S15 Helical Diff Explained

[Topper]

Further to this thread: - http://forum.s15oc.com/viewtopic.php?t=494

I did some digging into the S15 Helical diff, as you can see from a recent Drift Uk event i attended (link above), the diff would apear to be not working correctly, only spinning up the inside wheels. However, this is what its meant to do. I will let someone who knows what there talking about explain -

The picture does show up that one wheel is clearly spinning up.

First of all the S15 diff is very poor for drifting as it never actually locks up both wheels. Just because one wheel is spinning doesn't mean the diff is broken. What you have to think about when we talk about limited slip diffs is, it is called a limited slip diff because it limits the amount of slip between each wheel but eventually it will allow slippage. An open diff has 100% slip i.e. it never locks up. If we just consider an open diff, its job is to allow one wheel to spin faster than the other during cornering / turning. When you accelerate hard enough on an open diff during cornering the inside wheel will always be least loaded due to the weight being transferred to the outer wheels due to the centrifugal forces of the vehicle. The inside wheel will lose grip easier and when it does all the power gets transmitted through this wheel as it has the lease grip, hence the one wheel spinning. With all diffs there is a certain percentage of slip, i.e. open diff has 100% slip, a viscous diff may have 75% slip therefore 25% lock, a clutch drift diff may have 30% slip and 70% lock. The percentage relates to a breaking away torque i.e. the amount of torque the diff transfers to both wheel before the diff begins to slip each wheel.

Helicals never lock up as the power is constantly varied between each wheel, but at the same time some slippage occurs and one wheel will always spin once the breakaway threashold is met

 

[Nicely]

I believe that the main theme of that was covered in our conversation yesterday (if somewhat more basic... ) :thumbs:

 

[Topper]

Agreed :nod: , this is more for everyone else's info really

 

[S15R]

well when you drag the car out, both of the wheel should spin, no matter 1 way, 1.5way, 2 way, Viscous or Helical LSD. it will spin both wheels due to under load.
Try use thicker diff oil that would help abit too.

one wheel spin under load appears on NA S15 because they didnt come with LSD, they have open diff(no LSD)
Autech S15 is different

 

[Topper]

well when you drag the car out, both of the wheel should spin, no matter 1 way, 1.5way, 2 way, Viscous or Helical LSD. it will spin both wheels due to under load.................

This i agree with, i.e. both wheels will still turn, there is, as Barky said, a level of slip where both wheels will spin, i.e. the diff locks, but with the Helical this figure is alot lower than the likes of any others you mentioned. And once this figure is passed the diff will act like an open diff, i.e. spinning one wheel, the inside wheel :nod:

 

[Barky]

well when you drag the car out, both of the wheel should spin, no matter 1 way, 1.5way, 2 way, Viscous or Helical LSD. it will spin both wheels due to under load.
Try use thicker diff oil that would help abit too.

one wheel spin under load appears on NA S15 because they didnt come with LSD, they have open diff(no LSD)
Autech S15 is different

You said DRAG :hurl: ! During draq, both wheels should get same loading and traction and therefore the relative speed of each wheel should be the same i.e. diff isn't doing much. But when we talk cornering / drifting the loads / traction / speed for each of the driven wheels are different and this is where the diff comes into play.

 

[S15R]

You said DRAG ! During draq, both wheels should get same loading and traction and therefore the relative speed of each wheel should be the same i.e. "diff isn't doing much"

i wonder why most drag cars use LSD?

try drag a civic, a starlet or NA silvia with open diff, if a car with an open diff, when it playing a role of drag racing both wheel wont spin at the same time cos there is no initial torque setting in open diff

 

[Barky]

You said DRAG ! During draq, both wheels should get same loading and traction and therefore the relative speed of each wheel should be the same i.e. "diff isn't doing much"

i wonder why most drag cars use LSD?

try drag a civic, a starlet or NA silvia with open diff, if a car with an open diff, when it playing a role of drag racing both wheel wont spin at the same time cos there is no initial torque setting in open diff

The note says "should get" or "should be" since road conditions and laws of physics of the mechanicals in the real world are never exact the same so both wheels have the same grip (i.e. the twisting motion of the rear wheels along the prop shaft axis, suspension movement, tyres etc etc). LSD's are used to over come the issues and provide drive through both wheels when you need it.

On FWD cars most of them have different lengths of drive shafts this causes torque steer as one wheel gets more torque and pulls the vehicle to one side and as it does the difference in wheel speed is becomes apparent and one wheel will spin faster than the other, the LSD reduces torque steer as it puts power through both wheels, thats why they are used so widely in drag on FWD cars so your not fighting with the steering wheel as much to try and keep it in a straight line.

Initial torque is not possible on an open diff because of the design of the diff and if it was it would be called an LSD anyway as the initial torque setting is limiting the slip.

I'm not doubting you in any way that both wheels should spin, yes they will both spin with an LSD, but there will be a point where slip occurs, thats why its called limited slip since the slip is limited. A locked diff is different in that it locks it 100% with no slippage allowed i.e. used on some 4x4's and some drift cars with a 3 Way welded diff

 

[Topper]

Well after about 3 read of that Barky i think i'm with you (Bit technical at 1st , till i thought about it)

 

[spoonman]

So if you do a skid the helical will only light up one wheel?? but apply controlled power to both wheels when cornering hard?