About our LSD

[Archyz]

I am in charge of our car's drivetrain. And it is our first time in the FSAE. I really need some help about our diff.

1) if our budget is not so tight, what kind of diff are you suggesting? I am currently look into the Cusco LSD. It's a plate form of LSD. Can I use this?

2) from some forum discussion I know that suspension set up has something to do with the diff choice and modification. How, exactly? Do you have any material or something to explain that?

3) How to determine the TBR? Though we may BUY a diff, we can still tune the TBR.

4) How to fit the spline? Is there easy way or tips.

Thanks for your help! My email is archyz710@gmail.com
we can discuss something else.

[Archyz]

I am in charge of our car's drivetrain. And it is our first time in the FSAE. I really need some help about our diff.

1) if our budget is not so tight, what kind of diff are you suggesting? I am currently look into the Cusco LSD. It's a plate form of LSD. Can I use this?

2) from some forum discussion I know that suspension set up has something to do with the diff choice and modification. How, exactly? Do you have any material or something to explain that?

3) How to determine the TBR? Though we may BUY a diff, we can still tune the TBR.

4) How to fit the spline? Is there easy way or tips.

Thanks for your help! My email is archyz710@gmail.com
we can discuss something else.

[Mike Cook]

For a first year team, I would recommend using something like the taylor-race quaife diff. They have all the half shafts and tripod joints already made which makes putting everything together pretty easy. It isn't the lightest diff around, but it works very well. For most diffs I think about the practical limit for TBR is about 4:1. Beyond this, the diff starts to lock/unlock more abruptly and upset the vehicle. The first step in determining how much tbr you need is to look at your wheel loads in a corner. This will be a function of your weight distro, track width, cg ht, LLTD. If you know you have 50lb on the inside rear, and 250 on the outside rear, you would need a 5:1 tbr to be able to put the power down coming out of the corner (of course after starting to accelerate and come out of the corner this should put the inside tire down but if it is already spinning this won't help that much). If your wheel weights are 100 and 200, a 2:1 tbr would work.

Good luck.

[scotty young Taylor Race]

Mike,

Thanks for the good word bud.....

Complete diff with bearings , stubs , sprocket flange and oil is around 16/17 pounds.

The mojor O.D is 3.500.

scotty
Taylor Race Engineering
scotty@taylor-race.com

[TorqueWrench]

Mike was right on with the TBR calculations. I have heard a good number of teams use that as a starting point and, as with everything, does track tuning to tweak the understeer/oversteer characteristics.

I personally am a fan of the toque sensing differentials. I don't have any experience with the Taylor Race ones, but I have heard good things in the past. I know a good number of teams use ones from Torsen (and last I heard they are taking orders again) because of how cheap they are. I know we are planning on running either an old T1 or buying a T2 for our next car.

Then again, you could always try getting away without a differential (I can't be from UB and not throw that idea out there).

[Archyz]

Thanks guys. So what I get here is that the only influence that suspension impact on diff is the transfer of load on wheels, right? If so, my job here is much easier.

Also, I am still confused about fitting a given spline. Anyone have some practical suggestions.

By the way, I wanna wrap our steel half shaft with CF to reduce the girth of the inner steel core and lighten the half shaft. Though we are about to do some strength test, I still need some experience.

Thanks!

[TorqueWrench]

Thats pretty much the main chunk of it, but your differential choice can affect your suspension. If you have too little TBR, the car will tend to understeer a bit more. From what I know, a higher TBR results in higher frictional losses in the drivetrain. A good example of this is solid rear axle teams (TBR=0) that understeer in corners badly unless you set the suspension geometry up correctly.

Best suggestion I have heard to the spline cutting is to send an item with the same spline (either male or female) to make sure the dimensions/clearances are right.

[Hector]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by TorqueWrench:
Thats pretty much the main chunk of it, but your differential choice can affect your suspension. If you have too little TBR, the car will tend to understeer a bit more. From what I know, a higher TBR results in higher frictional losses in the drivetrain. A good example of this is solid rear axle teams (TBR=0) that understeer in corners badly unless you set the suspension geometry up correctly. </div></BLOCKQUOTE>
Except TBR of a solid axle is infinite. (?:1)

Everything I've ever read about Torsens say that higher TBR is what contributes to understeer. Torque is always transfered to the slower spinning wheel on a Torsen. The inside wheel therefore gets more torque than the outside wheel. What does that do? It tries to push your car in the opposite direction that you are trying to turn in. I know this may not be the case for all diffs, but for the Torsen it's spot-on. This is also only for power-on (corner exit). On corner entry (engine braking) it's the opposite, and a higher TBR will help you enter the corner better because the inside wheel is getting more braking. We, however, run a slipper clutch so engine braking is limited.

[TorqueWrench]

My bad, haha. They always spin at the same speed and get the same torque from the engine, but the amount of force they can provide is then dependent on the load.

I may be wrong at this seeing as I first looked into differentials for the first time two weeks ago, but I was under the impression the entire point of the torsen is that it transfers more torque to whichever tire has the greater longitudinal capacity at a given time, which would be the outer tire normally during cornering. This was the big improvement when compared to differentials like the cam and pawl, which does apply only load to the inside tire during cornering. I just checked the taylor race tech document on differentials and it seems to say the same thing.

[Hector]

The Torsen is capable of transferring more torque to the wheel with greater traction, but it has no way of inherently sensing which wheel has the greater traction.

In low-grip environments, since the slower wheel has greater traction, it sends torque to that wheel. It helps in true low-traction scenarios, but can make tight cornering difficult.

[Hector]

From a Torsen white paper:
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">In turning situations, the outside wheels of a vehicle travel over more distance than the inside wheels. Accordingly,
the inside and outside drive wheels must rotate at slightly different speeds (i.e., differentiate) to maintain rolling
traction with the road. A torque division between drive axles at the bias ratio is a precondition for differentiation
under all circumstances of operation. Essentially, in order for one drive wheel to rotate faster than the other, the
drive wheel having greater resistance to rotation slows with respect to the differential case and transfers torque to
the other wheel contributing to its faster rotation. The Torsen differential resists transfers of torque between drive
wheels in proportion to the torque applied to the differential housing, and this results in a larger proportion of the
applied torque being delivered to the slower rotating drive wheel. Therefore, bias ratio should be selected to
provide the maximum traction advantage that will still allow both drive wheels to deliver significant portions of
engine torque in turns.
However, even in turning situations, the Torsen differential enhances traction management. Since
torque is already distributed in increased proportion to the inside drive wheel, it is exceedingly
unlikely that the outside drive wheel will ever exceed available traction and 'spin up'. Alternatively, should the
torque of the inside wheel exceed available traction in a turn, it is equally unlikely for this wheel to 'spin up' since
such a 'spin up' would still require a difference in traction between drive wheels which exceeds the bias ratio.
Ordinarily, when the inside wheel exceeds available traction, differentiation ceases and torque is divided in more
even proportion between drive axles determined by the maximum torque that can be sustained by the inside drive
wheel. Thus, in all directions of travel, the Torsen differential will resist 'spin up' of either drive wheel by instantly
dividing torque between drive axles in proportions up to the bias ratio to match prevailing traction conditions.
</div></BLOCKQUOTE>

[Ian_N]

Torsens are a nice option, but have a look for differentials for bike engined cars, since these have come onto the market recently and a company in Germany called Drexler do a bespoke LSD for the FSAE compition (not cheap at 1200 Euros)

Since you mentioned possibly using a Cusco Differential (expensive and heavy) though it'd be a good idea to use a plate differential, using one from a car of S15 Silvia size on a SAE car is a tad overkill and would be rather heavy, especially since you'll either have to use the S15 output shafts or make bespoke ones and also have to account for the fact it's not a sealed unit.

There's a good few companies over here in the UK whom now do sealed units for motorbike powered cars and are about the same price or less as a cusco but easily half the weight or less (Sub 4kg).

Elite Transmissions, Drexler, Gripper are 3 i've looked into, and recently found the williams one as well.

Just a few considerations for you to think about when you look at diffs, is maximum tractive effort based upon maybe last years car or something similar to what you're proposing if you can get the weight split , CoG height and unsprung weights (build a spreadsheet with a free body diagram - big headache but well worth the effort!)
Maximum torque of your proposed engine and gear ratios of your first reduction / gearbox / final drive.

[Archyz]

I have discussed this question with a professor who has done a lot of research in the clutch type LSD. It is confirmed that LSD add more traction on the inner wheel. Just think of a car whose rear wheels are set on ice and dirt with a normal gear diff. The wheel on ice will spin faster than the one on dirt just like the outer wheel spins faster than the inner wheel in a corner. A LSD will enhance the torque on the sllow side, like the one on dirt or the one on the inside of a corner. So a understeer situation is bond to happen when using a LSD. However, I was told that by properly adjust the suspension can balance the steer situation. Unfortunately, I haven't found any reletive materials in this field. Can anyone help me with that? If it's a SAE paper, just tell me the NO.. I can get that from our database.

Thanks!

[Ian_N]

Theres 2 good SAE papers on a aluminium LSD, not sure if it'd be any help but the calculations do help a fair bit.

2000-01-1156 and 2001-01-0883

[TorqueWrench]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Hector:
The Torsen is capable of transferring more torque to the wheel with greater traction, but it has no way of inherently sensing which wheel has the greater traction.

In low-grip environments, since the slower wheel has greater traction, it sends torque to that wheel. It helps in true low-traction scenarios, but can make tight cornering difficult. </div></BLOCKQUOTE>

Thanks for clearing that up. I knew there was a reason I gave the differential to someone else to do.

[Archyz]

Thanks Ian_N, I have look into those three company and their diffs. Definitely better than the Cusco for a FSAE car. However, here is the ugly situation. In China it is not possible to make the gears in diff in any company. And the only already-made LSDs we can purchase nationwide are Cusco, the Torsen on Focus and the LSD on BMW M3. There is a dealership of Cusco in my town. So, if it is not so bad, I think I will go with the Cusco. Luckily enough, I got enough money for that.

The real question of Cusco is the weight. I wanna get rid of the original heavy steel shell, and rebuild it with aluminum. Also we need to design an light outer cover of the diff, to seal the lubricants. Maybe, we also need to throw out some of the friction plates and also a pair of pinion gears.

As for the half shafts, we plan to use the original CV that link with the Cusco diff and choose another rather small CV that link with the upright. Make the shaft with thin 40CrNiMoA steel wrapped with CF. Of course since we have limited resource of this method to build a half shaft, we ought to do some strength experiment about it.

By the way, the TBR of a Cusco clutch type LSD is very easy to be modified by changing the number of friction plates. If I can make sure the TBR of our car. It would be rather easy for us to get the TBR that we want.

Here is almost all the plan of our rear axle. Any suggestions is welcomed. Especially on details of the TBR design, spline made and ANYTHING else you wanna say.

And also, thanks for the advice of those SAE papers. I have already get them and begin to dig in.

[Hector]

Glad to see you're willing to read. Papers on TBR/tuning diffs/drivetrain are few and far between. When you find a good paper, you really get excited.

The aluminum housing is a great idea. We've run a setup similar to what you're describing now with great success. As far as TBR goes, if you can get adjustability in the 2:1 to 4:1 range, that would be as much as I think you would need. I could be wrong though, so other feel free to chime in.

It might be worth finding the same CVs for both the diff and upright so you aren't running more types of parts on your car. Your call. I understand the tight budget completely.

Good luck with the project. You're going to love FSAE.