Re: Gear Ratios 101 [was <MISC> Overdrive]

[SMILLER@xxxxxxxxxxxxxx]

Again I apologoze if this has already been covered, I'm now 5 days
down on Digest reading and sinking fast.

>Date: Fri, 16 Oct 1998 12:48:49 -0700
>From: "Boyd, David (Neighbor Boy)" <boydd@domain.elided>
>Subject: <MISC> Overdrive

>This brings up a question that I've always had.  What constitutes an
>overdrive gear.  I always assumed it meant a ratio of less that 1:1.
>Kinda like .80:1 with my 5th gear.  Is that all it is, or are the
>gears
>actually physically different?  Hope it's not too off topic.

>- --David Boyd, boydd@domain.elided
>'84 325e 200k
>BMW CCA
>BMW ACA

More important questions are:

1.  What is the overall gearing, and
2.  Why do some cars have OD and some not?

You can get the total gearing by multiplying the transmission gear ratio by
the diff gear ratio.  Just making up some numbers, say one car has 1:1
5th gear and 3.23:1 final drive ratio.  Say another car has 0.79:1 5th gear
and 4.10:1 final drive ratio.

Car A:  1 x 3.23 = 3.23
Car 2:   .79 x 4.10 = 3.24

The 2 cars have almost the same overall gearing (you couldn't tell the
difference).

So why did BMW switch to 1:1 5th gears and taller final drive ratios (E36
comes to mind, probably others, too)?  Because there is less gear wine
with a 1:1 ratio.  You can verify this if you have an OD transmission. 
Drive in 4th gear for a while (all BMW OD transmissions that I know of
use 1:1 4th gear), then shift to 5th gear (OD) and listen to how much
more gear noise there is in 5th.  So, assuming that you cruise around in
5th gear when at highway speeds, in the E36 you're cruising more
quietly than in, say, the E30.

Another question:  Why wasn't this done sooner?  Well, most early 5
speed transmissions were basically just existing 4 speeds with an OD
gear added.  Overall cost was kept low by using existing tooling for
some of the transmission's parts.  However, the early close ratio
transmissions used a 1:1 5th gear, don't ask me why those were
different.

OK, class is over for today, and NO TESTS!  Go enjoy driving your
Bimmers in whatever gear you choose.

Scott Miller
GGC BMW CCA #44977

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