[rocwandrer]

I have a specific, pressing reason for asking about this, but it would probably be interesting data for everyone if we can get enough responses to sort of see the trends....

What I'm interested in is STR spyders, with and without LSD. I know I was able to setup my Spyder in DP (admittedly under prepped) so that there was no need for an LSD, and the car was faster because of not having one. It will be tougher, but I'm trying to get a sense of whether that might be possible (in a compromised way) in STR.

Questions/Poll (responses contribute even if some of it doesn't apply to you):

If no LSD
1.)Do you encounter 1st gear corners on the courses you run?
2.)How often/how much is wheel spin frustrating you or hurting times, etc?
3.) What are your springs and (if applicable) anti-roll/swaybars?

If LSD,
A.) Do you encounter 1st gear corners on the courses you run?
B.)Do you find you wish the car had less understeer in your lowest speed corners?
C.)Spring and bar rates?

 

[sldnrck]

No LSD.
1. Never encountered a first gear corner yet.
2. Wheel spin causes issues on faster mid corner and corner exit and not being able to control the rear. Kind of used to wheel spin on slow corner exits from front wheel drive. It would be nice to be able to get on gas earlier.
3. front 500, rear 450, 22mm front sway, no rear sway.

Can you explain how the car was faster by not having a LSD? I don't see how this is possible.

 

[rocwandrer]

I can try to explain. Some of this goes against conventional wisdom. I'm not infallible, but conventional wisdom also isn't necessarily a good source of info

The explanation also varies depending on the type of LSD, but the conclusion is similar.

First, how a differential actually works (from a rubber meets the road perspective, not the mechanical inner workings, which are cool but irrelevant to going fast):

Lets start with the case of torque application below the critical torque for the LSD to bias the correct direction:

Say you want to put down 200 ft-lb (after leverage multiplication through the drivetrain) at part throttle on corner exit on a path that is (instantaneously) a 50 ft radius corner.

Say the inside tire can support 200 ft-lb worth of resistance.

OPEN DIFF:
The inside tire turns at 9% lower speed than the outside tire, and both have a scrub percentage around 3%. Energy wasted in the diff is negligible, extra heat in the tires from acceleration torque is negligible, extra wear on the tires from acceleration is negligible. The torque on the inside and outside tires is 100 ft-lb. The sum of the moments about the Z axis due to drive thrust is 0.

Clutch type LSD:
The inside tire turns at 8% lower speed than the outside tire, and the inside tire has a scrub percentage around 4% with the outside at 2% (wild guesses on the numbers). Energy wasted in the diff is not huge, extra heat in the tires from acceleration torque is negligible, extra wear on the tires from the fight going on in the LSD is probably not huge. The sum of the torque on the inside and outside tires is around 200 ft-lb, with the INSIDE tire carrying more torque than the outside.

Gear type LSD (several simplifications here, but the gist of it is correct):
The inside tire turns at around 8% (wild guess) lower speed than the outside tire, and the inside tire has a scrub percentage around 4% with the outside at 2%. Energy wasted in the diff is negligible, extra heat in the tires from acceleration torque is negligible, extra wear on the tires from the fight going on in the LSD is probably not huge. The sum of the torque on the inside and outside tires is around 200 ft-lb, with the INSIDE tire carrying more torque than the outside congruent with the bias ratio.

What's going on here? The LSD doesn't know which direction you are turning or if tires are spinning or anything. It only knows 6 parameters, 1 input speed, 2 output speeds and 3 torque values. In the above case, the torque is going the wrong way until the inside tire spins up to match the outside. Neither clutch type nor gear type can leverage enough torque to spin up the slower tire, but they try. This is why an LSD adds understeer in tight corners on throttle at a torque below that which would have broken free on an open diff. Below the critical torque, the inside tire is turning too slowly, and gets extra torque, which generates a moment about the Z axis that tends towards understeer.

Next the case of the upper limit of the open diff (well above the critical torque for the LSD):

Say you want to put down 400 ft-lb (after leverage multiplication through the drivetrain) at part throttle on corner exit on a path that is (instantaneously) a 50 ft radius corner.

Say the inside tire can support 200 ft-lb worth of resistance (the outside can support maybe 1200, but you'll never see anything over 200 under these conditions with an open diff).

OPEN DIFF:
The inside tire turns at 9% lower speed than the outside tire, and both have a scrub percentage around 3%. Energy wasted in the diff is negligible, extra heat in the tires from acceleration torque is negligible, extra wear on the tires from acceleration is negligible.

working limited slip DIFF (other than VLSD):

The inside tire turns at roughly the same speed as the outside tire. The inside tire has a scrub percentage around 12%. Energy wasted in the diff is negligible. Extra heat in the inside tire from acceleration torque is significant (can even get greasy if this corner is followed immediately by one the other way). Extra wear on the tires from acceleration is significant. How much horsepower does it take to spin up a tire (one that can pull 1.4g's+ in acceleration) to 40 mph when the road speed is 35? I don't know, but given I don't have a lot to start with..... I mean, at 4000 rpm, where I would be at the bottom of second in this example, I've only got ~75 rwhp to work with....


In this case, what's going on, and what do we care about? Why would changing on open diff to an LSD make a car slower?

Tire wear, power lost due to the scrubbing process and its effect on lap times, handling bias effects that come with this effect (moment about the z axis that counter cornering input.) They are all significant.

More in my next post....

 

[rocwandrer]

The primary effect I'm talking about when I say a car can be faster without LSD is in the moment about the Z-axis. If you can put enough of the power down without needing the LSD, you can always put it down sooner on corner exit (because the handling bias is further from understeer, so you can open the throttle more, sooner, and still hit your track out point, instead of the cone marking it). If you can put power down sooner/earlier in the corner, you can have a higher corner exit speed. A higher corner exit speed allows for a higher speed down every single foot of the following "straight" (whether it is straight or not, is unimportant, any place where you can use full throttle is a "straight"). Having an LSD adds understeer in tight corners. Note that this effect goes away as the speeds increase, because the radius is bigger, meaning the speed differential between the inside and outside tires goes towards zero. Adding a bunch of understeer in the tightest corners (which at autocross are almost always found immediately preceding the longest straight (IE the most important corner of the course)) hurts times.

If you have an LSD, you can counter this with the tiniest bit of throttle on oversteer (not enough to need countersteer, or it is slower than just living with the understeer). But that is very difficult to nail perfectly every time, and can often cost you in tire temperature problems going into the next corner. You can also counter this with a setup biased a little further away from understeer, but then you will have more oversteer bias in the higher speed corners than is ideal, costing you either time, or consistency.

The more power you have, the more likely it is that a setup built around a limited slip differential will net faster times than one built around an open differential. The STR Spyder is right at or just above this limit, I think. Figuring it out better was the point to this thread.

There is a lot more to this, but I just typed through lunch at work and now I've got to get back to it.

 

[sldnrck]

So luckily we don't normally see 50 foot radius corners at national events. Locally we don't allow for anything smaller than 60 foot radius corners.

So what about being able to get on the gas say earlier because the inside wheel won't spin up by itself loosing all power output? I know when the clutches went in my Z06 diff turning it into an open diff I chased all kinds of suspension issues because I was spinning the car on concrete. Once I replaced the clutch pack the car felt phenomenal coming out of corners and I stopped spinning the car. I feel like I have a similar issue in the MR2 along with the spins when applying power. It feels like a snap oversteer, complete loss of car control when I don't feel that I was doing anything unusual. No sharp steering inputs and no sharp throttle responses.

 

[rocwandrer]

One more thing that will help my perspective make sense... IN DP I was on bias slicks. The bias slicks don't need a lot of static camber, so the inside rear tire is flatter, meaning it had more acceleration grip available.

I plan to try out using a similar scheme in STR, despite the lack of bias plies. More front stiffness bias, less static rear camber, wider than ideal rear wheels (wider wheels allow the tire to work as well with less static camber), widest front track width possible, and so on.

 

[rocwandrer]

So luckily we don't normally see 50 foot radius corners at national events. Locally we don't allow for anything smaller than 60 foot radius corners.

So what about being able to get on the gas say earlier because the inside wheel won't spin up by itself loosing all power output? I know when the clutches went in my Z06 diff turning it into an open diff I chased all kinds of suspension issues because I was spinning the car on concrete. Once I replaced the clutch pack the car felt phenomenal coming out of corners and I stopped spinning the car. I feel like I have a similar issue in the MR2 along with the spins when applying power. It feels like a snap oversteer, complete loss of car control when I don't feel that I was doing anything unusual. No sharp steering inputs and no sharp throttle responses.

Some of that was cut and paste from an email discussion from a couple years ago, I picked 50 ft radius and did the math to figure out the actual speed differential and so on to avoid getting hung up on that sort of approximation. It still applies in any corner, but less and less strongly, the larger the corner radius is. The effect is large enough to be felt if you can compare two otherwise similar cars in a 150 ft radius corner.

You are lucky. Some of our local venues are so tight that we have to have 36 ft as our minimum allowed radius. 45 ft is the minimum recommended by rule with our local club. Our event chairs are typically the course designers and people always seem to want to push the corner radius down somewhere in order to fit in a longer straightaway.

As I said above, my feeling is that there is a power level for a given chassis where the setup built around an LSD will be faster than the setup built around an open diff. What you describe is the exact, simple, perfect explanation for why the limited slip differential exists. I didn't say open diff is always faster. The STR spyder is on (maybe just above on a nationals style course) the point where it is obviously better to have an LSD. Any corvette is going to be way above that power threshold in favor of LSD. I'd be willing to bet that part of your problem in that case was that the inside tire would spin (unobtrusively) to the point of being quite a bit overheated on a left hander, then on the next right hander it would become the outside tire, and be easily overwhelmed because of being above its ideal temperature range. The other part of that is that in a vette, there is going to be a lot of load on the inside rear tire to begin with, so when it spins and loses lateral grip, that transitions towards oversteer. With the working LSD, oversteer is a function of throttle position, stay below the threshold, and you are ok, so it is easy to know whats going on. At higher power levels, it is worth a little handling imbalance to be able to accelerate. You drive around it. You turn more sharply earlier in the corner, and then accelerate in a straighter line with both rear tires handling a lot of the torque. The same can be done without the LSD, but it doesn't pay a big dividends, and as soon as you are doing it anyway, why not get the lsd?

 

[sldnrck]

LSD should be on the car in the next month or so. So i can let you know what I think about it then. The car is very drivable without one, but I am driving around the wheel spin.

 

[Levi]

If no LSD
1.)Do you encounter 1st gear corners on the courses you run?
Only once, it was a 300 degree turn around a cone. 1st gear pulled out of it fine. I usually spin one tire a little while launching at the start of a run.

2.)How often/how much is wheel spin frustrating you or hurting times, etc?
Getting on the gas early has been my weakness since switching to the spyder. I drove a open diff V6 MR2 on street tires and had to get on the gas really late to ensure I didn't light up the inside tire. In the spyder, I don't have a problem coming out of corners...yet.

Where I really feel the open diff hurting me is when I hit bumps mid corner. A rear tire lifts up for a second because of the bump, starts spinning, then when it comes back down I am spinning that one tire. It takes a second for the tire to get grip again. I stay on the gas while this is happening.

3.) What are your springs and (if applicable) anti-roll/swaybars?

500 lb/in front and rear
22mm front swaybar
No rear swaybar
Running more rake than oem (my rear sits lower than front)

 

[rocwandrer]

that's it? Nobody else willing to step up and put some data out there?

 

[SuperWhiteGT4]

that's it? Nobody else willing to step up and put some data out there?

If no LSD
1.)Do you encounter 1st gear corners on the courses you run?

Very rarely. When we do, they're always pin turns.


2.)How often/how much is wheel spin frustrating you or hurting times, etc?

Very, very minimal amounts. Maybe 1/3 of the pin turns showed inside wheel spin, which equates to maybe 3 times in the last 5 years. Beyond that, I've never encountered inside wheel spin.

3.) What are your springs and (if applicable) anti-roll/swaybars?

Was on 3 kg/5kg with a 25mm front sway, stock rear sway. Will report back with the new 5kg/7kg w/ 22mm front sway setup when it arrives.

 

[rocwandrer]

If no LSD
1.)Do you encounter 1st gear corners on the courses you run?

Very rarely. When we do, they're always pin turns.


2.)How often/how much is wheel spin frustrating you or hurting times, etc?

Very, very minimal amounts. Maybe 1/3 of the pin turns showed inside wheel spin, which equates to maybe 3 times in the last 5 years. Beyond that, I've never encountered inside wheel spin.

3.) What are your springs and (if applicable) anti-roll/swaybars?

Was on 3 kg/5kg with a 25mm front sway, stock rear sway. Will report back with the new 5kg/7kg w/ 22mm front sway setup when it arrives.

with no lsd and no wheelspin issues, I'm wanting to clarify that those are 3kg/mm front springs and 5 kg/mm rear springs you are saying you run, or the other way around?

 

[SuperWhiteGT4]

with no lsd and no wheelspin issues, I'm wanting to clarify that those are 3kg/mm front springs and 5 kg/mm rear springs you are saying you run, or the other way around?

Correct.

 

[SuperWhiteGT4]

Running a tight, technical, medium-speed course this weekend with the new shocks and springrates:

If no LSD

1.)Do you encounter 1st gear corners on the courses you run?

Very rarely. When we do, they're always pin turns.


2.)How often/how much is wheel spin frustrating you or hurting times, etc?

I was able to detect trace amounts of inside wheel spin in the tightest corners, but not enough to disrupt the speed of the car

3.) What are your springs and (if applicable) anti-roll/swaybars?

5 kg/mm Front, 7 kg/mm Rear, Front 25mm Swaybar, Stock Rear swaybar.

 

[Levi]

Bumping this up for 2 reasons, everyone probably has read this first article but here it is for those that haven't.

http://grassrootsmotorsports.com/articles/whats-diff/

Second, I've been told that in I think the August 2011 SCCA sportscar mag issue that the GRM project STR NC miata tested a OS giken vs stock LSD and actually went slower with the OS giken installed. Does anyone have this issue or know the details of the testing? Here is the snippet on the GRM site:
http://grassrootsmotorsports.com/project-cars/2010-mazda-mx-5-miata/osgiken-lsd-install/

 

[rocwandrer]

And to go right along with that, don't forget that LSD performance by type will not necessarily be the same story for FF, FR, MR. You'll never unweight an inside rear wheel with MR anywhere near as light as you can with FR (or front with FF).

 

[sldnrck]

Just as a point of reference the Mineral Wells Tour had a big fast sweeper towards the finish. I was getting all kinds of wheel spin in that fast corner. I was getting under-steer in all the slow stuff and not able to put any power down in slow corners because it would just cause the car to push more. Trying different setup for San Diego, and LSD won't be in until at least after San Diego. I think the OS Giken will take some effort to get setup correctly for autocross in the MR2. I don't think it is going to just be drop in and go faster. Almost everyone I talked to said putting in a LSD will need a different suspension setup, so I am unsure how much faith I but in the back to back tests. It was interesting to read, as I had not seen it before.

 

[Levi]

Just as a point of reference the Mineral Wells Tour had a big fast sweeper towards the finish. I was getting all kinds of wheel spin in that fast corner. I was getting under-steer in all the slow stuff and not able to put any power down in slow corners because it would just cause the car to push more. Trying different setup for San Diego, and LSD won't be in until at least after San Diego. I think the OS Giken will take some effort to get setup correctly for autocross in the MR2. I don't think it is going to just be drop in and go faster. Almost everyone I talked to said putting in a LSD will need a different suspension setup, so I am unsure how much faith I but in the back to back tests. It was interesting to read, as I had not seen it before.

Agreed reality seems to be conflicting with theory, I also get inside wheel spin where I feel I shouldn't. I've already reduced my rear camber and this weekend am going to try 672 lb/in springs upfront +no bar instead of 500 lb/in+22mm swaybar. Theory says a stiff front end+less rear camber should cure inside wheel spin. The only other option after that is softening the rear springs, if I still get inside wheel spin then I guess an LSD is the only option. Andres complained of inside wheel spin with his 672 lb front/448 rear setup. Maybe the lightness of the rear combined with the huge 1.8L torque is causing it? Or are we just missing something completely? Maybe the kooks just suck at putting down power?

PS: the GRM tirerack ppl adjusted their OS giken for less lockup I was told, this is why I want to find that article!

 

[sldnrck]

Maybe the spring setup is just working a lot better at generating cornering force causing you to left the inside rear wheel. I was pulling over 1g when I was getting inside wheel spin. I am not sure softer rear will eliminate rear wheel spin. I think that would transfer weight more unloading the inside rear tire faster.

 

[silversprint]

My NC miata had stock suspension and stock LSD. It's probably the worse handling sports car I have ever driven. The stock sport suspension was very soft and had a ton of travel. In a sweeper the car would roll A LOT. It would almost lift the inside rear tire off the ground. At which point the LSD would abruptly kick in and jerk the car back in. The miata LSD if not bad. I did several track days in the wet and the did fine. I think the car benefits much more from better suspension than a different LSD.

I never really got massive wheel spin the spyder. Probably because I don't run massive rear camber, run bigger than normal tires, and dont' see a lot of 2nd gear corners.