Getting round the spool diff understeer

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KPS
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Joined: Tue Oct 11, 2016 10:20 pm

Getting round the spool diff understeer

Post by KPS » Fri Oct 06, 2017 3:54 pm

Hi everyone,

I was thinking about the spool diff on a Formula Student. One way to get around the horrible understeer was to apply jack up the inner rear tyre.

That's all good, but usually the problem is fighting that initial understeer and getting the car to roll and actually lifting off.

I thought of a few ways to counter it such as softening the front springs, increasing front negative camber, or increasing the roll centre rake.

Are there any tricks to fight that initial transient? I was thinking something in the lines of decreasing the front damping maybe?

Also considering that the rear inner tyre is now lifted, would it be of any use to introduce roll steer into the equation or would this make the car too unstable?

Would appreciated your thoughts on this!

Greg Locock
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Joined: Fri Jun 29, 2012 11:48 pm

Re: Getting round the spool diff understeer

Post by Greg Locock » Fri Oct 06, 2017 8:10 pm

Wish I was the judge for your team. Why did you select a spool diff? Fashion?

KPS
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Joined: Tue Oct 11, 2016 10:20 pm

Re: Getting round the spool diff understeer

Post by KPS » Fri Oct 06, 2017 9:55 pm

Hahaha good question :D

No, the main reason was it's easier and A LOT cheaper to make, and less mass, rotating mass that is. Only problem is that you have to work around the spool to make the car acceptable :?

bill shoe
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Location: Dallas, Texas, USA

Re: Getting round the spool diff understeer

Post by bill shoe » Fri Oct 06, 2017 10:56 pm

One way to work around spool-diff understeer on the skidpad is to simply use massive amounts of toe-out at the rear of the car. It should be easy to set lots of toe-out in a couple minutes with adjustable rear toe-links.

For the more transient events like autocross, endurance, etc.-- One way to reduce understeer from rear spool is to move weight distribution forward, more like 50/50 instead of traditional rear-bias. This adds weight to the front tires that cause yaw rotation, and it takes weight off rear tires that (with a spool) resist yaw rotation.

Another approach is to make rear track width extra tiny, and depend on front track width to pass the roll-over stability tests.

mrluke
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Re: Getting round the spool diff understeer

Post by mrluke » Fri Oct 06, 2017 11:29 pm

Its quite easy to get a rear wheel off with an overly soft front but it makes the back quite snappy. In practice I have found it nicer to have a stiffer front end because during initial turn in, the push from the fixed diff goes to the front outside wheel and tends to make that corner squat.

Its only on tight corners (full lock) that it really impacts your ability to just drive around it, anything higher speed is fine you just get slightly heavier steering.

Caveat being this is from a lardy bmw rather than Formula Student.

Greg Locock
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Joined: Fri Jun 29, 2012 11:48 pm

Re: Getting round the spool diff understeer

Post by Greg Locock » Fri Oct 06, 2017 11:59 pm

Thanks for the explanation. I think unloading the inside rear wheel vertically is the most natural form of reducing its steering effect, but that relies on rolling the body, which only occurs in response to latacc, which is too late subjectively, I think.

KPS
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Joined: Tue Oct 11, 2016 10:20 pm

Re: Getting round the spool diff understeer

Post by KPS » Sat Oct 07, 2017 3:26 pm

Thought so, I guess that's the only problem that will be very difficult to deal with, low speed corner entry...

Greg Locock
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Re: Getting round the spool diff understeer

Post by Greg Locock » Sun Oct 08, 2017 12:02 am

It might be worth looking at the tuning table in Milliken.

Jersey Tom
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Location: Huntersville, NC

Re: Getting round the spool diff understeer

Post by Jersey Tom » Sun Oct 08, 2017 12:43 am

My off-the-cuff take - you're pretty much boned here. Differentials can have a tremendously overpowering effect on handling (as you're noticing). Spool diff and tight radii turns - no bueno. I don't think shock changes or most conventional changes aren't going to amount to anything - you pretty much have what you have.

If I had to guess I'd think that you'd get the most out of driver line, more rear brake bias, and trail brake technique as far as being able to dump load on the front tires and unhook the back to get some rotation on early/mid entry. Beyond that, just realize that there are things you can't tune out - and think of it as, "Well if we're going to lose time entry-to-center, how can I make the most of this and gain time center-off back to throttle"
Grip is a four letter word. All opinions are my own and not those of current or previous employers.

Scootin159
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Re: Getting round the spool diff understeer

Post by Scootin159 » Sun Oct 08, 2017 2:40 am

I've been racing vehicles with one-piece rear axles (Formula 500 and Shifter Karts) for the better part of a decade now. Turn-in understeer will always be a challenge, and anything you do to fix it will tend to lead to corner-exit oversteer. However, it's not all doom and gloom. F500 is probably your best analog, since the cars are similar sizes (55" width, 80" wheelbase, 10x8" wheels), but we do carry considerably more weight (800# with driver), and have slightly more power (~135hp). None the less, F500 autocross speeds tend to be pretty close, albeit slightly behind, FSAE cars. According to the SCCA PAX system, F500 should be about 95% the speed of FSAE, and shifter karts (KZ2 spec) about 97%.

One of the biggest tricks that many people overlook is front end caster. We use a TON of it. Pretty much the more the merrier, as long as your driver can still turn the steering wheel. This is done to promote weight jacking - in the shifter kart I have enough of this that when the car is at full steering lock, the inside rear tire will be 2" off the ground while sitting still. With a soft suspension the effect will be minimized, but still remember you don't need to actually get the rear tire airborne, just reduce the weight on it so it can slide.

Since you'll have an independent rear suspension, another trick you can do is to run a massive rear sway bar. Combine this with a lot of body roll in the front, and you'll be lifting the rear tire in no time. However, one of your biggest benefits to a spool will be reduced weight - you may find that you'll be able to drop the weight even further by having your rear axle serve double-duty as your sway bar as well. We effectively do this (albeit as a requirement of the rules) in F500 by running a 50mm kart axle suspended by two rear uprights connected via a 4-link + watts link setup.

Tim.Wright
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Re: Getting round the spool diff understeer

Post by Tim.Wright » Sun Oct 08, 2017 1:13 pm

The only other things I can think of are droop limiter on the rear axle and to deliberately build torsional compliance into the driveshafts.
Not the engineer at Force India

KPS
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Re: Getting round the spool diff understeer

Post by KPS » Sun Oct 08, 2017 9:45 pm

I guess there aren't many options to fight that, and as Jersey Tom said, try and make up for it in another area. Thank you very much everyone for your input, interesting and helpful stuff :)

jschalch
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Re: Getting round the spool diff understeer

Post by jschalch » Mon Oct 09, 2017 12:00 am

Many teams in FSAE and Formula Student run a spool. I honestly would say it's not a bad choice for a young team that doesn't know how to tune a diff, is a young team with few members, or just is focusing on another part of the car and don't have the resources to get a diff. Some of the best teams have run a spool, and if you want to win, get the car done quick and drive it all day and night. If running a spool will get the car done a lot earlier then I'm sure it would be a better option and your drivers will be faster just from the extra seat time.

Many of the other replies have had great info just wanting to add. You can change the balance of the car if you're running aero on the car for low and higher speed corners. You can adjust the toe as stated above. You could even change roll center heights in your suspension to induce more oversteer. I have seen many teams use a spool and pick up the rear wheel by using softer springs in the front. I have also seen teams not pick up the rear tire and still oversteer. I would say look at the 2016 GFR car they ran a spool and was sliding all over the place, maybe that can spark some ideas.

roon
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Re: Getting round the spool diff understeer

Post by roon » Tue Oct 10, 2017 2:38 am

-Removing a driveshaft on one side would eliminate your problem (assuming IRS) while possibly causing others...

-If linking corners is legal, mechanically or hydraulically, a compressed front corner could simultaneously lift up the inside/unloaded rear wheel.

-If the inside rear wheel could alter its camber drastically when needed, its tire contact patch would reduce, reducing it's available traction & influence. I'm thinking of something like a variable length control arm or moveable attachment point. Actuated by compression of the outside suspension members, or via a second rack-and-pinion attached to your steering. Turning the wheel counter-clockwise would pull in the upper rear control arm on the driver's left side, and vise versa, for example.

factory_p
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Re: Getting round the spool diff understeer

Post by factory_p » Tue Oct 10, 2017 10:33 am

roon wrote:
Tue Oct 10, 2017 2:38 am
-If the inside rear wheel could alter its camber drastically when needed, its tire contact patch would reduce, reducing it's available traction & influence.
Perhaps if you can fix a rear geometry where camber and toe variations are important on droop travel, as soon as you'll start unloading the inside rear, it will lose grip. Be careful not to make a dangerous car under braking though, especially with toe variations.