Spool Diff

[N.Tsuji]

Hi, I am a Japanese student.
Our team is thinking about using spool differential next year.
So, I want to know about spool diff.
What is the fastest car using spool diff?
If I remember correctly, there is a team which is using spool diff with very special CV joints (carbon plate?).
But I can't remember which team it is.
Could someone tell me about it?

[N.Tsuji]

Hi, I am a Japanese student.
Our team is thinking about using spool differential next year.
So, I want to know about spool diff.
What is the fastest car using spool diff?
If I remember correctly, there is a team which is using spool diff with very special CV joints (carbon plate?).
But I can't remember which team it is.
Could someone tell me about it?

[Jersey Tom]

My first question would be, why are you thinking about using a spool 'differential' ? Especially if you're looking to know more about it.. would make more sense to me to learn about differentials in general.. spool, open, HLSD, VLSD, and go from there..

[N.Tsuji]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Jersey Tom:
My first question would be, why are you thinking about using a spool 'differential' ? Especially if you're looking to know more about it.. would make more sense to me to learn about differentials in general.. spool, open, HLSD, VLSD, and go from there.. </div></BLOCKQUOTE>
We want to maximize the drive efficiency.
We do not want to lose some power with normal differential.
So, we are thinking about using a spool diff.

[Mike Flitcraft]

have fun in turns....

[Chris Allbee]

First off, list out the compromises a diff must make. Look at what would be the most beneficial in the different phases around a race track, including forward acceleration, braking, and cornering. What would you like the rear axles to be doing in each of those phases? Which of those phases do you think is more important to your over-all performance? Obviously, your car set-up will be a compromise intended to provide adequate performance in each of those phases on the track and the diff will be just another compromise and variable to tune around.

Once you have a better idea of the vision for your car performance-wise then you can begin to decide what kind of character you want to have the car to have while it is being driven (for example: do you favor high speed corners, tight corners, slaloms and chicanes, or the straights?)

after that look at the type of diff that will best give your car the character you want it to have, then learn how to tune around the drawbacks that the diff will have.

So, after you see what you would like the car to be doing look at the spool again and see if that is what you would really like to have.

And remember, more detailed and specific questions will get more detailed and specific answers, so if you have already considered all of this, let us know so we don't tell you something you already know.

[N.Tsuji]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Chris Allbee:
First off, list out the compromises a diff must make. Look at what would be the most beneficial in the different phases around a race track, including forward acceleration, braking, and cornering. What would you like the rear axles to be doing in each of those phases? Which of those phases do you think is more important to your over-all performance? Obviously, your car set-up will be a compromise intended to provide adequate performance in each of those phases on the track and the diff will be just another compromise and variable to tune around.

Once you have a better idea of the vision for your car performance-wise then you can begin to decide what kind of character you want to have the car to have while it is being driven (for example: do you favor high speed corners, tight corners, slaloms and chicanes, or the straights?)

after that look at the type of diff that will best give your car the character you want it to have, then learn how to tune around the drawbacks that the diff will have.

So, after you see what you would like the car to be doing look at the spool again and see if that is what you would really like to have.

And remember, more detailed and specific questions will get more detailed and specific answers, so if you have already considered all of this, let us know so we don't tell you something you already know. </div></BLOCKQUOTE>
We know pros and cons of many types of diff.
And we are thinking about using the spool diff in spite of all the fault it has.
We want to see as many example of F-SAE car using spool diff as possible.

[murpia]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by N.Tsuji:
We want to maximize the drive efficiency.
We do not want to lose some power with normal differential.
So, we are thinking about using a spool diff. </div></BLOCKQUOTE>
While it's true that some energy is wasted in a limited slip diff in the form of friction heat, this is insignificant in terms of drive efficiency. Do the calculations and you'll see for yourself. This is not a good justification for a spool, in fact I expect you'll waste far more energy in tyre scrub than you'll ever save in diff friction.

The whole reason for having a diff is related to cornering performance and car balance. Again do the calculations or a simulation, and you can make an informed decision.

Regards, Ian

[drivetrainUW-Platt]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Jersey Tom:
My first question would be, why are you thinking about using a spool 'differential' ? Especially if you're looking to know more about it.. would make more sense to me to learn about differentials in general.. spool, open, HLSD, VLSD, and go from there.. </div></BLOCKQUOTE>

Drifting.

With a properly tuned suspension a spool can work ok. You dont need oil in it so no leaks. It will also be cheaper/easier to make.

Will you be runing cv joints?

[Welfares]

A university running carbon plates and a spool diff? That would be us i think, unless there's somone over in the states with the same combination.

I might also tentatively claim that we are the quickest, although we are still a long way behind that yellow rocket from out west.

And we don't call it a spool, we like to refer to it as our "infinately biasing differential".(which gives me an idea for a thread)

What do you want to know about it?
Basically is.... basic, just a member that transmits torque with no speed biasing whatso-ever. Its main advantage is obviously mechanical simplicity/lack of mass.

We tested with it as we thought it would solve an issue we were having with strange things going on in our suspension.

For anyone who didn't already notice, the torsen IS NOT, a limo diff, its torque biasing, but if you lift a wheel, 4*0 = 0, so in will act like an open diff if you lift a wheel.

We use it because it works for us, but beware that you will have to tune AROUND it. Reducing drivtrain losses is not really a reason to use it, a torsen will act like a spool and loose you nothing and a limo will loose you not much.

So, there's something, there is literature on diffs, check it out, and good luck. Hope to see you guys down under again in December.

[Travis Garrison]

How many of you lift your inside rears in corners?

http://dot.etec.wwu.edu/fsae/HostedPics/2007_FSAE_West/Best_Of/Best_Of_Dynamic/images/IMG_2422_big_lift.jpg
(courtesy of James Waltman)

...granted I wasn't there this year and there maybe some other reason for KU lifting it's leg on that cone.

A lot of schools run narrow(ish) tracks to help them optimize lines, a lot of those same schools aren't really using the torsen in the corners. *Edit: I should say it would apear a lot of schools aren't using their diff in the corners, I may not be seeing the whole picture.*

[N.Tsuji]

Thank you guys.
I really appreciate your replies.
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Welfares:
A university running carbon plates and a spool diff? That would be us i think, unless there's somone over in the states with the same combination.
</div></BLOCKQUOTE>
Yes, I think it is your team.
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Welfares:
What do you want to know about it?
</div></BLOCKQUOTE>
Is there some special suspension geometry for the spool diff?
Of course, you do not need to say it if it is a secret.
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Welfares:
Hope to see you guys down under again in December.
</div></BLOCKQUOTE>
Unfortunately, our team do not have enough money to go abroad.

[Ashley Denmead]

One approach for susp geometry that works with a spool could be to have an inclined roll axis so that the lateral weight transfer is different from the front to rear during cornering. Similar effect to a narrower rear track. This of course assumes your goal is to unload a largish percentage of normal force off the inside rear tyre to allow it to slip.....maybe not quite so much as the car in the pic above though...http://fsae.com/groupee_common/emoticons/icon_wink.gif

Ash

[Welfares]

Well, like i said, it solved a problem we had.

Look at those discs, and assume they are reasonably stiff, (say 20% of wheel rate) and think about what they will do to your effective spring rate in droop.

[PatClarke]

Hi all,
Several teams have used a spool over the years, some highly successfully when they understood how to maximise it.

A spool choice can be driven by several considerations, and is a very good way to take several kilograms and a thousand bucks out of the car. Rotational MoI is also reduced and there is one less thing to leak or break, and the car can be driven with drive to one wheel if necessary (As done by Welfares team successfully at an event in the dim distant past)

An example would be the University of Queensland car that was fastest in the dynamic events at Formula Student Germany in 2006. Several knowledgeable observers watched this car intently to see the performance characteristics of a locked rear end.

In simple terms, the car was plain FAST and unspectacular. It did nothing untoward in corners as the team had learned to set it up and learned to drive it. It looked very easy for the driver to catch oversteer slides, and put the power down very well in the acceleration event.
The car did NOT hike a wheel as illustrated by Travis, nor was it nasty when the course was wet.

I understand how the team have done this, but it is their knowledge, not for me to discuss here.
Simply said, the car DOES have a differential, but it is in the tyre slip angles rather than located between the rear wheels.

Mike, don't be so quick to dismiss the idea...It can be made to work very well and save you enough to buy an AK47 and a couple of banana clips of 7.62 ammo.

Cheers
Pat

[Mike Flitcraft]

I come from an off-roading background, and started to deal with drag racing in high school. Spools in those two arenas, absolutely awesome. I simply don't see a car where the two rear wheels are forced to spin at the same speed will handle better than the same car where the two rear wheels are allowed to differentiate the speed differences. I've been wrong plenty of times before, and could be just as wrong about this one but I just don't see it working as well as some of the other options.



Oh, and that AK47 with ammo costs less than a chicken in some parts of the world. However, I'm more of a fan of the H&K H416 or the FN FAL myself.

[James Waltman]

I think that there's a flaw with the assumption that the inside rear tire on these cars is heavily loaded through the entire corner.

That picture of Kansas that Travis posted above was not an isolated incident.
To further counter the idea that these cars must have a differential to get around corners, I've assembled a few pictures from last week in Fontana. (Pics by Dan G, Flexicoker, and myself).

For your consideration I present:
Diff'd Cars That Pretend They Have A Spool:
Texas A&M (http://smg.photobucket.com/albums/v237/Flexicoker/FSAE%20West%202007/?action=view&current=IMG_1721.jpg)
Oklahoma (http://smg.photobucket.com/albums/v237/Flexicoker/FSAE%20West%202007/?action=view&current=IMG_1652.jpg)
Kansas again (http://smg.photobucket.com/albums/v237/Flexicoker/FSAE%20West%202007/?action=view&current=IMG_1788.jpg)
Michigan State (http://evilengineering.com/gallery/v/SAE/FSAE/WEST07/IMG_4296.jpg.html?g2_imageViewsIndex=1)
Michigan State again (http://dot.etec.wwu.edu/fsae/HostedPics/2007_FSAE_West/Best_Of/Best_Of_Dynamic/pages/IMG_2376_lifting.htm)
Ohio State (http://dot.etec.wwu.edu/fsae/HostedPics/2007_FSAE_West/Best_Of/Best_Of_Dynamic/pages/IMG_2322_engine_smoke_and_air.htm)
Chalmers (http://dot.etec.wwu.edu/fsae/HostedPics/2007_FSAE_West/Best_Of/Best_Of_Dynamic/pages/IMG_2164_Chalmers_air_under_rear.htm)
Maryland (http://dot.etec.wwu.edu/fsae/HostedPics/2007_FSAE_West/Best_Of/Best_Of_Dynamic/pages/IMG_2393_lifting.htm)
Lehigh (http://dot.etec.wwu.edu/fsae/HostedPics/2007_FSAE_West/Best_Of/Best_Of_Dynamic/pages/IMG_2437_big_lift_rear.htm)
Toledo (http://dot.etec.wwu.edu/fsae/HostedPics/2007_FSAE_West/Best_Of/Best_Of_Dynamic/pages/IMG_3271_rear_lift.htm)
Tokyo Denki (http://dot.etec.wwu.edu/fsae/HostedPics/2007_FSAE_West/Best_Of/Best_Of_Dynamic/pages/IMG_2498_big_lift.htm)
Tennessee Tech (http://dot.etec.wwu.edu/fsae/HostedPics/2007_FSAE_West/Best_Of/Best_Of_Dynamic/pages/IMG_3620_lifting_rear.htm)

These images are not miracle shots of a one time event. It was happening a lot and it was happening to more than these cars. These are just the ones that are fresh in my mind and have photographic evidence.

So, for mid-corner, I think that we can safely say that the spool won't be a hindrance. I mean, these cars all had differentials (as far as I know) and they couldn't be bothered to keep all four tires on the ground. Surely a team with a spool wouldn't have too much trouble getting the inside rear tire to unload a little.

Now, this isn't the whole story. On corner entry the tires are more evenly loaded. You'll want to design and tune for this – it can be done. You'll might want different caster, stiff rear roll, limited rear suspension droop, etc.

The driving style is a little different for a FSAE car with a spool. Yes, I know from experience – I've driven two different FSAE spool cars. WWU cars all had spools before we got there and thought we knew better (funny how often that happens huh).

I'm not saying that there won't be trade offs with a spool. There are a lot of advantages too though.

[drivetrainUW-Platt]

Awsome pict collection James.

I think this is also a good argument to having duel rear brakes. If you have one wheel in the air and one on the ground, and a differential with a brake on it, guess what, the wheels will counter rotate. I saw this a number of times being a corner worker in Autocross.

[mtg]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Chris Allbee:
First off....&lt;lots of technical jargon&gt; </div></BLOCKQUOTE>

I'm suspicious....all of that sound engineering coming from this guy? Ha ha, joking.

http://dot.etec.wwu.edu/fsae/HostedPics/2007_FSAE_West/Wednesday/images/IMG_1318.jpg

[John Stimpson]

Kettering's Mahle transaxle used a Torsen, which locked up after 1 hour of use... We realized this after the AX, and retuned our suspension to limit our rear droop travel, so that we too, could pull the inside rear off the ground... This helped our understeer condition considerably.

[Kirk Feldkamp]

Could this lifting wheels fiasco be a rear damping issue as well? The spot where James took all the wheel lift photos was right where there was a quick uphill section (read: the front right wheel is hitting a big bump before the rear gets there). I'm thinking it could be possible this is being caused by front shock bump being relatively soft compared to rear shock rebound.

-Kirk

[VFR750R]

were these cars lifting their inside rears from apex off? Not likely(although inside front could have been), and there a diff is still a performance variable.

[James Waltman]

Yeah, I was set up at the sharp/uphill/bumpy corner for many pictures.
That's where the action was.
That doesn't explain the cars that were doing it on the flat sweepers though.
In the images above, these ones were on flat sections: Oklahoma, Kansas again, Michigan State?, Chalmers, Toledo, and Tennessee Tech.

[mtg]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by turbotwig:
Could this lifting wheels fiasco be a rear damping issue as well? The spot where James took all the wheel lift photos was right where there was a quick uphill section (read: the front right wheel is hitting a big bump before the rear gets there). I'm thinking it could be possible this is being caused by front shock bump being relatively soft compared to rear shock rebound.

-Kirk </div></BLOCKQUOTE>

I think it boils down to sticky tires and a car with a small footprint- you're going to have enough grip to pick up tires. The damper issue could help it out though.

[Welfares]

Ah, damn, how could i forget the rocket that Queensland took to Germany, i was in the same shed while Luke spend a week there putting it together. A good car handled by some Very good drivers.

The long ago incident Pat refered to would be when someone made the driveshafts too short and the end of the splines sheared off the driveshafts for the torsen.

This was solved by bolting one CV directly to the sprocket. http://fsae.com/groupee_common/emoticons/icon_biggrin.gif Good for fuel economy.

NB: I do not recomend this solution.

[Ashley Denmead]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Welfares:
Ah, damn, how could i forget the rocket that Queensland took to Germany, i was in the same shed while Luke spend a week there putting it together. A good car handled by some Very good drivers.

The long ago incident Pat refered to would be when someone made the driveshafts too short and the end of the splines sheared off the driveshafts for the torsen.

This was solved by bolting one CV directly to the sprocket. http://fsae.com/groupee_common/emoticons/icon_biggrin.gif Good for fuel economy.

NB: I do not recomend this solution. </div></BLOCKQUOTE>

That wasnt a shed....it was a series of shipping containers strewn together!! You're right though it IS amazing how luke managed to put that car together and not mess it up too badly.... http://fsae.com/groupee_common/emoticons/icon_wink.gif he did learn one valuable lesson though, that being that it does matter which wire you connect to which race scale! lots of good memories.

[Chris Allbee]

Nice pic http://fsae.com/groupee_common/emoticons/icon_wink.gif

Anywho, at least for our car the inside wheel issue is definitely related to the castor (sp?). When testing the thing we noticed that beyond a certain steering angle the inside tire would unload dramatically and we would get lovely whisps of white smoke upon power application. The onset occurred at different steering angles for different lateral G turns.

hmmmm....gives me an idea...Steering Angle Vs. Lateral G vs. Weight Transfer plot. Might help lowly engine guys like myself make more sense of this dynamic foolery called suspension http://fsae.com/groupee_common/emoticons/icon_smile.gif