Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Right biased LSD for circle track

  1. Aug 16, 2014 #1
    I know that some AWD vehicles used the gear ratio of a planetary or other transfer gear combined with a gear type torque biasing diff that would allow a natural mechanical advantage to apply more of the torque to the front or rear axle.

    Why not apply this to a rear diff for circle track racing. the bias would put more torque on the more highly loaded right rear tire of course. I was thinking that the gears inside a Helical LSD could be arranged for the bias, or a reduction gear added to the right side output.

    Or, ideally, you could just replace the diff gears with a planetary set that has the left axle connected to the sun gear, the right axle on the planet carrier and the ring gear attached to the uh.. ring gear or diff case. You would have an open differential that is biased to the right side.

    If you can get the ratio right for the grip difference for a given lateral acceleration or corner speed and radius, and car geometry, you could have an open diff with equal or better traction coming out of a turn to a LSD. Of course once you start to straighten the wheel and side loading is decreased, the right rear wheel may spin before the left side when a lot of power is applied. This would probably only be an issue for very high powered cars, as most cars would have limited torque output at the higher speeds once you are coming out of the turn, and the available grip is higher since there is less lateral load on the tires.

    This also should give you a little bit of overall reduction in gear ratio when turning left, raising the engine rpm slightly for a given speed. It might free up a car that is bogging getting on the gas.
    If you were to turn right, the gear ratio would effectively increase.

    I suppose you could also get by some rules that require an open differential. You would still have an open diff, it would just have a different amount of reduction to each side.
  2. jcsd
  3. Aug 17, 2014 #2

    Ranger Mike

    User Avatar
    Science Advisor
    Gold Member

    Normally I would be jumping on the "can do" band wagon. I love new ideas and this is one of them. Sanctioning organizations have pretty much mandated floater type differentials in racing. These full floaters are axle housings that retain the wheel even when the axel breaks. These were based on truck rear ends. They also pretty much dictate t he Ford 9 Inch rear end or in some cases a quick change rear end that permits gear ratio change without the ring and pinion being replaced. Mean while the Wiley crew chief tries to take advantage of every rule. Since the rear end is unsprung weight it is the biggest evil on the race car but there is not a lot to be done with it since it has been developed to handle the 600 plus Horsepower put pout these days. About all we can do is lighten the rotating components and gun drill the axels. You have 50 pounds of tire and wheel and 200 plus pounds of weight of the rear end...

    Limited Slip diff- LSD- the Detroit Locker is about the only diff used that is not a locker. A locker is just that. A device to replicate the planetary gears of an open differential but the gears have been welded so no rotation occurs. You must run staggered tires to be able to turn the car in the corner. This means one tire is significantly larger diameter than the other tire. Think of a Styrofoam cup on its side. Move push it and it “ goes around the corner”.

    LSD like Sure grip and Positraction have discs that just wont hold up to the racing. This was tried in the 1960s.

    If we can find a traction system that is more effective than the simple locker or spool arrangement being used, that will not add weight or have more parasitic horsepower loss, i am all for it. I fear the current method is about as good as it is going to get.

    Because of the rules, availability of proven components and cost, I think a venture into development of a better differential will not be supported by the small market of racers who can take advantage of the new design. Maybe off road competition?
    Last edited: Aug 17, 2014
  4. Aug 17, 2014 #3
    I know a lot of tracks and sanctioning bodies allow either open or welded open differentials, as well as the detroit. A few even allow the helical type, and those are quite popular where allowed, often referred to as "torque sensing or biasing diffs". they are often called a "Torsen-Gleason" type, after the original designer. I think the clutch type that has angled ramps is probably the best for road racing though. I have seen about every type of differential used in both the 9" and quick change units for road racing. Even the clutch type units hold up well enough for 24 hour races in fairly heavy GT cars. I think stagger can kill one pretty quickly on a superspeedway, but you don't really need it there.

    What I am suggesting though should qualify as an "open" differential though, since either wheel can spin freely if it is raised off the ground. It should also be able to fit inside a carrier for either a 9 inch or quick change.
  5. Aug 17, 2014 #4
    Oh yeah, with the planetary I described, the left wheel would spin backwards when the right side is held, and the right side would turn at reduced speed if you held the left side.

    Even if it is banned in classes that only allow open or welded or spool type diffs, there are plenty of classes that allow any LSD or differential, like PASS, for example, as well as any super late model class I have seen, and most local Pro late model classes.
  6. Aug 17, 2014 #5
    I have looked at the Torsen designs, and the single pinion T-3 is what I am looking for. It provides up to 65% torque bias. It is used in a wide range of AWD applications. I will contact them about what spline outputs are available, and the case drive mounting. It should be simple enough to either mount in an existing carrier of some type or machine a new one to mount it, and if the splines are not compatible with currently available axles, it would still be no problem to get custom splined axles.

    I think I will look into the quick change in either the 10" or 8" ring gear carrier as the best unit to adapt to.

    Am I right in assuming the 8 3/8" quick change is not as desirable as an 8" carrier in a 10" housing these days?
  7. Sep 7, 2014 #6
    Don't circle track cars use locked diffs and use tire stagger so the rear wheels can exert force to help the car "turn in" on the corners?

    If not for that, I would assume we would see a lot of understeering cars pushing all around the track.
  8. Sep 9, 2014 #7
    a spool or welded diff with stagger is the simplest and cheapest way to go, and works quite well, except for the tire wear and slight loss of speed from friction when on the strait sections of track. Also stagger is set up for a particular radius of turn. A lot of tracks have turns of various radii, and even within a turn, the radius can decrease or increase. The track nearest me is a sort of triangle shape of sorts. The front straigh is not straight, and it gets tighter and tighter radius into turns one and two, then the back is straight, and turns 3 and four are pretty wide. The torsen style diff is pretty good at handling anything with or without stagger though, so this type of diff would only be able to get a possible slight advantage on that type.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook