[Fot] Rear End Ratios

[TR4 Tony]

Hi again fellas

You need to be aware that using overdrive in 1st gear with an a type unit will
give it a very short life.

Apart from the fact that you will cane the hell out of the bearings as you
load up on exit, the less obvious issue is the rear bearing set in the a type
unit, which is rated to about 9000 rpm.

This bearing set runs at 3 times engine speed. So do the maths, but in a 6000
rpm max engine, it's double the designed load rate on those bearings. With a
steel crank and 8500 rpm plus it's nearly 3 times. I regularly run upwards of
6000 rpm.

If you run at full revs in any gear for any pronged time this bearing seizes
and the overdrive cooks, and using it on first gets it to that failure point
very quickly. its worth remembering that the inside of the overdrive does not
recover coolness very easily as its snuck up inside the car, so the heat issue
is cumulative - ie once it gets hot it stays hot and the heat builds. I've had
my gearbox so hot after a half hour rest on a rally that it was a gloves on
job to check the oil level and the forward UJ on the prop.

The answers are firstly a higher rated bearing set to deal with those extra
revs (and by that I mean larger individual bearings with bigger gaps between
them for the oil to flow through and get that cooling done), secondly and / or
a gearbox oil cooler.

I was getting about 200 miles running out of an a type because of this revs
issue, firstly manifesting itself in the speedo drive melting like wax on a
stick (so both my speed and tripmeter failed - which is what alerted me to the
problem)  and secondly by the cage round the bearing melting and causing the
unit to seize (oh and the smell and racket wasn't great either) - the first
time the gearbox and overdrive unit were scrap, the second I was a little more
clued up and got it back to base intact, albeit utterly knackered.

So, if you do plan to use OD on first in an a type certainly, do make sure the
unit is suitably uprated otherwise you'll be taking it out again shortly
afterwards.

Regards

Tony

Sent from my iPhone

On 2 Jul 2013, at 21:01, Larry Young <cartravel at pobox.com> wrote:

> That was with a close ratio gear set that has a high 1st gear and
> Hallett had one turn that is called "Stop and turn left".  Yeah, I did
> my first autocross in years in Waco last month with the street TR250.
> It's like Goldilocks and the Three Bears.  1st was too low, 2nd was too
> high, but 1st overdrive would have been just right. Maybe I will rewire
> the TR250 the same as the John's TR3.  There is a very simple way to
> wire them using only one switch on reverse.
>
> On 7/2/2013 8:29 AM, BillDentin at aol.com wrote:
>> In a message dated 07/01/2013 7:28:09 PM Central Daylight Time,
>> cartravel at pobox.com writes:
>>
>>
>>> At
>>> Hallett there is one very tight turn and with a close ratio gearbox,
>>> first is too low and 2nd is too high but 1st overdrive is just right.
>>
>>
>> "Overdrive in 1st!"  What a concept.  Mike Belfer would be rolling
>> over in his grave to hear that.  He used to lecture me about not using
>> it in 2nd.  But it sure worked slick in traffic coming out of Turn 5
>> at Road America.
>>
>> My overdrive unit has been on the shelf for years now.  No longer
>> searching for faster laps.  Just trying to enjoy still being out there.
>>
>> Bill Dentinger
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