The original question was what constitutes an overdrive. Simply put, an
overdrive is a gearing mechanism which allows a lower than 1:1 ratio on the
driveshaft.In fact, the overall intent is to provide a speed to engine RPM
ratio which achieves a more comfortable cruising speed for any given engine
RPM. Anything that can achieve this is desirable.
However, one CANNOT achieve this ratio and desired effect simply by changing
the differential gearing, since to do so affects all gears, not just top
gear.
The real problems in driving a TD or TF at highway speed are twofold. First,
the ratios between 2nd and 3rd gear are pretty useless. In second gear the
engine is racing with no more than about 40 MPH possible. Yet third gear is
way above that, so you have a gap where the RPMs aren't in the power range
where they are needed. The original gearing is something like 1st- 3.5, 2nd
- 2.07, 3rd - 1.385, 4th - 1:1. A more ideal set of numbers would be
something like 2.86, 1.84, 1.33, and 1.0. Note the closer spacing between
2nd and 3rd.
The second problem is that with a 1:1 ration in top gear, the engine is
running at much too high an RPM for comfortable cruising at 60 - 65 MPH.
This is where the application of different rear end gearing comes in.
A fifth gear or overdrive simply drops the ratio in top gear to something
like .87.
Just my tuppence worth.
Lew Palmer
-----Original Message-----
From: Simon Matthews [mailto:simon_matthews@avanticorp.com]
Sent: Thursday, January 07, 1999 10:19 AM
To: mgbob@juno.com
Cc: GMGoodman@aol.com; mg-t@Autox.Team.Net
Subject: Re: 5 speed question
I think your definition of what is an overdrive gear is wrong -- I don't
believe it is simply a function of the ratio in the gearbox but rather I
had always understood that an overdrive gear was one in which the overall
gearing (engine to wheels) gave a higher gearing than the gearing required
for top speed. In other words, the gearing is designed for comfortable
cruising, not absolute top speed.
Thus one can create an overdrive gear by changing the rear axle ratio. This
definition allows a 1:1 top gear to be an overdrive gear.
There is then the separate question of how many gears are required for
convenient driving.
Simon
At 10:32 AM 1/7/99 EST, Bob Howard wrote:
>Jerry,
> For years and years, it was the practice to have top gear at 1 to 1. An
>overdrive unit was then bolted on in order to achieve the OD ratios, .87
>to 1 or whatever.
> Making the top gear with an OD ratio is a simpler and less expensive
>way to achieve the desired ratio.
>Bob
>
>On Wed, 6 Jan 1999 20:24:34 EST GMGoodman@aol.com writes:
>>I have seen the use of 5 speed transmissions with the Midget--and some
>>discussion of 5 speeds in Bs---but today I had a thought which crossed
>>my
>>mind. Help me out -----I have always been under the impression that
>>no matter
>>how many gears you have, the final gear is 1 to 1. That is for every
>>turn on
>>the input side, you get one turn at the rear. Sooooo how is it that
>>a 5
>>speed has the effect of an overdrive -----or does it? That is, lower
>>rpms at
>>high speed in the final gear. Example hihway driving. I'm
>>puzzled----------guessI don't understand what I thought I did.
>>
>>Your thoughts please Jerry G '72 B
>>
>
>
>
---
Simon Matthews MailTo:simon_matthews@avanticorp.com
'57 MGA
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