Overdrive losses
Overdrive losses
(OP)
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





RE: Overdrive losses
RE: Overdrive losses
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.
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RE: Overdrive losses
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.
RE: Overdrive losses
Mike Halloran
Pembroke Pines, FL, USA
RE: Overdrive losses
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.
RE: Overdrive losses
Mike Halloran
Pembroke Pines, FL, USA
RE: Overdrive losses
Cheers
Greg Locock
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.
RE: Overdrive losses
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
RE: Overdrive losses
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.
RE: Overdrive losses
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.
RE: Overdrive losses
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