How does a final drive affect dyno figures?

[Kellerman]

I'm assuming a wheel dyno, which measures wheel torque and power, is affected by changing a gearbox final drive?

In that the estimated flywheel torque and horsepower will be wrong (overestimated)?

[geeks]

I have asked a friend who is wise in such things but I am pretty sure the answer is no

[Rowe]

Yes it's affected, much like it would be by using 3rd instead of 4th (for example)

[E-bmw]

IIRC they generally use 3rd gear on most dynos as it is frequently the closest to 1:1 ratio, but I may have heard that wrong.

[Mr MXT]

E-bmw said:

IIRC they generally use 3rd gear on most dynos as it is frequently the closest to 1:1 ratio, but I may have heard that wrong.

4th

[E-bmw]

Mr MXT said:

E-bmw said:

IIRC they generally use 3rd gear on most dynos as it is frequently the closest to 1:1 ratio, but I may have heard that wrong.

4th

Knew it was something like that, for that reason.

[mmm-five]

5th gear is 1:1 in mine

So on a dyno at max RPM of 8000 would mean the car would be showing 170mph

[GreenV8S]

The gearing will affect the amount of energy lost at the tyre, and the amount of kinetic energy absorbed by the wheel.

If you're looking for really precise figures, it will also affect which part of the RR calibration map you're in and the tolerances allowed by the various standards.

[stevieturbo]

You would not be changing the diff during a dyno session...or at least it would be strange to do so. So doesn't really matter.

But overall drive ratio can affect readings....but as it's a constant for the vehicle and session....again, would not really matter

[Super Sonic]

Lowering the final drive ratio increases wheel torque. Unimog uses a final drive reduction at each wheel so wheel torque is much higher than gearbox torque.

[anonymous-user]

Kellerman said:

I'm assuming a wheel dyno, which measures wheel torque and power, is affected by changing a gearbox final drive?

In that the estimated flywheel torque and horsepower will be wrong (overestimated)?

Power is unchanged, because power = Speed * Torque and fitting a lower final drive drops the output speed by exactly the same amount as it increases the output torque.

All chassis dyno runs start with a calibration run, where the cars engine speed is recorded at a known wheel speed (or vise-versa), which effectively encodes the gearing of the transmission.

[DVandrews]

If there were no losses through the tyres then that would hold true, roller speed (and therefore overall gearing) will significantly change the losses experienced at the wheels, the main losses are through unnatural compressions of the tyre on a relatively small radius roller , the at the wheels figure will be higher if roller speed is lower, tyre pressures have a significant impact too. If you want a high power figure run your tyres at 80PSI and run the test in first gear, you wont see very good traction though..

Dave

[anonymous-user]

DVandrews said:

If there were no losses through the tyres then that would hold true, roller speed (and therefore overall gearing) will significantly change the losses experienced at the wheels, the main losses are through unnatural compressions of the tyre on a relatively small radius roller , the at the wheels figure will be higher if roller speed is lower, tyre pressures have a significant impact too. If you want a high power figure run your tyres at 80PSI and run the test in first gear, you wont see very good traction though..

Dave

It's much more complicated than that when you consider tyre hysteretics!


Under load, the tyre deforms and the hysteris of the rubber causes it to absorb power.

As torque increases, tyre deformation increases, especially on dual rollers when the tyre tries to climb the front roller)

As wheelspeed increases tyre deformation can DECREASE (because the CF helps the tyre support the normal mass of the vehicle)

In all cases, basic effects like tyre temperature or transmission oil temperature will have a far bigger effect.

Chassis dynos are always a blunt instrument unless you do a large amount of careful control of all the boundary conditions and spend a lot of time characterising your vehicle agaisnt known good figures. This is why chassis dynos are only really useful for A_to B type mods, where a change in wheel power should be more consistent.....

[DVandrews]

So that confirms that overall gearing and therefore roller speed affects measured power at the wheels.

A coastdown test, while not 100% accurate makes a pretty good fist of measuring losses across the range of roller speed and to a degree copes with the change in losses due to roller speed. It’s a better way of getting to grips with losses than just estimating a percentage.

Dave

[gazza285]

E-bmw said:

Mr MXT said:

E-bmw said:

IIRC they generally use 3rd gear on most dynos as it is frequently the closest to 1:1 ratio, but I may have heard that wrong.

4th

Knew it was something like that, for that reason.

On RWD vehicles it traditionally was straight through, meaning the input shaft connects directly to the output shaft, so no power is lost in the gearbox. That is all, it is to maximise the power figure.

[GreenV8S]

gazza285 said:

On RWD vehicles it traditionally was straight through, meaning the input shaft connects directly to the output shaft, so no power is lost in the gearbox.

Have you got an example of that? On every gearbox I've seen, all gears are driven in the same way ie via a fixed gear pair to the layshaft and then through a constant mesh gear pair to the output shaft.

[bigothunter]

GreenV8S said:

gazza285 said:

On RWD vehicles it traditionally was straight through, meaning the input shaft connects directly to the output shaft, so no power is lost in the gearbox.

Have you got an example of that? On every gearbox I've seen, all gears are driven in the same way ie via a fixed gear pair to the layshaft and then through a constant mesh gear pair to the output shaft.

Ford Type 9, Ford MT, BMW RWD manual boxes...

I am unaware of any conventional RWD gearbox which doesn't lock the mainshaft to provide one gear. Do you have any examples please?

[stevieturbo]

GreenV8S said:

Have you got an example of that? On every gearbox I've seen, all gears are driven in the same way ie via a fixed gear pair to the layshaft and then through a constant mesh gear pair to the output shaft.

Almost every inline rwd transmission has 4th, or one gear which simply locks the mainshaft, No gears are used for the transmission of power.

But that does not mean there is zero drag or losses

It would actually be almost pointless for such a box not to have a 1:1 drive in this manner ( although some race sequentials may not ? But perhaps for other reasons )

[bigothunter]

stevieturbo said:

Almost every inline rwd transmission has 4th, or one gear which simply locks the mainshaft, No gears are used for the transmission of power.

But that does not mean there is zero drag or losses

It would actually be almost pointless for such a box not to have a 1:1 drive in this manner ( although some race sequentials may not ? But perhaps for other reasons )

Although parasitic losses are lower with the locked mainshaft compared to those ratios using the laygear.

Quaife boxes tend to be based on production designs (eg Type 9) so engage one direct gear (ie locked mainshaft) whether sequential or H-pattern.

[Panamax]

Note that in a torque converter automatic it's not just the gear ratios that count. The torque converter can actually increase the torque.
https://en.wikipedia.org/wiki/Torque_converter#:~:...
One of the many reasons that auto's can be very quick.