Query about transmission losses and top speed

[Mr_Sukebe]

I'm hoping that someone can explain something to me.

Having seen a few rolling road print outs over the years, every one of them shows transmission losses for cars. From what I remember is often something like 18% for FWD, 25% for RWD and upto 40% for 4WD.

So there I am earlier today reading about the new RWD only Gallardo. According the mags, it's slightly slower than the 4WD version. Now going back to the transmission losses, I'd expect the RWD only version to have something like a good 10%+ less transmission losses. Shouldn't that mean it's got more power going to the rear wheels, and thus should be faster.

What's going on then?

 

[nicRob]

IIRC the rwd version is slightly less powerful then the 4wd one (only 10-20bhp or so, though)

 

[Peter Diffey]

Slower off the line?

 

[NeverWinter]

Traction?

 

[Mr_Sukebe]

IIRC the rwd version is slightly less powerful then the 4wd one (only 10-20bhp or so, though)

The rwd version is only around 10hp less powerful. If transmission losses are normally say 10% higher for a 4wd car, then surely the car has effectively around 40hp more going to the wheels. Wouldn't that increase the cars top speed?

 

[Tomsk]

Probably due to different gearbox ratios.

 

[Matt82]

Probably due to different gearbox ratios.

this is where my bet goes

 

[danza]

Errrrm....weight!

No, that doesn't work.

Errrrrmmm...maybe the 4WD simply has more ultraness?

 

[BigglesPiP]

Transmission losses on a rolling road includes tyres vs the rollers, which includes how hard it is strapped own.

 

[olv]

Transmission losses on a rolling road includes tyres vs the rollers, which includes how hard it is strapped own.

lolwut

 

[BigglesPiP]

lolwut

lolwut what?

The machine doesn't know anything about how hard you've connected it to the rollers. Like the car carrying a heavy load, this causes more rolling resistance in the tyres. Resistance which is still there when rolling down. You cannot compare transmission loss on the rollers to transmission losses on the road.

 

[Westyfield2]

The machine doesn't know anything about how hard you've connected it to the rollers. Like the car carrying a heavy load, this causes more rolling resistance in the tyres. Resistance which is still there when rolling down. You cannot compare transmission loss on the rollers to transmission losses on the road.

This.

[Rant]

Can we please stop using rolling road figures, and using transmission corrected rolling road figures used as "dyno". It's not. A rolling road will tell you how much power it recorded at the wheels. And the coastdown will do a bit of maths to guess what it was at the fly. But tyre pressures and strapping will affect it etc.

I've seen a car get 180bhp on a rolling road as the "transmission corrected at the fly" figure. That was even with sticking people over the rear axle to try and make it stay on. I'd seen that same engine a few weeks previously get 215bhp on a state-of-the-art calibrated dyno cell (with the same exhuast manifold and system).
[/Rant]

RR figures don't bother me anyways, I've seen the calibration cert and the printout for the Westfield's latest engine dyno runs.


@ OP: In theory, the same engine/box/exhaust etc will give a higher at the wheels figure on a 2wd drive-train than a 4wd, as they'll be less transmission losses. But whether that gain is enough to counter the reduction in traction...

 

[olv]

lolwut what?

The machine doesn't know anything about how hard you've connected it to the rollers. Like the car carrying a heavy load, this causes more rolling resistance in the tyres. Resistance which is still there when rolling down. You cannot compare transmission loss on the rollers to transmission losses on the road.

On the road you increase the pressure inside the tyre on heavier cars so that the surface area of the contact patch on the road is roughly the same. Heavier cars aren't slower to accelerate because of their increased rolling resistance. Up to a point it will help with traction.

With your logic, a 1.8 k-series engine would make more power on a rolling road in a Lotus Elise then it would in a Rover 200

 

[BigglesPiP]

On the road you increase the pressure inside the tyre on heavier cars so that the surface area of the contact patch on the road is roughly the same. Heavier cars aren't slower to accelerate because of their increased rolling resistance. Up to a point it will help with traction.

With your logic, a 1.8 k-series engine would make more power on a rolling road in a Lotus Elise then it would in a Rover 200

What are you talking about?

I'm, saying the "transmission loss" measured on a rolling road is not the same as the transmission loss of the same car on the road.

I'm not talking about how much power would be measured, because this wouldn't change within the standard deviation of the calculated flywheel power. As this is what the run down is actually for.

I'm not talking about how fast anything accelerates.

I'm not talking about traction.

 

[olv]

lol, as you were.

*whistles*

 

[Biohazard]

lol, as you were.

*whistles*

haha

 

[The_Dark_Side]

The amount of power lost by the transmission isn't linear. ie you can't expect a 1000bhp 4wd car to lose the same percentage of it's power through it's transmission as a 300 bhp 4wd car.