turn they are acting as a flywheel. It is easier to spin up a lighter flywheel than a heavier one.

Larger diameter wheels generally have a larger contact patch with the road which will generally improve grip. Drag cars have large diameter tyres which give them a large contact patch enabling them to get all that power onto the tarmac.

A good sportscar (Lotus of course) will have lightweight large diameter wheels with low profile tyres.

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The stylist of S1 Elise, Julian Thomson,  put bigger (111S) wheels on his 118bhp S1 when new in 2000. I do not think it adds to the driving experience but clearly he thought it improved the look of 'his' car. 

At least he left off the 111S additional spoiler which really does live up to its name on a S1 Elise. Mind you, he hated the moulded-in spoiler added by the aero team during development. 

Justin 

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