Stupid Question?

[bvanetten]

Why do transmission's (stick or auto) have overdrive's. Why doesn't the big three gear the diffs to run the rpm/speed you want in the last transmission gear and space the rest of them accordingly. Am I missing something?



Bruce

 

[Peter Campbell]

I think it has to do with the peak driveline torque. The ring and pinion are rated for a certain amount. So when you increase the numerical ratio ( to a point) the ringear can be stronger. The weight of the vehicle being accelerated may not let the engine reach full power in a low gear ( cars). Pulling a trailer you could be at full power in a low gear like second or third that boost has built up fully. A cheap and lighter way to get more capacity is to increase drivline speed and decrease torque in the highest gear. Hence overdrive. An extreme example of this is the HUMMER ( real one ) they use a super highway "pumpkins" (2. 05 or something??) and wheel transmissions ( another 1. 7??) to both lighten the driveshaft- lower torque/ higher speed and get it out of harms way by rocks and such. Of couse this fact is balanced by the most efficient transmission speed being direct. So it is likely optimized for that. If you want to travel a little faster shift to OD. You get more speed and slower engine, but more drag in the trans. Those who play with superbikes know it is hard to break first gear- the tire is the weak link- engine can not reach full power. Second and third break more often. At least in the old days before they made them more bulletproof.

So they do this to avoid the axle and transmission and frame from getting too heavy, and overdrive helps on the other end.

 

[SRadke]

If I had my way I would have a six speed transmission with 3:1 differentials. Five gears to get you there and most of my time spent in direct. Sixth gear is just there if you need to run. Instead I buy taller tires to try to accomplish the same thing.



Peter does have a good point about reducing the torque on driveline parts though.

-Scott