Final BHP

[dogvet]

What is considered to be the percentage loss of power between flywheel and rear wheels?

Thanks in advance.

[Peter Lydford]

too much whatever it is teeth.gif

 

[Mike Bees]

Depends... on... gearbox, gear ratio, diff type, diff ratio, engine speed... If you're talking about losses between the flywheel and the *road* then it's also dependent on tyre size/type/pressure as well (not to mention the road surface). There is no percentage figure that 'works' for all conditions. A further point to note is that losses on a rolling road tend to be higher than on a (real) road due to:

 

* The rollers aren't flat, and there are 2 of them resulting in significantly lossy distortion of the tyre

* Powerful cars have to be strapped down hard onto the rollers, resulting in even more of the above.

* More powerful cars have to be strapped down hard and have 1-3 fat bastards sat on the back leading to even more of the above

* Sticky stuff squirted onto the tyres to help them grip the rollers

 

As a rule of thumb say 15-25% for all of the losses (the high end of that on a rolling road), of which a significant proportion is in the tyres themselves.

 

Fun eh?

 

Mike

[dogvet]

Funny I thought it would not be that easy!! The loss of 15-25% seems a good starting point!

 

[Roger King]

I'd put the range of losses 10% - 35% depending on all the conditions outlined by Mike Bees. The best I ever saw was a FF1600 and the worst was a 2.8 Capri.

 

As a further illustration, we recently tried a rather powerful 7 on the rollers in 4th gear and then 3rd gear at 6000rpm. On the face of it you might expect the power at the wheels in 4th to be higher than in 3rd because 4th goes straight through the gearbox whereas 3rd goes via the gear cluster. In practice the car gave 6bhp MORE power at the wheels in 3rd despite the gearbox losses. This is simply down to the fact that the back axle is running more slowly and therefore absorbing less power.

 

The difference between 3rd and 4th shows how much the rear axle ratio, wheel/tyre size, etc can also make to this figure.

 

 

[Mike Bees]

"...the worst was a 2.8 Capri". Oh how many different situations that phrase could be applied to smile.gif

 

Expect the significantly higher tyre losses in 4th gear than 3rd.

 

Mike

[Peter Carmichael]

... but the calculated flywheel figures came out nearasdammit spot on (question.gif)

[Tony Martyr]

The phenomina Roger describes is the working of the 'Road Load Equation' of the vehicle, the second term of which covers the change in vehicle losses with speed and the third term of which explains why the Seven is so top speed limited. As shown the effective road speed is not the true variable rather the rotation speed of the transmission components. Of these the most inefficient is the differental.

On the point about tyre losses. Most serious development work and all emission tests are done on single (48 inch dia) rolls chassis dynos to reduce the artifical tyre losses and the heating damage it does to tyres.

I don't agree quite with the notion of lashing cars down hard on the rolls. It is obviously true that it does increase the tyre distortion. Modern machines have high friction surfaces but even on plain steel surfaces considerable power can be transmitted so long as the acceleration rates are kept low and the rolls and tyres warm and clean. Trucks in low ratio do require pulling onto the rolls.

As anyone who has driven cars at speed on CDs knows, it is the front wheel drive ones that are scary because of the sudden sideways screwing effect you get if the wheels get out of line; the lashing has to consist of cross bracing.

The danger of jumping out of the rolls at speed is more apparent than real because the car has apparent road speed but no momentum. Since the momentum has been transferred to the rolls if you jump on the brakes at speed the car shoots off the rolls backwards.

[Roger King]

Tony,

 

You ain't kidding about front wheel drive cars. I've tested the odd one myself and can confirm that without lashing the things down to restrict side to side movement the job is practically impossible. Even on fairly low powered vehicles.