Hypothetical mpg question fo a 90 mile journey.

Ultrasonic

WTFH wrote: »

Would it be higher wear?

To maintain a higher speed there needs to be a greater force between the tyre and the road (enough to oppose all of the resistive forces including air resistance), which will surely increase tyre wear? The only complicating factor is that this will also cause the tyre to warm up, but I thought this would also increase wear?

WTFH

Ultrasonic wrote: »

To maintain a higher speed there needs to be a greater force between the tyre and the road (enough to oppose all of the resistive forces including air resistance), which will surely increase tyre wear? The only complicating factor is that this will also cause the tyre to warm up, but I thought this would also increase wear?

See, I'm not sure, which was why I questioned it.
Here's my thinking/questioning...
The "force" that makes the tyre grip the road is dependent on several factors, such as the road surface, the tyre material, tyre size & pressure, torque being applied to the wheel and the weight of the vehicle (amongst others).
At higher speeds will the aerodynamics push the car down or lift it up?
If the effect of the weight of the car is reduced at higher speeds, then the force between the tyres & the road is reduced too.

Then there's the tyre wear situation (which also kinda ties in, I think). If the tyre is running below it's optimal temperature range then the level of grip is reduced. While this is more obvious under braking/steering, it will also be true when the transmission is trying to drive the vehicle.
If the tyre is above its optimal temp, then the wear is increased.

You could then question whether it makes a difference which wheels are being driven and the type (and straightness) of the road.

Ultrasonic

accidentalspender wrote: »

With the above in mind, the 30MPH car's engine will have turned over 240,000 times more than the 90MPH car in the same journey. Every time it turns over, it has fuel injected & experiences all the same inefficiencies as the 90MPH car, other than air resistance.

I'm not quite sure what point you are making here? Also, I don't think the inefficiencies are exactly the same? Won't more air be forced into the engine at 90 mph thereby increasing the pumping losses?

Ultrasonic

WTFH wrote: »

See, I'm not sure, which was why I questioned it.

I'm not certain either and I'm sure you know more about this than me...

The "force" that makes the tyre grip the road is dependent on several factors, such as the road surface, the tyre material, tyre size & pressure, torque being applied to the wheel and the weight of the vehicle (amongst others).
At higher speeds will the aerodynamics push the car down or lift it up?
If the effect of the weight of the car is reduced at higher speeds, then the force between the tyres & the road is reduced too.

We're talking about two different (components) of force I think. I was talking about force parallel to the road surface (the bit needed to push the car foward), whereas I think you're talking about component normal to the road surface?

I would have thought that the component parallel to the road surface must increase with speed for the reason I gave above, and it's not immediately obvious to me that it matters if the car's downforce increases or decreases with speed. Unless there is an amount to which wheels naturalls 'slip' on the road surface and this is reduced by increased downforce?

Then there's the tyre wear situation (which also kinda ties in, I think). If the tyre is running below it's optimal temperature range then the level of grip is reduced. While this is more obvious under braking/steering, it will also be true when the transmission is trying to drive the vehicle.

If the grip is reduced, does this increase or decrease tyre wear, or make no difference?

WTFH

Ultrasonic wrote: »

I'm not certain either and I'm sure you know more about this than me...

I doubt that - it's 20+ years since I studied mech eng, and I've hardly touched it since I left uni.

Ultrasonic wrote: »

We're talking about two different (components) of force I think. I was talking about force parallel to the road surface (the bit needed to push the car foward), whereas I think you're talking about component normal to the road surface?

Yes, perpendicular to the surface, but I think it's relevant. The weight of the vehicle affects how much energy is required by the engine to maintain the speed. Now, if you can reduce the effect of the weight, you can reduce the energy required by the engine. The weight is "downforce". Reduce that and your engine is more efficient in a straight line. You'll also have less tyre wear.

Ultrasonic wrote: »

I would have thought that the component parallel to the road surface must increase with speed for the reason I gave above, and it's not immediately obvious to me that it matters if the car's downforce increases or decreases with speed. Unless there is an amount to which wheels naturalls 'slip' on the road surface and this is reduced by increased downforce?

Ah, friction. Your friend and your enemy all at the same time. It needs to be high enough that you aren't spitting stones off the tyres all the time, but low enough that you can move easily. Consider this - get a car with no PAS and try turning the steering wheel when stationary, then try it when driving slowly, and finally when driving quickly. Which is easiest?
And if you're going too quick, then the steering causes you to lose grip - the friction is less than the force pushing you forward.

Ultrasonic wrote: »

If the grip is reduced, does this increase or decrease tyre wear, or make no difference?

Eek - excellent one. If the grip is reduced because you're below optimal temp, then at a constant speed it might have decreased tyre wear. Under acceleration it will increase it, definitely, but I'm not sure at constant speed.

Ultrasonic

WTFH wrote: »

I doubt that - it's 20+ years since I studied mech eng, and I've hardly touched it since I left uni.

My degrees are all physics/electrical engineering related so you win .

WTFH wrote:

The weight of the vehicle affects how much energy is required by the engine to maintain the speed.

Hmmm... I suspect this is probably true, but I have to admit that I'm not 100% sure why. Presumably it must be caused by an increase in tyre rolling resistance. Narrower tyres tend to have lower rolling resistance, so I guess it comes down to changes in frictional losses between the tyre and the road?

I've never thought about this before. I'm sure it's true that drivers who accelerate faster, break harder and corner faster will wear tyres out quicker. But I see what you mean about the straight line speed question possibly being not as clear cut as I'd thought. Although my guess remains that higher speed means increased tyre wear.