Spools or welded diffs on track cars

Just following on from Ron's comment on his "Jensen Healey failure" thread.

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"(just read Mark Donahue and crew were running welded diff Hewlands in some of their cars back in the day) "
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Was this a common practice back then? Any one got anything to add to Ron's statement? Specifically as applied to high power, big rubber, mid-engined applications? Eg CanAm, F5000, Le Mans

I am toying with the idea, later on, after the car is sorted, of doing away with the diff in the 930. The 930 has 40% lock up power on,, and 80% trailing throttle. Really almost like a locked diff on trailing throttle turn in, but only 40% under power. I thought a locked diff by comparison may be not a lot different under turn in but heaps better under power?

Experience any one?

Cheers,

Russ Noble said:

J Really almost like a locked diff on trailing throttle turn in, but only 40% under power. I thought a locked diff may be not a lot different under turn in but heaps better under power?

Experience any one?

Cheers,

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I don't have experience in the mid-engined cars with a locked diff, but I do in two race cars. They can work well, but you have to realize the chassis setup is different because they create a push on the front. And if you don't take care of it you'll be washing out of the turns as I did a couple years back at CMP in our 260Z. Once it was sorted (well mostly, still some work there) it is good, just be aware your setup will change.

Another aspect, although not directly racing related, is how hard they are to push around with a welded diff. The Jensen is light, 2200lbs, but it is impossible for one guy to move around with the steering wheel anywhere but straight. It is easier in a sandy paddock or on grass, but on pavement or a driveway it is rough!

Now, another thing I think that should be considered on this is tire. Welded diffs "back in the day" were on "back in the day" tires that don't stick as well as modern race rubber. I, and others I know at the track, have had some concern over how long a welded diff and axles would last with high power and super sticky tires. The solution, of course, is to not weld them as a poor man's fix but to get a proper full spool. A spool can have some ring gear support on it as well to add strength.

And one more thing I was thinking of about tires. Lots of folks are fitting some super wide rubber on the rear of their cars. Has anyone thought about rear steering effect and how these are going to affect a car on track? I'd imagine they have a tendancy to track groves in the road(not a huge issue on track) and due to size just flat out want to go straight, thus inducing a little front end push on track.

R

Pay rates for Crew are going too have a major increase if you think we are going to push you around in the pit area with a locked diff. Save your welding rods for the chassis and then save some more $$ for a, DPI, Tru Trac, Torsen or similar.

Jac Mac

OK Jac Mac, I'll double your pay rates, but I take a dim view of union standover tactics! oke:

jac mac said:

Save your welding rods for the chassis and then save some more $$ for a, DPI, Tru Trac, Torsen or similar.

Jac Mac

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I'd agree with that! The Z ended up getting a Quaiffe and the 930 I have has a ZF LSD. If the JH ever comes back to life it'll have a LSD of some type as well.

If I remember right, you are not stuck with 40/80 in your ZF LSD either. There are ways of adjusting that.

Lynn

Thanks for the comments Ron.

I have had experience with lockers, all or nothing type things, which many people advised against running. There were some inherent minor problems with the way it'd suddenly lock in but nothing that couldn't be coped with from the drivers seat. Just meant changing ones technique to suit. Lap times majorly improved and that's the bottom line.

The only thing with a fully locked diff that concerns me (since I've got a now overpaid crew to push it round the pits :lol: ) is turn in to hairpins, not a problem if the track is grippy as you would just turn in under brakes. But in the wet, or if it was slippery, where it would be difficult to get sufficient weight transfer to the front, I'd be worried about massive understeer. Maybe the extra brakes required on the rear in the wet would dial that out? And how much worse would it be anyway than an 80% Porsche lock up?

That's why I was wondering if locked diffs were the norm way back then and how the guys coped with them. BTW I have to run the historic race rubber of that period.

A guy told me that Denny told him they ran Detroit Lockers in the McLaren Can Am cars. Whether that's true or not I don't know. I think the LG600's would normally have a Salisbury slip. Be nice to know what everyone did run and any problems they encountered.

Just thinking about it, but I could be wrong, I don't think F5000's would run locked diffs because those DG300's are pretty fragile anyway behind 5 litres.

Cheers

llarsen said:

If I remember right, you are not stuck with 40/80 in your ZF LSD either. There are ways of adjusting that.

Lynn

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Should be able to tighten or loosen to suit by shimming it up.

From a discussion on our race board, actually today from a crew fellow on Zs and he knows his stuff - K Thomas from the Atlanta area.

"Anyway, any LSD would be faster than a welded diff. We were. You're adding an element of drag, all the time to some degree, with a welded diff. And forget about racing in the rain (which you ought not do in an old Datsun anyway, but that's another subject). Many in the ITS Z community ran welded R180's through the early 90's, and experienced several stub axle failures, some of them spectacular in a bad way. I think a lot of us eventually migrated to clutch or Quaiffe LSD's and our stub axle issues disappeared."

llarsen said:

If I remember right, you are not stuck with 40/80 in your ZF LSD either. There are ways of adjusting that.

Lynn

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Lynn,

I think 40/80 is the maximum you can get. Other adjustments are less.
Let's hope Joel sees this thread. He would know any tricks.

Cheers

Russ - I raced an SCCA American Sedan (class) Camaro back in the late 90's. Certainly not mid-engined, but relatively high HP and weight.

All I can tell you is this -
During qualifying at an event, I blew my 9" Ford differential with a LSD in it and had to go to my backup differential. This was with 3.50:1 gearing and had a spool in it. I was racing at Brainerd International Raceway in northern Minnesota. This is known to be the fastest dedicated roadcourse in the USA. I would enter turn 1 at 160mph (in traffic but hopefully not). With the locked rear end, the car was so un-predictable it would go from PUSH/LOOSE/PUSH every other heartbeat. I have a video that I will someday capture to digital that shows the white line around the inside of the turn that was snaking all over the place. Talk about PUCKER factor! After that race I vowed to **Never** run a locked differential again. I did, indeed, win the race - but I think I scared the heck out of my competition as I (my car) was so un-predictable..

Other factors to consider is the wear on your CV/U-Joints in your half-shafts. If you have one of them fail, you would be a very busy person as the car will turn in the direction of the failed joint and rather quickly. With an LSD, a broken axle will do something similar but it will be buffered by the slippage.

Well - that's my $.02 worth...

I'm amazed you blew a Ford 9"... I ran a Detroit locker in my Shelby GT350 and it gave me a heart attack everytime it locked and unlocked going into and coming out of bends but it wouldn't break. In fact the blasted thing unsettled the car so much it was more dangerous than the driver!

When I acquired the Penske Camaro, it already had another Ford 9" in it but with a Posi. It was like champagne versus brown ale...

My 2 cents...

Keith, that was pretty much how my locker was too, but at the end of the day the times were faster with it than without it.

Randy, that was one of the main reasons I was advised not to run a locker, if something broke on one side, the wheel with the drive would spear you into the wall. But in a race car if you worried about what happens if something breaks, you'd never get in the bloody thing!

Thanks for the replies.

Guard Transmission makes different "ramps" for the Porsche 911 transmissions so you can set different lock percentages. They have 40/60, 50/80, and 80/80 for example.

I agree with that last sentiment of yours Russ. When I was racing the 240z the LSD (clutch type) let go and I decided to repair it myself. I packed the clutch up but went too far.End result was basically a locked diff with associated hastle in the pits.Driving impressions were ok though and maybe because I didn't have massive power I found the understeer/push easy to control.
Ross

jmracecar said:

Guard Transmission makes different "ramps" for the Porsche 911 transmissions so you can set different lock percentages. They have 40/60, 50/80, and 80/80 for example.

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Thanks for that info Joel. I guess 80/80 would be a good compromise AND keep the pit crew happy!

Nevertheless it would be good to be able to find out what the CanAm teams etc actually ran in their day. Presumably Mark Donohue found a locked diff did the business and if McLarens were running a Detroit Locker, it can't have been too bad. I wonder whether these guys were the exception or the norm? There must be some guys with that sort of first hand knowledge that could enlighten us....

Cheers

I ran both a detroit locker and a spool during the time I raced my GT350 and latterly found the spool to be at least as fast as the locker. As the car basically understeered in tight/slow corners the technique required to get the car to turn in played to the strengths of a locked diff - throttle steer. On the exit the differential action available from the detroit locker was wasted since the car could easily spin both tyres in all four gears if you were too brutal with the loud pedal ,so that the diff was locked pretty much the whole lap save for braking. The car ran a very soft rear spring to help traction on less than smooth circuits so the lighter weight of the spool should have been an advantage. So in a car with big horsepower, narrow tyres (limited rear grip) and less than ideal weight distribution a spool might work ok. In a car, such as the GT40, where there is plenty of rear tyre and the weight distribution is much better I cant see it giving any benefit, other than a bit of weight saving, and the driving style it would dictate would not play to the handling stengths of the mid engined layout.

sfm6s said:

I ran both a detroit locker and a spool during the time I raced my GT350 and latterly found the spool to be at least as fast as the locker. As the car basically understeered in tight/slow corners the technique required to get the car to turn in played to the strengths of a locked diff - throttle steer. On the exit the differential action available from the detroit locker was wasted since the car could easily spin both tyres in all four gears if you were too brutal with the loud pedal ,so that the diff was locked pretty much the whole lap save for braking. The car ran a very soft rear spring to help traction on less than smooth circuits so the lighter weight of the spool should have been an advantage. So in a car with big horsepower, narrow tyres (limited rear grip) and less than ideal weight distribution a spool might work ok. In a car, such as the GT40, where there is plenty of rear tyre and the weight distribution is much better I cant see it giving any benefit, other than a bit of weight saving, and the driving style it would dictate would not play to the handling stengths of the mid engined layout.

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The 9" that I blew was a conventional posi. For some reason one of the pinion gears split and jammed, then broke loose and the ring and pinion ate one of the "chunks" - Car stopped pretty quickly with both rears locked up. I was at about 60mph coming off of a pretty tight corner.

What I do recall about the handling of my car with the locked diff was that there was no trail-braking that the car would allow. Literally all braking was threshhold braking and you had to deliberately throw the car into the corner's apex and then stand on the throttle and hope the outside rear would give enough traction to get you off the corner while still tracking out.
One other thing that was very evident was I could run a lot more rear brake with the locked rear end. Braking was improved (straight-line) but corner exit speed was substantially reduced. My lap times were up about .5 seconds overall (I ran on the track record and held it for a while). Other drivers I talked with told me that it took them a few races to get used to the locked rear and were staying that way either for economic reasons or because they liked it. Suffice it to say those that I spoke with were not on the podium that weekend but as long as they enjoyed their drive - that makes all the difference in the world.

In a mid-engine car I would have to believe that the corner entry would be worse as they all typically exhibit understeer conditions even with an LSD with less than a full load of fuel. I'd hate to be driving one in the wet with a locked diff.

Russ,
While not wanting to side-track this thread, I'm interested in the percentages. What exactly do the 40% and 80% relate to? Does 80% lock up mean one wheel can be receiving 20% less torque from the diff. than the other? Or 80% of the torque input to the diff is output usefully (ie. to the wheel not slipping), or what?

Dalton

Dalton,

Not a side track.

Look at it this way, an open diff transfers 0% tq to the wheel with grip, a spool transfers 100%.

Actually that is not strictly correct, the wheel that is spinning usually has some grip and an open diff applies that same amount to the wheel with grip but the rest is lost to wheelspin.

Limited slips are just that, they limit the amount of tq lost to the wheel without traction and are generally progressive and smooth in operation. Most limited slips as I understand it work equally under power and overrun but the Porsche ones are set to have more effect on the overrun than under power. That is where split ratios like 20/80 or 40/80 come from. That is in an attempt to compensate for the quirky tail heavy handling of those chopped and channeled VW's.

Interestingly, if you achieve fitment to a mid engined setup by flipping the crownwheel (as opposed to inverting the box) you then get the ratio split reversed 40 front/80 rear then becomes 80front/40rear. Because the direction of rotation of the diff is reversed. Much more desirable but the additional motor height involved is not.

As a side issue a locker is a 100/0 setup and not progressive. It works as an open diff until traction is lost to one wheel under acceleration, then it just instantly locks both wheels together. This can cause "interesting" handling issues.

For racing it is my understanding that Torsen type LSD's are not as good as clutch or pawl types.

Also just to digress, the first time I ran a locker was in the wet, it was tricky to drive, but it got the job done. At one meeting I was 9th on the grid after dry qualifying, it hosed down just before and during the race. In the race I came through to 2nd place and set fastest lap. I don't think the bad press that lockers get is necessarily fully justified! Most if not all the opposition were running conventional LSD.

Hope that clarifies things.

Russ Noble said:

Dalton,


As a side issue a locker is a 100/0 setup and not progressive. It works as an open diff until traction is lost to one wheel under acceleration, then it just instantly locks both wheels together. This can cause "interesting" handling issues.

{{{ Not stricty correct-- There are different versions of the 'Detroit Locker' with different springs/ramps/pawls that change the engagement characteristics}}}

For racing it is my understanding that Torsen type LSD's are not as good as clutch or pawl types.

{{{ Comes down to Driver Preference-- For my money the Torsen types are the best thing since sliced bread, A lot easier on rear axle components as engagement is smoother, very good in wet or dry conditions, excellent for ageing pit crew's who are unfortunate enough to be around when our hero driver needs a push start!}}}

Hope that clarifies things.

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Jac Mac