having an arguement with my mate, im saying trans loses on a fwd car around 20-25%, hes saying alot less, im also saying 4wd losses are around the 40% figure?

is that about right or what?

 

I have personally never believed that transmission loss can be calculated as a percentage.

My RST has always shown around 25-30bhp in transmission losses regardless of progression in power. OK, so it was measured using coast-down, but surely that's fairly accurate.

Also, it seems odd that most 4wd cars lose 80-100bhp, I have even seen 500bhp 4wd cars quote those kinds of losses, but suddenly 600-800bhp 4wd cars magically lose 250-300bhp somewhere. The amount of 600-800bhp cars that only quote 500-600 ATW is untrue.

 

My xr2 made 64 atw recently and 99.8 at the fly. Dunno what percentage that is. But I dont believe for a second its actually making 99.8 think the RR was a tad optimistic

 

thatll be a 35% transmission loss....

seems a bit high but then again i guess it all depends on youre gearbox, mounts and suspension etc etc.

 

Ive never really undetstood why 4x4 cars can loose 100bhp at least through the transmission, if a 2wd car looses 25 - 30bhp wont it just be doubled for a 4wd car

 

It all depends how good stuff like your wheel bearings etc are as well. WRC cars use ceramic bearings or something that are supposed to free up something like 5bhp per wheel.

 

What part do these compnents play in getting a power at the wheels figure

 

i thought it was roughly
11% fwd
24% rwd and
45% 4wd
all rough but you get the idea

 

Transmission losses MUST be relative to the amount of power transmitted.
Coastdown test is all very well for "fixed" losses, but do not take into account "dynamic" losses.

I'll try to give an example...

Take one clutch.
Torque rating is calculated around: Co-efficient of friction; contact area (both sides), CLAMPING PRESSURE.

Your transmission will have a certain co-efficient of friction per gear.
What can vary however is torque. Equate that to clamping pressure.

Take one car. with a certain engine/transmission/wheel bearings/ etc.
Increase the torque level of the engine, and the losses will go up.

Think of it like this: lightly put your hands together. Now start rubbing them backwards and forwards.
Makes them warm yes? Now push your hands together harder. Rub them backwards and forwards at the same speed as before.
Fuck me that's hot!

Now, the co-efficient of friction didn't change, you used the same pair of hands as before. What changed was apply pressure.

This is the same as transmission losses, as you increase torque.

We can reduce these losses to some degree by altering your co-efficients of friction through your transmision components, but, on a given transmission train; the more torque, the more the transmission losses.

I've also got a feeling that a graph of this would not have a straight line...

 

gearboxman


Good read that.

I've been meaning to pm you about a 205 gti gearbox actually as I was wondering if its something you would rebuild and with what sort of costs etc. If you've got time to send me a pm with a few prices that would be brilliant. Don't worry if you dont have time though as I will phone you to find out for definite when i've found a 205 to buy.

Cheers

Stu.

 

Good info Bernie, ive always viewed coastdown as utter bollox.

Im of the opinion that gearbox losses are more along the lines of:

X + (Y * bhp)

As opposed to just a percentage, but i think even then its probably a FAR more complicated formula than that and no doubt varies with oil temp etc.


But as a percentage only type quote, The ballpark figures ive always found to be about correct are:
FWD = 15-20%
RWD = 20-25%
4WD = 30-35%