Overdrive losses

[SMOKEY44211]

I'm curious as to why vehicle manufacturers have universally adopted overdrive transmission gearing for high gear driving. On all the chassis dyno tests that I've conducted I've always observed about 3% better power #'s at the drive wheels with 1:1 transmission gear selection.-------Phil

 

[Metalguy]

They probably haven't figured that out yet. One look at their trans. ratios indicates they don't have a clue about gearing.

 

[patprimmer]

I think it is to keep the final drive ratio reasonable.

Also, overdrive has a nice ring to it from a marketing point of view. It implies fuel economy to the uninformed for some reason.

I suspect that sometimes top gear ratios are chosen for quiet running rather than fuel efficiency and first gear ratios are chosen so as not to exceed torque limits on axles and half shaft CV joints rather than good very low speed tractability.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[GregLocock]

I'm not sure I really agree, Pat. We have no problems offering diff ratios from 2.8 up to 4.1 (from memory), and I can think of some that are as low as 2 or as high as 5.

Before we went to 2 piece driveshafts (damn foolishness IMO) it would have been very nice to run a non overdrive top gear, so we could have run up to the true max speed rather than speed limiting.

I'm thinking it could be one of two things.

1) first gear gets a bit unwieldy - it has to fit into the same space as top gear, in a manual box.

2) Inertia. (Corporate inertia that is)

I'm sort of betting on (2)



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[MikeHalloran]

My Z28 has a 6th gear of 0.5:1, and an axle around 3.50, I think. To get an equivalent direct top gear, you'd need an axle ratio below 2, and the driveshaft would be running at twice the torque level as now. I don't think there's that big a margin in the u-joints.



Mike Halloran
Pembroke Pines, FL, USA

 

[GregLocock]

Torque capacity of the U joints is a consideration, for sure, but you've still got a much worse problem in the half shaft design, both the halfshaft itself, and its cv joints.

I guess the gearbox is torque limited, so that would have to get bigger.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[MikeHalloran]

No worries; Z28s don't have half shafts or CV joints.



Mike Halloran
Pembroke Pines, FL, USA

 

[GregLocock]

Ha. Great catch. I always liked the Camaro's driveline. What does it have at the back of the gearbox? Looks like a hookes joint in the only photo I could easily find. Interesting. A CV would be my choice.







Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[MikeHalloran]

Just a regular driveshaft, two Hooke joints. Some are aluminum.

I think the real trick part is the long torque arm bolted to the diff and pivoted near the front u-joint. The axle can't wind up without lifting the whole car, and the motor isn't quite _that_ strong. It _does_ lift the car enough to notice on a hard launch.








Mike Halloran
Pembroke Pines, FL, USA

 

[GregLocock]

Oh OK, I thought it was like that old Peugeot that had a torque arm from the front of the diff to the gearbox, and eliminated the rear UJ.

Well in that case 2 Hookes joints is fine.



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[Fabrico]

I have wondered the same thing for a very long time. It is very unlikely it is connected to noise, torque, or smoothness. It's as if they just never want to change from the original transmission gear ratios.

 

[MikeHalloran]

One upside to high production numbers, on the order of 10,000 units a day, is that you can afford very specialized 'hard ' tooling to keep the unit cost down.

One downside of hard tooling is that a major change, e.g. increasing the torque capacity of an axle pinion by a factor of 2, costs a lot of money.

If a major change doesn't bring the unit cost down by a whole bunch, or open up a new market, implementing it would be irresponsible. A 3 pct power increase on the dyno isn't going to produce 3 pct fuel economy improvement, and even if it did, the saved money wouldn't go into the manufacturer's pocket. So where is the incentive to do it?



Mike Halloran
Pembroke Pines, FL, USA