Bhps Vs Acceleration Vs Gear Ratio's Vs Rr Results

[huckleberry 0]

I've been thinking about all the RR threads and some other things I heard a few times. I KNOW I'm getting it wrong somewhere but maybe we can have a nice thread about this.

 

Rolling roads measure wheel BHP by the rollers. But it wouldn't be by the acceleration of the rollers because otherwise you should have different results in 3rd of 4th gear. How come?

Then again tyre size shouldn't affect the acceleration of ones car. But gear ratio's seem to do. Why is that? Is there any use changing the gear ratio's except for being in the better rev-range on a certain track most of the time? And is there any use chainging tyre size?

[TEKNOPUG 3]

I've been thinking about all the RR threads and some other things I heard a few times. I KNOW I'm getting it wrong somewhere but maybe we can have a nice thread about this.

 

Rolling roads measure wheel BHP by the rollers. But it wouldn't be by the acceleration of the rollers because otherwise you should have different results in 3rd of 4th gear. How come?

Then again tyre size shouldn't affect the acceleration of ones car. But gear ratio's seem to do. Why is that? Is there any use changing the gear ratio's except for being in the better rev-range on a certain track most of the time? And is there any use chainging tyre size?

 

Have you thought this through Tim, you usually post quite sensible topics?!

 

Tyre sizes have a huge effect of a cars dynamic ability. People tend to dismiss tyres when calculating gearing or suspenion changes but they are the only part of the car which is in contact with the road. They are the first part of the suspension and the last part of the drivetrain.

 

Imagine that you swapped from 60 profile to 50 profile tyres. Effectively you are reducing the size of the wheel by almost an inch and in turn lowering the overall gearing. The opposite would happen if you increase the profile of the tyre. Hence why many people run 45 or even 40 profile tyres when fitting wheels larger than 15" - to keep the gearing the same.

 

With regards to people changing the gearing in their cars; you are right to say that you can keep a modded engine in the power by altering the gearing, particularly if you are radically changing the engines power characteristics (such as turbo-charging). But there are other reasons too, such as providing better low-gear traction or better top-gear cruising or fuel economy.

 

Tyres are also the first part of the suspension as they initially absorb the in the road. So higher profile tyres will give a more comfortable ride as there is more travel in them before the energy/movement is transferred to the suspension components. However, higher profile tyres will also flew more on corners (tyre wall flew) and thus lead the car to slide. So you may choose to fit a lower profile tyre to counter this. If you drive a car with very stiff suspension and you find that the car is nervous or skittish over bumps, it may simply require the tyre profiles to be increased to balance this out.

 

I'm sure other, more qualified people will add their thoughts to this in due course.

[huckleberry 0]

Thanks for the reply.

 

I kind of knew the facts you point out. I had some different questions about it. I believe someone on this forum once said that he used 45 in stead of 50 tyres so he would accelerate faster like having lower gearing. And I believe other people said it wouldn't make any difference. Somewhere in my mind I thought it wouldn't matter but I'd like the theory behind it. Does lower profile tyres make you car accelerate faster?

 

Same thing about the RR results. What does it measure to get the wheels bhp? If it measures the acceleration then gear ratio should matter. Yet it doesn't so somewhere I'm missing one or more links.

[M3Evo 0]

Guess all it can be measuring at the wheels is the speed of the roller, and the torque required to prevent acceleration of the roller.

 

From this the power can be calculated, and then related to engine speed.

 

Thinking about it, if it were done on torque at the wheel and engine speed, you would get giant steps in the power curve every time you change gear.

 

(That's probably wrong though )

 

EDIT> Oh and yes, if you fitted smaller wheels and tyres which reduced the rolling radius over original kit, you'd be able to accelerate quicker, but your top speed would be reduced.

Edited March 16, 2006 by M3Evo

[TEKNOPUG 3]

Thanks for the reply.

 

I kind of knew the facts you point out. I had some different questions about it. I believe someone on this forum once said that he used 45 in stead of 50 tyres so he would accelerate faster like having lower gearing. And I believe other people said it wouldn't make any difference. Somewhere in my mind I thought it wouldn't matter but I'd like the theory behind it. Does lower profile tyres make you car accelerate faster?

 

Same thing about the RR results. What does it measure to get the wheels bhp? If it measures the acceleration then gear ratio should matter. Yet it doesn't so somewhere I'm missing one or more links.

 

 

The increase/decrease in acceleration when changing wheel diameter will be more noticable the greater the change is. For example, just changing from 55 to 50 profile tyres won't make much of a change to the wheels diameter and therefore gearing. However, changing from 14" wheels with 45 tyres to 18" rims with 60 tyres (and vice versa) will obviously make a far more noticable difference.

[jonah 1]

M3Evo, that sounds pretty much spot on. Power = torque x angular velocity (rotational speed), so as long as these are both measured at the same place - at the rollers, at the wheels or at the flywheel - then you can get a meaningful power figure. Multiplying torque of the rollers by speed of the engine would be meaningless. Engine speed is only measured to give the power curve a scale on the x-axis, it is not used in the power equation itself.

 

Gearing exchanges torque for speed or vice versa. A higher gear generates less torque at the wheels but higher speed (for a given engine rpm), so the product is the same.

 

Don't get confused by "acceleration" of the rollers. When doing a power run the speed is ramped up, but it is not the rate of speed increase that is being measured - in fact that's controlled by the machine itself. The rollers are constantly being braked to prevent acceleration, and it is the amount of braking that needs to be applied that lets the machine measure the torque - hence the term "brake" horse power. It's perfectly possible to measure power with the rollers at a constant speed - acceleration has nothing to do with it.

 

And yes, smaller tyres have the same effect as lower gearing, so yes you will accelerate faster (in a given gear and at a given engine speed). But you will also be going slower when you reach the red line and need to change up. These effects almost cancel each other out when looking at overall acceleration times from standstill to a given speed through the gears.