Locked diffs for circuit racing?

[Al Weyman]

I have an old Road & Track article with a test on an IROC race car similiar to the one I am trying to renovate and it said that for road courses (circuit racing to us UK types) they used a fully locked diff but for the ovals used a slipper. I was surprised at this as thought it would be the other way around.

Does anyone have any experience with locked spools (diffs) on rear wheel drive race cars as I have just found out that the diff on my older Camaro is not working as should which could account for the sudden oversteer condition (would it?) and was wondering about trying a fully locked configuration as its all wheel spin at the rear anyhow.

[goforit500]

Locked diff + RWD usually = Understeer

Suppose that going around in circles means that you have wheels spinning at different rates all the time which woul destroy the tyes PDQ on the ovals..

I've used diffs that were too tight on my car (RWD Escort Cosworth) and they have been a right handful. I imagine a locked diff would be great fun but ultimately not the best option... :-)

[dtype38]

I used a Jag Powerlok diff on my XJS and now in my D-type. As the name implies, it'll slip on over-run or off power, but locks up solid when you put your right foot down. Agreed it takes a bit of getting used to, but once you get the hang of "power-on understeer" you can play with the angle of the car using your right foot. I don't think its actually any quicker or slower than LSD's just a different driving style that suites some better than others.

[ian_w]

Sounds backwards to me as well. Indy cars run spools on the ovals but a conventional LSD on the road courses.

A spool will give you lots of turn-in understeer. Once you get the car in the corner it sounds like you have enough power to unstick the back end and neutralise the understeer.

I know Legend cars run spools on road circuits, not sure how they handle. I imagine you have to throw it at the corner to get it to turn in.

[Al Weyman]

When I used to do 1/4 mile ovals we always used to lock the diff as they spun the wheels up through the tight bends anyhow and I don't recollect any problems which is why I wondered if the article was incorrect.

[Chucky]

Locked diffs or spools on road courses have no place. The negative effects on turn in far outweigh any traction benefit. V8 Supercar uses them but that's not out of choice. It was one of the cost cutting rules from the beginning.

Besides, any decent clutch pack or torque biasing diff will put power down as well as a locked diff.

[Notso Swift]

I thought they used stagger on the ovals to help the tyres/turn, and ran a locker

Originally Posted by Chucky

Besides, any decent clutch pack or torque biasing diff will put power down as well as a locked diff.

Torque biasing are no good if you tend to get any air, they will spin up.
Plates are effectively the same as a locked once you get to the preset limit anyway, at least a locker is consistant.
CIG Locker = Free, any other diff = $1000+
There is a place for them

At the ultimate level you would always choose to use a diff, but it would mostlikely be hydro/electrical

[Chris Y]

Originally Posted by ian_w

I know Legend cars run spools on road circuits, not sure how they handle. I imagine you have to throw it at the corner to get it to turn in.

And how! I've driven one, and you have to slide them everywhere to get them round corners. However, that's easy to do as they have good power to weight ratio, and very hard treaded tyres. The Knockhill hairpin is a particular favourite in a Legend - opposite lock and a steady drift to the exit

[1200Datto27]

V8 Supercars are setup to unload the inside rear wheel when cornering (hence the ability to adjust the rear roll center quickly in the pits) as this helps the car to turn in.

With the indy car style oval setups, I though that they used the stagger of the tyres in a manner that during steady state cornering the difference in diameter allowed the wheels to rotate at close to equal speeds, and then when in a straight line they deal with the turn issue. I would presume that stability when cornering would have a higher priority on the high speed ovals than straight line stability.

[Al Weyman]

Well on hard and skinny 8 inch wide Dunlop control tyres on this car it lights up the tyres to easily and gets power oversteer to the extent of spinning it as I did last time out at the Thruxton chichane admittedly in damp conditions, so would a spool help?

[VIVA GT]

The Group A RS 500 Cosworth originally had a Ferguson rear differential (from the RS200) which used the very progressive viscous coupling LSD. Unfortunately, the CWP sets for these were weak and an alternative was produced, known as The Australian Diff.
This used much stronger internals, but was only available with a Detroit Locker LSD, or a spool.
The Detroit Locker was not at all progressive, and could suddenly lock mid-corner, throwing the car sideways. this was most un-predictable, and kept the driver on his toes!
Using a spool instead obviously caused handling problems (severe understeer), but once the chassis was altered to compensate for this (running toe out for example), the car became perfectly driveable.
(An unfortunate side-effect of the spool is that the cars are horrendous to push around in anything other than a straight line, as both wheels want to go at the same speed all the time. If only we'd had Go-Jacks in those days!)