FSAE general diff selection

[BrendanS]

I am a student in a FSAE-A team currently doing a thesis on a custom diff build, so far I want to go with a Torsen styled diff. I have spoken to some past members of the comp and they said that the Torsen "FSAE special" was quite popular a number of years ago, yet was phased out by spool and clutch type lsds (particularly the drexler unit). I was wondering if anyone care to share their personal experiences/opinions on why this is the case?

Also any issues anyone found with using the FSAE special would be helpful.

[Paul Achard]

Hi Brendan,

Which team are you from?
When I first started studying differentials, I found this document by Taylor Race Engineering to e super helpful:
http://www.taylor-race.com/pdf/understanding_differentials.pdf

From there a few extra good sources have been:
-RCVD section on diffs
-The rest of RCVD
-OptimumG techtip on tuning differentials (http://www.optimumg.com/docs/DifferentialAnalysis_BertaReport.pdf)

Most vehicle dynamics books have their own section on diffs and most have their own little flavour to it. This isn't a bad thing since diffs are hard to get your head around.

To answer your question, I would say many teams choose the Drexler since it presents a lightweight (relative to any other diff out there) and easily packageable solution.
It does not necessarily perform better than Torsens, but it is more easily adjustable. It sometimes has trouble maintaining its adjustment however.

Personally if I were you I would buy the Drexler and design a reliable mounting arragement. I would then use the extra time understanding what the hell you want the diff to do and implementing simple tests on the track.

If you guys are running data acquisition try to get wheelspeed sensors on all wheels as well as TPS and steering angle, there is a lot to work with right there.

[mech5496]

Basically what Paul said. I know my team switched from a Torsen to a clutch-type LSD back in 2007, because our car at the time had a tendency to heavily unload (essentially lift) its' inner rear wheel, which caused no power to be transmitted on the wheel that had traction during corner exit. The Drexler unit is really small and light, but IMO really overpriced. There are numerous options out there, either you choose a torque-biased or a clutchpack LSD, an you can design your own housing. Hint: take a look at utility ATV diffs... ;)

[BrendanS]

Thanks for the help guys.

I'm from UQR in Queesland. Paul those documents were very helpful, I hadn't yet found a decent document relating different types of diffs and their affects on vehicle dynamics. We run a very tight budget so a drexler is definitely out of the question for us. After doing some more research I'm leaning towards a quaife design particularly the wavetrac design (http://www.wavetrac.net/technical.htm). I believe this will overcome the problem of no power to the grounded wheel whilst cornering as our car this year has been built for a spool (I realise this isn't ideal for running a diff, however my plan is to obtain data from installing the unit in this year's vehicle and building the following year's car to use a diff).

[Z]

Brendan,

The type of differential you choose is a major decision in the FSAE design process.

They can have a bigger effect on general performance than many of the "higher profile" parts such as engine, suspension, etc. For example, fit a spool to a car that is not suited to a spool (eg. wrong weight distribution, LLTD, steering geometry, amount of power, aero or not, +++), and the car will be a dog. Likewise if you fit an open-diff to a car not suited to it (well, in this case the car is just slow...).

As Harry said, a Torsen style diff is similar to an open-diff. It will spin the inside wheel whenever that wheel is off the ground, and then no (or very little) thrust to the other wheel.

Anyway, lots of threads on this Forum covering diffs. Search for "Weismann diff" to see one that was up recently.

Z

[AxelRipper]

For some more papers, there's a few good sections on diff design in Tune to Win and RCVD. However, there's no "This is better than that" explanation anywhere. It all comes down to how it effects the rest of the system (just like everything else in, well, everything else). The real 'University Special' diff has falled out of fashion in recent times doe to the weight and ease of use of the ready built Taylor diffs. The Torsen was originally designed for an Audi IIRC and is a little overbuilt for the application. I think a scaled down version of it would be sweet, but we always had other, more important things to worry about on the car other than designing a custom diff.

[scotty young Taylor Race]

I posted a paper on our face book page giving some insight to the mechanics of our TRE/Quaife ATB differential.
Unlike a Salbury type diff there is no adjustment. All the adjustment is made through the chassis.
The diff characteristics will remain the same from day one to day 1000. There are no parts to wear out. It is all gear driven.

I also have a pretty good article that Mr Craig Taylor wrote just before he retired explaining the different types of differentials.

https://www.facebook.com/permalink.php?story_fbid=600239743360473&id=139126679471784&comment_id=6087619&offset=0&total_comments=1&notif_t=feed_comment

[BrendanS]

Scotty,
I've been playing with a differential the team currently has and it turns out to be from Taylor Racing! I intend to use this as a platform to analyse and try and incorporate the Wavetrac design.
http://www.wavetrac.net/technical.htm

I have next to no information about it and I can't seem to find any from your website. In case there is more than one designed for chain drive, we bought it in 2009 for roughly $1200AU, would you have any info on that unit still? I was particularly interested in the torque bias ratio and torque rating the unit is designed for.

[scotty young Taylor Race]

Brendan

Bias ratio is 0 to 80% . It is rated at 200 ft-lbs . The Wave track is a Quaife knock off... That is Mike Quaifs design.
Paul, the Taylor Race Engineering MK diff is not only lighter . it is complete... there is no need to to make stub axle , sprocket flanges and source bearings. It is the only complete package you can buy .. and it is less then 10 pounds.
The new silicon fluid eliminates inside wheel spine , Ask Kansas , Marlyand , SDSM , UTA just to name a few what they think,.
And speaking of presents.... We have attended every FSAE comp here in the sates for last 13 years. When is the last time you saw someone from Drexler....
I will now step off the soap box.......

[jlangholzj]

I'll step back up on the box for scotty. We've been using several different revisions of the Taylor units for the last 8 years. Always have been happy with the quality and level of service from them. Personally, I prefer the quaif "style" differential anyway. From what I've seen they provide a much better torque bias between your rear tires and have no chattering or excessive locking issues. It is also very entertaining to talk to one of our better drivers about car performance. When an inside tire *does* lift he's able to feel it almost instantly...not due to loss of torque but just from the nature of the Quaif. Durability is also amazing. To name a few failures: bent/broken diff mounts, Grenade'd sprockets, failed tripod bearings(that one was our fault) and broken rear a-arms the diff has always held up.

all things aside, as a personal preference I ALWAYS will work with a company that has superior customer service. Companies like Taylor, Precision Auto Research, MoTec and SKF have always treated me (us) on a 1-on-1 basis. They've never been late and are always willing to work with the customer based on their needs. I've worked for companies personally that are on both sides....both amazing customer service and terrible customer service. From an employee side as well as a customer side stuff always gets done better and faster when you work with a "good" company. On that note, I'll gladly pay "extra" for a "similar" product from a company like that. Note how (extra and similar) are in quotes....thats mostly because their both typically "less" or "the same as" for a "better" product ;)

All in all, I think one of the more valuable points being made is by Z. Make sure you talk to your frame/suspension guy and make sure you've got a setup that works!