Wheel size physics question

  • Thread starter Thread starter KSaiyu
  • 12 comments
  • 1,404 views
KSaiyu

KSaiyu

(Banned)
Messages
2,822
We all know a larger diameter wheel, or a larger tyre with more surface area is better for racing, but why is this?

It sounds simple, but I've looked up on so many sites and books, and they give different answers. I originally thought, more surface area = more available grip, but then I was reminded about the physics that would be involved. If you increase the surface area, the pressure exerted would be less, and I read that since it's the pressure that forces the downward force for the grip, increasing the surface area of a tyre wouldn't lead to more available grip. I also wondered, why would a bigger diameter wheel create more grip too?

I also have one other quick question. I was reading a road test in Evo for a luxury car (I think the Bentley), and it said something like "With all this torque, 4 gears would have been sufficient, at the most". Why do bigger, more torquey engines not need as many gears?

Any help would be great :D
 
torque is the AVAILABLE power to accelerate. A big engine with lots of torque at low rpm means that you can still accelerate hard despite a higher gear. If you have a high tuned engine with a high-lift camshaft with very little torque at low rpm will require a low gear to pull hard and accelerate. This is why a motorbike will need to downshift several times to pass a car very quickly.
Therefore, if you are rolling along behind a truck in a high gear on a motorway, if your car has little torque a downshift will be required. The Bentley, on the other hand, would be able to use a high gear ratio and still pull hard past the truck.

I'm not too sure about the grip thing, I'll try and find out nore.
 
Large wheels and tires on a car not ment for them aren't good. It makes it slower since the gearing is throwin outta wack. As well as the brakes. Thats why when you see these Escalades rollin on 24's they usally have bigger brakes. Also they won't go nearly as fast as they used to due to different gear ratios at the wheels.
 
I've seen several lifted trucks with big wheels and tires that can't go faster then 70 because of gears. You need to switch gears when you make bigger tires.
 
Ahh haa. Well thats because they are overgeared. Perfect gearing means that top speed is achieved just below the limiter in top. The car will not pull any more because the actual limiting factor is the aerodynamics.
As for brakes they don't affect top speed at all unless they are being used. Gearing only needs to be changed if the car becomes overgeared, and therefore will not accelerate in top gear to the redline.
 
Originally posted by KSaiyu
We all know a larger diameter wheel, or a larger tyre with more surface area is better for racing, but why is this?
Click to expand...
Sometimes it's not. Autocrossers in the Prepared classes, where suspension components are often very stiff, frequently step down a wheel size in order to lower the car, shorten the overall gearing, and reduce sidewall height.
If you increase the surface area, the pressure exerted would be less, and I read that since it's the pressure that forces the downward force for the grip, increasing the surface area of a tyre wouldn't lead to more available grip.
Click to expand...
OK, your thinking is correct as far as it goes. Increasing the surface area does decrease the pressure per unit on the contact patch, theoretically decreasing grip. That, however, assumes infinite grip on the part of the rubber. In point of fact, if the contact patch is too small, the acceleration forces (cornering or accel/deccel) will overwhelm the total available grip the contact patch can generate. Increasing the size of the contact patch increases the total amount of grip you can generate.
I also wondered, why would a bigger diameter wheel create more grip too?
Click to expand...
The overall diameter of the wheel/tire combo doesn't affect grip. The point in upsizing the wheel is to maintain the same outside diameter of the tire. This means that the sidewall height of the tire is reduced by the same amount that the wheel diameter is increased. Shorter sidewall heights mean less flexibility and greater resistance to lateral forces, keeping the contact patch in better shape on the road. That's where the increased grip comes in.
I also have one other quick question. I was reading a road test in Evo for a luxury car (I think the Bentley), and it said something like "With all this torque, 4 gears would have been sufficient, at the most". Why do bigger, more torquey engines not need as many gears?
Click to expand...
Others have addressed this pretty well, but I'll add my two bits.

The torque output of an engine, like the horsepower, can be graphed against the rpm of the engine. A big, torquey engine will make the majority of its power over a broad range of revs. For instance, a Viper makes peak torque of about 500 ft-lbs somewhere in the 4000-5000 rpm range. However, it makes more than 70% of that peak amount starting at a mere 1200 rpm, which is not far above idle. This is referred to as a broad or flat torque curve. By contrast, an S2000 makes 200-odd ft-lbs, but you don't really get into the thick of it until you get above 5000 rpm. This is a narrow or peaky power band.

The narrower the power band, the more gear options you need in order to keep the engine in the sweet spot for a given vehicle speed. Broader power band engines don't require as much ratio coverage because they are more tolerant of a wide range of rpms.
 
Originally posted by KSaiyu
We all know a larger diameter wheel, or a larger tyre with more surface area is better for racing, but why is this?

It sounds simple, but I've looked up on so many sites and books, and they give different answers. I originally thought, more surface area = more available grip, but then I was reminded about the physics that would be involved. If you increase the surface area, the pressure exerted would be less, and I read that since it's the pressure that forces the downward force for the grip, increasing the surface area of a tyre wouldn't lead to more available grip. I also wondered, why would a bigger diameter wheel create more grip too?
Click to expand...


Hi again, KSaiyu. You're on the right track. The larger the contact patch is, the more opportunity for grip there is. Note I used the word opportunity.

Remember that a modern racing tire sticks more the harder you push on it. Each square inch of the tire's contact patch as a potential to turn its portion of the car's download into grip. The more surface area a contact patch has, the more opportunity to transfer download into a lateral load.

Of course, the car has to produce enough download in order to take advantage of the larger surface area. F1, Indy and Champ car tires are super wide compared to the size of the car, but mostly because these cars have the aerodynamics to produce enough download to take advantage of the wide tires. Take the wings off an F1 and its cornering ability drops signifigantly.

This is why wider tires give diminishing returns on a non-ground effects, non-aero car. (ie, vast majority of street cars) With the trade off between rolling resistance and weight, it becomes counter-productive after a point.

I know you've already read my suspension helper in the Settings Forum. For a more in depth explanation, I highly recommend Brian Beckman's excellent site.


M
 
Originally posted by neon_duke
The narrower the power band, the more gear options you need in order to keep the engine in the sweet spot for a given vehicle speed. Broader power band engines don't require as much ratio coverage because they are more tolerant of a wide range of rpms.
Click to expand...


^^^ Yep. Exactly what he said.

Also, sometimes 4 gears, spaced wide with a long final drive is used to preserve some semblance of good gas milage.


M
 
The more surface area, the less weight gets transfered to the ground and is more spread out. I really don't know, I'm only in grade 10.
 

Latest Posts

Back