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Re: [Fot] Rear End Ratios

To: Tony Drews <tony@tonydrews.com>
Subject: Re: [Fot] Rear End Ratios
From: TR4 Tony <tr4.tony@virgin.net>
Date: Thu, 4 Jul 2013 10:19:12 +0100
Cc: "FOT@autox.team.net" <FOT@autox.team.net>
Delivered-to: mharc@autox.team.net
Delivered-to: fot@autox.team.net
References: <e5e9d.3ef1aff0.3f042fac@aol.com> <51D33184.5020009@pobox.com> <0F3E9407-C8D0-4F1A-BDE2-866DCFCBBD2D@virgin.net> <51D4A9EB.2080202@pobox.com> <51d4fd62.674e3c0a.1b1a.ffffca41SMTPIN_ADDED_MISSING@mx.google.com>
Hi both

This issue relates to the bearing in the epicyclic gear (around which the
three planet gears spin) - which therefore runs at three times engine speed.

The mainshaft bearings are not such a problem, although using a high first and
hefty revs shortens their life too as the shaft bends in the casing, which is
weak. You'll see a series of failures recently with those using dog gear sets
rather than the gentler engaging bevel sets as the shafts push apart bending
and cracking the main segment of the casing - this is even happening with the
late TR 6 cases which are the strongest.

In the case of the overdrive bearings It may not be happening because you're
not running for long periods of time - remember that I'm running for 4 to 6
hours at a stretch with out a break.

Peter Eggington at Overdrive Repair Services can supply these S78 bearings at
modest cost, so it's not a big money fix. If you talk to him on the phone he
remembers this failure particularly as I have him the mess to post mortem as I
didn't know where to start.

Regards

Tony

Sent from my iPhone

On 4 Jul 2013, at 05:37, Tony Drews <tony@tonydrews.com> wrote:

> 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
> <snip>
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