Diffs vs solid rear axles

[Big Bird]

Was just re-reading the 4WD thread, and noticed that Queensland was considering ditching the diff completely. Sam if you are out there, would you like to expand? Are you guys going to a solid rear axle, go-kart style? Delft crew, is this one of the ways you got down to 135kg? Has anyone else got opinions to offer on this?

Cheers all, I look forward to your responses,

Geoff Pearson
RMIT FSAE 2003

Design it. Build it. Break it.

[AukeBaas]

hey geoff,

We did use a differential. We saved a lot of weight in our drivetrain using carbon fibre axles, flexplates and high grade aluminium differential. Not speaking of the weight savings of the rest of our car. http://fsae.com/groupee_common/emoticons/icon_smile.gif

Auke Jongbloed
Delft University of Technology
www.dutracing.nl (http://www.dutracing.nl)

[George]

Yes we are going no diff - a solid axle. All the parts have been made except the bearing hangers, which to be done next week. Nice weight saving of ~5kg.

We'll be assembling the car for the first time in a few weeks, so it won't be too long before we find out how its going to go. Should be quite interesting... but either way its going to be fast http://fsae.com/groupee_common/emoticons/icon_smile.gif

George
UQ Racing Team Leader 2003
www.uq.edu.au/fsae (http://www.uq.edu.au/fsae)

[awhittle]

After driving A-mod for years with a Quaif and now autocrossing a 125 shifter kart...I am not convinced that a spool would not be a better solution. The chassis would have to be set up to carry the inside rear in corners (or at least close) Just the opposite from the standard chassis setup. The Quaif setup does have less of a learning curve involved. A solid axle car only feels good at near full cornering power. Any less, and understeer sets in. At the limit, it feels great. Last year the winner of B-mod would have been 6th in f125.

AW

[Big Bird]

<BLOCKQUOTE class="ip-ubbcode-quote"><font size="-1">quote:</font><HR>Originally posted by George:
Yes we are going no diff - a solid axle. All the parts have been made except the bearing hangers, which to be done next week. Nice weight saving of ~5kg.


George
UQ Racing Team Leader 2003
http://www.uq.edu.au/fsae<HR></BLOCKQUOTE>

Thanks for that George, should be interesting to see how your car handles. I look forward to seeing it at the comp. As we've mentioned elsewhere we are going the lightweight single route this year, and everything else is coming down in weight - except the rear end. A solid axle was considered, but shelved due to conservatism!

I can see a few advantages. Light weight, cheap, easy to build, less setup hassles, reliability (less moving parts/seals etc), good camber control and a big advantage in packaging the rear of the car. Considering manufacturing and design issues we've experienced with the diff, I reckon the solid axle would put us two to three weeks ahead in the whole design & manuf phases. Which means two to three weeks of extra testing.

Disadvantages - potential understeer, tyre wear, wheelspin out of corners, having to run an aggressive steering axis geometry to get the outside front to drop / inside rear to lift going into corners. All obvious stuff really, but would be nice to hear from someone who has done it to see how much it really matters.

Thanks too awhittle for your input, although I have no clue what an A-mod is - are we talking oval track stuff here? I'm just a dumb Aussie!

Cheers all,

Geoff Pearson
RMIT FSAE 2003

Design it. Build it. Break it.

[awhittle]

Are we talking about a solid axle (all 50" or so) or are we talking about a 1 pc dif?

see www.ncs-stl.com/racecar (http://www.ncs-stl.com/racecar)

AW

[This message was edited by awhittle on August 02, 2003 at 03:14 PM.]

[karter]

Steering axix inclination 10deg, castor 10 to 16 deg, and a light enough car, ditch the diff. If the steering is too heavy start taking out some castor.

[Frank]

excuse George,

it's spool (IRS)....

yes, we're going for a stack of castor at the front (in the hope of getting the rear wheel up quick)

almost flat roll axis, in the hope of turning in faster

smaller camber gain in roll (that's small ratio of kingpin vertical distance to chassis pickup vertical distance.. sorry Claude you are wrong)

0 initial front camber

smaller track, minimum wheelbase

100% Ackerman at full lock, minimum "Ackerman progression"

Basically no roll centre movement

we're building "bizarro car" the exact opposite of our previous car

yes, some of the decisions are made to compensate for the spool...

we are building 5-laser alignment attachments to enable castor camber set-up changes, and a quick toe re-alignment (really hoping this will work)

the biggest problem we had with the Torsen is the need for rolled splines (which we couldn't find.. feel free to enlighten us on where to get 'em Aussie teams http://fsae.com/groupee_common/emoticons/icon_smile.gif )

i expect 4 weeks (my 3rd car) til the first drive

regards all

Frank

[George]

Woops.

Yeah bad terminology on my part, sorry guys. Spooled diff is where its at.

UQ Racing Team Leader 2003
www.uq.edu.au/fsae (http://www.uq.edu.au/fsae)

[MikeWaggoner at UW]

Think oversteer on corner exit from tons of castor (weight transfer under acceleration). You can drive around it, but...

http://dot.etec.wwu.edu/fsae/v28/image008.jpg

That is the OUTSIDE wheel.

UW FSAE
The views of Mike Waggoner are not necessarily the views of the UW FSAE team.

[OPP]

Ran our 2002 car without a diff in Feb. Driver feedback was positive, but near the end of the test session the drivetrain cacked itself. The decision to run with the Torsen again was made then.

[Test Driver]

Frank,

Your car won't be so "bizzaro". The description you provided is quite similar to our 98 car; low weight (~400lb), single, spool, nearly horizontal roll axis, low roll camber gain, narrow track, smallest WB allowed, etc.

When your car is done, put up some pics for us. Here is something to shoot for. Our car got 4th on skid pad http://fsae.com/groupee_common/emoticons/icon_wink.gif

Ramon Mendoza
BRIDGESTONE/FIRESTONE North American Tire, LLC
Ride and Handling Group
Texas Proving Ground
915-336-4834