[Fot] Rear End Ratios

[Tony Drews]

I'm interested in which bearing(s) this refers to.  I've been beating 
the heck out of an A-type for 10 years now.  I did manage to frag the 
one way clutch arrangement by loading the driveline with rear lockup 
and a detroit locker (which is quite a jarring thing to experience - 
I thought someone hit me).  Didn't know I had a problem with the OD 
until I changed the oil and significantly sized chunks came out with 
the oil.  I do use 2nd OD semi-frequently although lately I've been 
more prone to lug it in 3rd and be able to skip the time-wasting 
shift from 2nd OD to 3rd.  It's normally on just one corner per lap 
that 2nd OD is the "perfect" gear (turn 5 at Road America, 3A at 
Blackhawk, the hard right entry to the esses at VIR).  But, I don't 
run for more than 30 minutes at a stretch and normally only about 20 
minutes of that is under full race conditions so I may be allowing 
enough heat to dissipate before bad things start to occur.  I know 
the driveshaft tunnel is uncomfortably hot after a 30 min session on 
a hot day, so I would imagine that the Tranny and OD are pretty 
toasty by then.  I also generally shift at 6000 even though I have 
the good crank / rods / etc. so that may help.

Is there some sort of beefed up bearing arrangement that can be implemented?

I'll need to look at the exploded diagram again to better understand 
this one.  Maybe it's the bearings in the planetary gears?  Those 
seem small and would be spinning pretty quickly...

Tony Drews

At 05:47 PM 7/3/2013, Larry Young wrote:
>I did not follow your logic concerning the overdrive bearings.  Are 
>you referring the the two rear bearings that support the annulus and 
>turn at drive shaft speed?  They will be turning slower when 
>overdrive is engaged.
>
>We used our A type for most of the 6 years we raced the car. Ours 
>also had the large accumulator shimmed to about 525 psi.  We found 
>it to be very stout and relatively trouble free despite the abuse. 
>Our only problem with it was a broken circlip costing about $1.  On 
>the rare occasions we used 1st overdrive, it was with a close ratio 
>gear set where 1st is nearly as high as 2nd in a normal gear set.  I 
>have no explanation for why our experiences differ.  Perhaps others 
>will comment.  Tony Drews?
>
>
>On 7/3/2013 4:10 AM, TR4 Tony wrote:
>>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.
>>W
>>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
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