Differential vs Spool

[LCT]

I'm well aware of the benefits of running a differential, but I haven't found anything in the way of actual test data comparing the two (lap times, braking, etc...). Does anyone know where I could find some actual test data or math that compares the two?

[LCT]

I'm well aware of the benefits of running a differential, but I haven't found anything in the way of actual test data comparing the two (lap times, braking, etc...). Does anyone know where I could find some actual test data or math that compares the two?

[Jersey Tom]

I cant imagine anyone has purpose built a car and back to back tested this, much less had data. I could certainly be wrong.

You could also certainly simulate this.

[Charlie]

Good luck finding any true back-to-back lap time tests of major design concepts in FSAE. As a former judge it's something I would love to see but have never found.

[LCT]

I realize that this data is pretty hard to come by. My hope is that a team who broke their differential at/before competition and had to use a spool might have some data or advice.

[J. Vinella]

We did it back in '07. Sent the car out with at the time a Torsen II, then changed out the diff and put in our Spool. Did this for two of our regular drives and an alumni drive to get good data and unbiased feedback.

One can argue the car was not totally set up for the spool each time we put it in. I wish we could adjust caster quickly...and a million other things. The data we got and the tire temps were interesting. Even the spool as a prototype was about 8 lbs lighter than the now "heavy" Torsen. Our current diff is much lighter.

If you run a diff, it is not hard to make a spool and test it. From memory UB runs a solid rear??? A bit harder to just make a diff.

As far as simulating it, I say this all the time: you cannot break the laws of physics on the racetrack like you can on your computer. Nothing beats testing and a stop watch.

[LCT]

We do run a solid rear at the moment, but I'm looking into the possibility of changing over to a live axle. The main concern right now is whether or not the extra 8-10 lbs of rotating (and un-sprung for our current suspension setup) outweighs the benefits of having a differential. Was there a significant difference between runs, or was the performance of the two about the same?

[J. Vinella]

We are currently running a diff and for '07 running a diff kind of answers the question. Is the weight worth it? Penn State (289 lbs at MIS) running a diff.


Don't do something just because others are doing it. Try it out for yourself you might learn something.

[rjwoods77]

The whole point behind going with a solid axle was to get the car to perform well without the cost or weight of a differential. To add a diff negates the concept completely so either run a solid axle or an IRS. The only other possibilty is to run a de-dion but that is a complitcated setup of its own. I think you need to take the advice of the people here and what I have told the team a thousand times. Test what you have to great detail and log what is going on and discover the true deficiencies and find out if there is a working solution. Dont go looking into other design alternatives until you have proven the concept can no longer be dealt with. An undertuned engine, undertuned cvt, undertuned suspension netted UB an 18th finish. 6 months of development would have landed you in the top 10. Instead reengineering a system that wasnt fully understood took place and for that reason the car wasnt completed and the team went from 18th to zero in a year. Any of the people on this site that have any experience will tell you that development of subsystems/vehicles wins competitions. Not the next bright and shiny. My suggestion would be to screw the diff idea, build a set of adjustable rear and front anti roll bars and go test the car for 3 months straight and find out what needs to be done. You will find yourself with a car that handles good without the waste in time and money on the next bright idea that was based on assumptions and not data.

[Mike Cook]

I ran our 08 car with both a taylor diff and a spool. The spool is better at some thing but a lot worse at others. To get it to work well you need to treat the car like a shifter kart, which means lots of caster and scrub, maybe droop limiters, stiff rear suspension, ect. The cool thing about karts is that they are really easy to adjust quickly, which makes dealing with the spool not that bad. If a fsae car doesn't have all that built in adjustment, it can be tough to get it to handle right.

Bottom line, the best solution is somewhere between a taylor and a spool.

Oh, and one last thing, if it rains your setup for the spool will completely change while with a LS diff, the setup won't change nearly as much. We had the spool in at the alabama redbull shootout thing and it was wet out and this just killed us.

Oh and a spool car sucks to push around a parking lot.

That is all.
For now.

[RacingManiac]

IMO if you have a incompetent drivetrain team or are hurting for manpower you might as well try to make a spool work....less stuff to get f-ed up....http://fsae.com/groupee_common/emoticons/icon_wink.gif

[J.R.]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Mike Cook:
To get it to work well you need to treat the car like a shifter kart, which means lots of caster and scrub, maybe droop limiters, stiff rear suspension, ect. </div></BLOCKQUOTE>

Caster for Load Transfer of Inside Rear, I'm guessing. Why the scrub? Droop limiter to avoid stress on dampers if IR lifts? Why the stiff rear?

[Mike Macie]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by J.R.:
Caster for Load Transfer of Inside Rear, I'm guessing. Why the scrub? Droop limiter to avoid stress on dampers if IR lifts? Why the stiff rear? </div></BLOCKQUOTE>

People always say caster to help diagonal LT but it seems pointless the way i try to calculate it. I think of it as a teedering affect as the contact patch would move vertically with steering. With 6-9 degrees of caster the contact patch barely moves. From my memory, around 0.1" with 9 degrees and 20 degrees of steering. Maybe there is something I'm forgetting?

[Mike Cook]

The contact patch is rotating around the king pin axis. Lets say you don't have any mechanical trail or scrub, then the contact patch lies on the axis of rotation. When you steer there will be no jacking. If you add scrub, the tire contact patch will now swing rewards and down (for the inside tire). This wedges the car and forces weight off of the inside rear tire. Lifting the inside tire off the ground is essential when running a spool because if there is much weight on the inside rear tire, it will tend to push the car straight ahead and make it understeer. This is why shifter karts have lots of different front wheel spacers, so they can change the amount of scrub they have which changes the handiling significantly.

Since we still have suspensions and karts don't, the caster will only do so much in terms of transfering weight from the inside rear to outside. Thus you need some other method to transfer the weight. Lots of spring or bar could do this. The problem is that both of those methods rely on the car rolling. In a slalom, the car doesn't have the time to fully roll, so your handiling is more decided by your dampers and roll centers. Think of droop limiters like running a ton of rear rebound. When the car starts to have roll velocity, the inside rear tire will not be allowed to droop. This will pick it off the ground and transfer all the weight to the outside rear tire. This will help it get through the slalom with out plowing.

[Mike Macie]

I guess my point is that with average fsae values for scrub radius and caster, the amount of "wedge" gained is so small that it has a negligible affect on weight transfer. About .1" of wedge. Played with some sketches I've done in the past and just checked in adams. With something high like 10 degrees of caster and a 5" scrub radius you will only add around .3" of wedge at 20 degrees. I just think it's too small to make a difference.

[billywight]

It's not the distance that matters, it's the weight transfer (though the two are certainly related). Can you look at the weight transfer difference in your Adams model?

[Mike Cook]

Rough math:

.1" * 150lb/in = 15lb /2 = 7.5lb on inside front and 7.5 on outside rear. with a 600lb car starting with a 50%cross, this would result in a 52.5% cross.