Fully locked diffs in RWD front engined live axled cars.

[Al Weyman]

I was surprised to discover that when the IROC NASCAR race cars were used on a road course they used a fully locked diff not a slipper but used a slipper on an oval course. I am having trouble getting power down to the ground on my reasonably powerful front engined RWD car and wondered if I should go the same route. I have spoken with a friend who raced a powerful SDI and he recon he locked the diff up so tight it may as well be fully locked so i wondered if anyone else has had any experience circuit racing (road racing in the US) with a fully locked diff and what were your impressions of the set up. I would imagine with a FWD car or it would be undriveable and with an IRS car unneccessary which is why I am asking about a front engined rear wheel drive car with solid live rear axle which by its very nature will tend to lift a rear wheel at high cornering speed and loose traction.

[EfiOz]

V8Supercar in Australia use a fully locked diff but not by choice. It was done as a cost saving measure.

[215maloo]

Wouldn't run anything else - its a tough to push it around the workshop, but its the only way to go for circuit racing. Running a full spool is not only cheap, but takes out a bunch of weight.

[R59]

You'll be asking for bags of understeer Al.

Brian Long ran one in his Sierra XR4i, and it made it very very pushy.

You'd have to compensate with loads of tail sliding!!

I run a Detroit Locker in the ASCAR, which is light, and cheap. They are fun on a banked speed way when you lift at high speed (the car pitches towards the wall!).

But I have a triple-trac in the Ford 9" that's under the Astra/Belmont. With 700bhp on tap, I wanted something more controllable, and that diff's a peach.

[GORDON STREETER]

Al. At least it would be cheap to try it if you have and old redundant diff laying around. But be prepared to possibly make major suspension changes to compensate for the inherent understeer. Locked diffs are all very well on a slippery and shale surface but are a bit brutal on a grippy track with big slicks etc. Also broken half shafts come to mind from my old Hot Rod days !

[davyboy]

Al, you could do worse than to take a leaf out of kart racing. We run without a diff [of course ] but have the front caster set up to cock the inside rear wheel so that we can get around tighter corners. It works perfectly. These days most karts are fitted with Ackerman steering systems which help to accentuate the inside rear lift while having less emphasis on tight steering radii.

The key question for you will be whether you can get that inside rear off the ground in tight turns... thereafter its a no brainer.

[Al Weyman]

I dont think that would be too much a problem as I have about 450 ftlb of torque to the wheels will light up any how especially on Trackday rubber. Its probably cocking up a bit anyhow courtesy of the GM style full length torque tube rear suspension in fact I think my old car with cart springs hooks up better.

[ian_w]

It does seem a bit odd to me that you are thinking of fitting a spool in order to gain traction. You are then talking about having to severly compromise the rear suspension to get the car to turn which must cause a major reduction in rear traction. Surely a limited slip diff is the correct answer - basically open on trailing throttle and closed on power?

To my knowledge, NASCAR 'Sprint Cup' cars can only run a Detroit locker diff, i.e. a type limited slip.

Indy cars run spools on ovals so they can utilise tyre stagger to aid cornering. On road courses they run LSDs ( plate type ? ).

[GORDON STREETER]

Also you can get away with a locked diff in a light car. I believe that some of the bike engined cars like Legends don't have a differential unit, but a heavy big HP Tank might be a different ball game ! .

[JamesH]

Global GT lights have a solid rear axle and chain drive like karts - they seem pretty nippy round a track. They are very light however.

James

[Al Weyman]

Originally Posted by ian_w

It does seem a bit odd to me that you are thinking of fitting a spool in order to gain traction. You are then talking about having to severly compromise the rear suspension to get the car to turn which must cause a major reduction in rear traction. Surely a limited slip diff is the correct answer - basically open on trailing throttle and closed on power?

To my knowledge, NASCAR 'Sprint Cup' cars can only run a Detroit locker diff, i.e. a type limited slip.

Indy cars run spools on ovals so they can utilise tyre stagger to aid cornering. On road courses they run LSDs ( plate type ? ).

Quite the opposite actually Ian, on road courses like Watlins Glen the NASCARS run a spool on ovals they run a slipper to compensate for tyre staggering.

If you see a video of my car at Mallory I thought the clutch was slipping coming out the hairpin but in fact it was smoking the inside tyre for about 70 odd yards. A mate of mine had the same thing in his big engined SDI and got round it by locking up the diff so tight in his words he may as well have locked it solid.

The cars are indeed heavy but the weight is biased towards the front and they also have an enormous amount of tyre traction breaking torque. I am not saying this would be good in a mid engined formula car which is why I was specific in the thread title. Maybe I should go back to the standard soft bushing in the lower arms to get more comliance.

[ian_w]

Al, I'm not disputing that you need a limited slip diff, I'm just saying that a spool is probably not what you want.

You are wrong about NASCAR 'Sprint Cup' cars here is an extract from the rule book:

E. Only magnetic steel Detroit locker-type differentials, acceptable to
NASCAR Officials, will be permitted. The locker-type differential, when
jacked up with the transmission engaged, must permit either wheel to turn
freely by hand for one (1) full turn – 360 degrees, while the opposite wheel
remains stationery. The locker-type differential must be from an approved
manufacturer and must be approved by NASCAR.

[ian_w]

Here is a quote from the IRL rule book:

The transmission must retain the following approved transmission supplied internal parts:

(1) Transmission case and castings
(2) Crown Ring and Pinion
(3) Spool ( Oval Events only )
(4) Differential ( Road and Street Course Events only ).....

[Al Weyman]

Things must have changed then because I can show you an old Road & Track road test on the car in my garage and that definitly used a spool at the Glen and a slipper on the ovals this fact was also verified by the Penske IROC crew chief in an email to him a few years ago when I was researching the car. Actually I was saying I wanted to not use an LSD but a fully locked diff as our regs say I have to use the original axle and I dont think there is a slipper available for the job apart from one from Auburn or someone which may just do it the GM one thats in it is certainly not man enough.

[ian_w]

I don't know much about american axles apart from the Ford 9" is undestructable! Is there nothing that suits the GM axle? As an alternative you may be able to fit a Ford diff unit into your GM axle with a little work, you may be suprised how similar they are - probably even manufactured by the same company.

I used to have a Clubmans car that had a real hybrid axle - Morris Mirror case and housing, Escort diff and half shafts and a Triumph CWP. With a little machining on each piece and the odd spacer it fitted together a treat!

[Al Weyman]

I have Ford 9 inch in the NASCAR IROC car must check what diff is in it! No I am stuck with the GM 7.75" axle although I have been reading that they put an Aussie Borg Warner in some of them (probably supply problem in period) and they are very strong although parts are a bit hard to get and expensive. I will probably go for an uprated Auburn and Strange shafts and give it a go from there.

[Supercar]

I welded my rear diff to give it a try, and here are my conclusions:

When road racing, you will notice a difference mostly during the turn entry. The car will be entering the turn more reluctantly, and less predictably. You will have to throw it into the turn, and that will be difficult to do the same way every time. But the turn exit will be better, faster, and more predictable that with an open diff.

When autocrossing (slower twisty tracks), the car understeers much more noticeably. A slalom is a PITA (pain in the butt).

[Al Weyman]

Thanks for the informative reply.

I have just pulled the engine on the car and fortunately it has only knocked out the crank and two rods so will hopefully have it back up and running for next season nice and early. It sounds like certain tracks in the UK this may work but prehaps on balance a tighter diff and not a fully locked one may be the way to proceed or get off the loud pedal more :-). I wonder if go lighter on the rear springs or less rear anti roll bar may help.

[retro_msport]

Al, can you get around 10 degrees or more castor and up to 7 degrees camber with your car? if not, run a proper LSD

[davyboy]

Al you'll need to find some way of getting the inside rear off the ground on corner entry if you run a locked diff otherwise the understeer will offset any gains you manage on corner exit. That's exactly the way a kart works.

[Al Weyman]

Originally Posted by retro_msport

Al, can you get around 10 degrees or more castor and up to 7 degrees camber with your car? if not, run a proper LSD

No chance, its maxed out at about 3 degrees even with special top mounts, anymore and the suspension hits the inner metalwork.

[retro_msport]

Originally Posted by Al Weyman

No chance, its maxed out at about 3 degrees even with special top mounts, anymore and the suspension hits the inner metalwork.

Scratch the idea of a locked diff then, it will be a nightmare.

[Supercar]

Retro, would you please explain how having a 10 degrees or more caster and up to 7 degrees camber could help?

A 10 degree caster with a large scrub radius would create a kart-like jacking, that's good. But I can't see any advantages or a 7-degree camber. That is at the top limit or reasonable even for a bias-ply tire.

And, what is that proper diff that you are recommending? What type? The reason why some people run a locked diff on some nose-heavy RWD cars is often because any other diff offers little to no advantages.

[retro_msport]

The reason i say it is because all the people i know with 'spools' have run that sort of camber, take a look at Greg Rose in his Falcon, he tried less and couldnt get the car to turn in.

When i was a poor, nothings changed, i used to weld up diffs and alos had to run loads of camber, making up TCA's and extensions for the rack, doubling up the roll bars as they now had bigger leveage acting on them, making brackets to move the ARB forward, modifiying the arch to the extra camber....

Bought my first ZF plate diff, and yippee, less camber/castor, could turn the car at slow speeds, and had shorter stopping distances due to the tyre sitting on the ground across its whole width.

As for Diff type, anything but a spool, ZF plate diffs have always been my choice, but then i'm stuck in a world of Atlas axles.

Funny, Greg and I was only speaking about his Falcon the other day .. the castor angle, 26 degrees.. i'm sure thats what he said, just to get even more camber in turn. And hes just going over to a plate diff in Martin Johnsons Thunderloon Sierra, reason.. get a flatter tyre at the front, Mcphersons on the front, under braking you'll gain neg camber under braking. and a tyre thats allready unloaded on the outside half of the tyre is going to loose even more contact.But with a locked rear, you need the front like Milikens Camber car to get it to turn.

That said, I don't know what Al's car needs, I've never checked it over, Al's driving style may suit a spool, maybe not, but if he cant make it like a Kart, then i can forsee problems.

[Al Weyman]

I dont know whether the tyres may also be a deciding factor as we run on Toyo 888's not slicks although I am running a wider tyre and wheel on the rear than in the front so I would imagine the push effect during cornering would be even worse as it is the balance is'nt too bad at all for what it is (no ex-Aussie supertourer thats for sure).

Anyhow the good news is I have just pulled the damaged engine and this time I will get away with just a crank and two rods (I have 5 spares so no big deal and a decently baffelled sump!!!) and everything else, block, cam, all pistons, 6 rods, heads etc is good so we will be running her again next season and try to get that power down again!!