4/4 rear axle questions (racing)

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Duncan Charlton
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4/4 rear axle questions (racing)

Post by Duncan Charlton »

I'm hoping the knowledge to answer the following three questions will be found on this forum:

A) Will I miss having a limited slip diff when racing a Series V 4/4? I will not be racing on slicks. Final drive ratio is 4.56:1 and engine power will be around 120-135 hp. Weight would be around 1450 lbs.

B) Is there any history of breaking rear axle half shafts on a Morgan? I am aware that TR3s and TR4s have suffered breakage with a very similar Salisbury axle (and this 4/4 currently has a TR3 axle in it). I know that the TRs outweigh the 4/4 by 550-600 lbs and am wondering if that's why I've never heard of a similar failure in a 4/4.

C) Does anyone know whether Salisbury axles in TR3s and Morgans have the same taper at the outer ends?

A bit of background: My Series V 4/4 has a Detroit Locker (a.k.a. "Detroit Unlocker") limited slip, which is reportedly great for drag racing but seems not to be the ideal setup for road racing although there are those that swear by them. I'm aware of other drawbacks to my current setup as well -- front and rear wheel stud patterns are different, and it is not unheard of for these axles to break under racing conditions (when mounted in a Triumph, at any rate), causing loss of a rear wheel.

I have before me several options -- 1) find a stock 4/4 rear axle with a 4.56:1 final drive and either a) use it without limited slip or b) purchase a Salisbury or Quaife limited slip. 2) Use the Triumph TR3 axle currently in the car but adapt Morgan 4-lug hubs to gain interchangeability of wheels fore and aft and either a) remove the limited slip apparatus entirely or b) replace the lsd with a Quaife lsd, 3) use a 7Ha Salisbury from a Plus 8, with Salisbury lsd but needing gear replacement to get the proper final drive ratio (this particular axle can be purchased with two 4-lug Morgan hubs that are not installed), or 4) use an MGA "banjo" type axle with a Quaife lsd unit.

Choice #1a sounds straightforward -- but I only have one (albeit highly experienced) racer's opinion that lsd is not necessary on a 4/4 race car. Option #1b would add at least $900 to the cost of the conversion. This does not address the question of potential axle breakage.

Choice #2 might be possible but #2b requires replacing the lsd with a different type and the question of likelihood of axle breakage remains.

Choice #3 likely would not be the least expensive way to go once I've purchased a new ring and pinion set and perhaps new axle half-shafts but if the axle tubes are of the proper length there would be few modifications needed. This option doesn't address the question of likelihood of axle breakage.

Choice #4 offers full-floating axles, removing the risk of losing a wheel when an axle snaps, and one of the available final drive ratios is probably close enough to 4.56:1. Quaife makes an lsd for this unit and the larger rear brakes (10" diameter, I understand) reportedly will not present a problem but altering the outer axle flange to fit the Morgan wheel stud pattern doesn't look likely (from photos it appears that the flange is not large enough), so adaptor plates would be required, introducing one more potential failure point. This option would also require redoing the spring mounting bracketry. Besides answering any concerns about losing a wheel, should an axle break, unsprung weight would be reduced since these axles weigh less than the Salisbury units.

Opinions?
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jack bellinger
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Re: 4/4 rear axle questions (racing)

Post by jack bellinger »

Well Duncan this would be my advice...
because of the lighter weight you are running the halfshafts are normally pretty good ( for road going)
But if you are racing you should ask yourself ..
how many rumble stones have the shaft suffered
how old are they
how many racing miles
how many wheelspinning starts

with that in mind I would personally fit new ones (having had a shaft break on me and putting me in hospital )its is just not worth the risk

Not sure if the tr taper is the same (cant answer that)

I would also fit a Quiafe diff (so much more progressive on and off the power)
you could run a std diff but the quiafe would be an advantage (my opinion)

I really cant see the point in running a Tr axle with different size PCD on wheels studs
and the reducing in the overall value of the car (I know you are already running one ) my opinion

If it were my car
I would try to get a Salisbury Morgan 4/4 axle with 4.56
fit a quaife diff
a pair of new shafts
lap the hubs on to the new shafts
weld the axle tubes all around the diff case
and race with confidence

I know that will be a lot more expensive but will increase the value of you car by more than you spend on sorting the axle

A Plus 8 Salisbury is to wide

your biggest problem will be to get an early 4/4 7HA axle or try to get a later 4/4 7HA with the same track

should be 44 inches wide from hub flange to hub flange

hope this helps

jack
Duncan Charlton
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Re: 4/4 rear axle questions (racing)

Post by Duncan Charlton »

Hi Jack,
Many thanks for the words of advice. This car was raced very successfully from 1967 to 1989 but I have no idea when the current TR axle was fitted, so, your point is well taken regarding unknown abuse it may have suffered. Judging by logbook entries, few if any standing starts were done in the last 10-15 years of the car's use but the car was raced hard with a 1340cc engine for most of its life.

I understand the Quaife will run cooler due to the lack of clutches. Hopefully I could avoid raising the inside tire too far, as I understand all power goes to a spinning wheel with a Quaife -- but that's no different from the current Detroit Locker, which always sends all power to the inside (slower-turning) wheel when unlocked. The car's past race success hints that the suspension is set up properly to avoid this and that the former owner adapted his driving style to it.

The TR axle was in the car when it gained its greatest fame in 1981 by taking first place at the SCCA (National) Runoffs, so I suppose that removing it might actually lower the value of the car!

Other than the axle width and the fact that it's not your #1 choice, is there a show-stopping reason not to use a Plus 8 axle? I am being offered one (48-13/16" wide, so I would want to narrow it) with a Salisbury LSD. I haven't estimated its full cost yet -- it needs new axle half-shafts and a different final drive ratio (it would come with Morgan 4-bolt hubs, since it's someone else's stalled project).

Yours,
Duncan
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jack bellinger
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Re: 4/4 rear axle questions (racing)

Post by jack bellinger »

yes you can narrow the axle to 44 inches

providing its done correctly

the Salisbury lsd will need to be stripped and checked
and the height of the crownwheel mounting face checked (may need a spacer made to move crownwheel across to mesh correctly)with longer bolts) depending on which lsd
8 bolt Crownwheel.
depending on what pinion you use the prop flange on the +8 axle is larger than 4/4 (larger prop shaft)and they can run different spline sizes (check that also)different sizes are available


Keep the tr axle in the garage because of the history

sounds like a plan to me
Duncan Charlton
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Re: 4/4 rear axle questions (racing)

Post by Duncan Charlton »

Thanks, Jack. I'm getting plenty of other opinions as well but the ideas are all over the place. Use a Ford 8" axle with new shafts and a Quaife unit, one says. I've heard mention of a Toyota 8" unit too (well built, LSD available, wide choice of ratios, etc.), but I'd put that quite a ways down the wish list.

Much to think about.

Duncan
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