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View Full Version : HLSD not working because of wheel lift



motonagam
08-23-2005, 09:26 PM
I'm an autocrosser, and my focus on setting up my S14 has been suspension, but I'm having a problem lifting the inside rear wheel off the ground during tight turns (especially tight hairpin turns), which makes my S15 HLSD go into open diff and makes it useless (the inside wheel just spins and smokes the tire). The HLSD works GREAT in mild turns and sweepers however. To get you an idea of what my car is like, these are the mods I have done to my suspension so far:

Kei Office XT-alpha coilovers (F-9kg/R-8kg)
Cusco front & rear sway bars
Cusco RUCA
Nismo front & rear strut tower bars
Nismo power brace
SPL rear subframe collars
SPL tension rods
NAMS fender brace
225/45/17 (front), 255/40/17 (rear) Advan Neovas

I've tried putting my stock rear sway bars back on, and even disconnecting my rear sway bar completely, but still lifts. I adjusted dampening full hard in the front, and full soft in the rear just to see the difference, and it seemed like it helped little bit, but understeers way too much -- I did not like how it handled at all, and I would never drive it with that setup. I've also realigned my car, which these are what I adjusted to:

FRONT- (left/right)
camber: -1.8 -1.8
caster: 6.4 6.6
toe: 0 0

REAR- (left/right)
camber: -1.2 -1.2
toe: 0 0

Could it be that my car is just too stiff with all the suspension mods I have done to it? The reason why I don't want my inside wheel to lift off the ground is so that my HLSD can work like it's supposed to (it only works when both wheels are on the ground). If I can't fix the wheel lifting problem, can I just upgrade my LSD to a clutch-type and cure the inside wheel spin problem? I'm thinking about getting a Nismo 1.5 way GT Pro LSD. Thanks!

Here's a pic of me inside my car at an autocross event...
http://www.geocities.com/masashi.geo/silvia/silviaongrid.jpg

BrianS
08-23-2005, 10:39 PM
correct me if i'm wrong, but i thought s15 LSDs were clutch types, 'helical' is just a fancy marketing word for it


torsional LSD's exhibit that problem, i.e. quaifs


anyway....your problem is you have too much tire and spring in the rear, a rear drive car should NEVER be lifting the rear tires completely

s14s understeer...that's what they do, when you put 225s in the front and 255s in the rear, they understeer a LOT...

so you need to compensate by putting stupid heavy springs in the rear (you have almost the same rates front/rear) and a hugh rear swaybar, right now the car probably feels relatively neutral, but is REALLY skittery during on-throttle exits

go with 245s at all four corners, or 225s at all four corners...there is absolutely no reason to stagger the tire sizes unless you have more power than your tires can handle

J_Rho
08-23-2005, 11:04 PM
I agree with the above poster.

The inside rear wheel is being sufficiently unloaded in cornering to spin. You need to keep more weight on that inside rear tire, by making the front "do more work" in the corner.

To do this, you need to make the front stiffer (more front spring rate or swaybar) and/or make the rear softer (softer rear springs, disconnect/soften rear bar).

As you noted, with the present tire stagger, doing so makes the car push (aka understeer).

Because of the 240's good balance and relatively low power levels, equal size tires at all 4 corners is a good bet. If you can fit 255's at all 4 corners, that would be killer.

p.s. Great choice of tires! I've had very good success on the Neovas this year in SCCA Pro Solo.

_Def_
08-23-2005, 11:32 PM
First of all, I'd go with slightly wider front tires. You have to be overheating those 225's.

Secondly, are you running any preload on your coilovers? If so, don't. I hated how my K-Sport coilovers handled with preload on them, and I suspect you might be having a problem with lack of travel.

As a general concern on your setup, I'd get more front negative camber. I'd aim for around -2.5*. Maybe put a hair of toe-in on the rear, like maybe 1/16" overall(1/32" per side).

I run an S15 HLSD(which is a "torsen-type", which IS the fancy marketing name for a helical diff, since torsen is a company, helical refers to the gear shape that transfers the friction to the outer case and thus the biasing). I have no problems with inner wheel spin. I can't see street tires generating enough grip to get the inside rear wheel off the ground in any properly setup car.

ItzGenX
08-23-2005, 11:38 PM
correct me if i'm wrong, but i thought s15 LSDs were clutch types, 'helical' is just a fancy marketing word for it


torsional LSD's exhibit that problem, i.e. quaifs




The S15 LSD is a torsen unit that you speak of. Helical is referring to the helical worm gears that make the LSD work just like a Quaif unit.

motonagam
08-24-2005, 12:12 AM
Thank you everyone for your response.

The reason I went with wider rear wheels/tires was because I was planning ahead to eventually modding my engine for more power after my suspension. I didn't want to have to purchase wheels twice, so I got 17x8s for the fronts and 17x9s for the rears (Work Emotion CR Kai). I don't think I will get to modding my engine for power anytime soon, so I will try your advice of not staggering my tire sizes so much the next time I need new tires. How small of a tire do you think I can mount on 9" wide wheels without stretching the sidewalls too much? Do you think 235s will fit on both 8" and 9"? And WOW... BrianS pretty much described how my car handles perfectly! Yes, it does handle relatively neutral, and it does skitter during on-throttle exits. But it does however handle awesome through slaloms! I don't feel the rear "whipping out" as I'm transitioning the car back and forth. The rear stays planted, which allows me to keep the throttle down to the floor and haul through it! The slalom is where my car is really fast.

Leaving the setup of my car the way it is, do you think a clutch-type LSD will help, despite the wheel lifting problem? Since clutch-type LSDs lock the two drive wheels together whenever I'm on throttle, it shouldn't matter if one wheel lifts off the ground, correct? Since I'm setting up my car for autocross, do you think a 1.5 way is the best to go? Wouldn't a 2way make my car understeer more because it won't allow the two drive wheels to uncouple during decel and let the car turn-in? Well, I'm hoping that the clutch-type LSD will solve the problem, because I'm actually doing pretty good in my class with my current suspension setup. But I do agree with everyone that I staggered my tires too much... I will lessen it when the time comes to buying new tires again.

As for my alignment, I was concerned about tire wear since I also drive it on the streets, so I didn't want to adjust my front camber too far. Thank you very much for all your help!

Masashi Motonaga

BrianS
08-24-2005, 12:41 AM
i duno, it might work (stiff rear + clutch LSD)

since the car only has 150hp you may be able to lose more time by carrying momentum through the corner like a front drive car.

As opposed to a more traditional 'point and sprint' rear drive setup, which tends tward understeer and requires heavy trailbraking to get the nose to bite on turn-in...then lots of traction on the rear to accelerate on corner exit ASAP


either way the more front tire the better



set your camber by practical experimentation... make an autoX run, then feel the contact patch of the tires, like, actually touch them with your hand (no need for fancy expensive pyrometers) adjust the camber accordingly until you get even heating across the entire tire surface



oK, now i have my LSDs straight, i think i was getting the torson brand name mixed into the wrong place

_Def_
08-24-2005, 12:57 AM
I've driven on the street with zero front toe and about -2.7* camber and it didn't wear the insides faster than the outsides, but I was also tracking those tires. I think you'll be fine with autoxing the car here and there, and it'll give you more front end grip.

You can easily fit a 235/40 tire on your front wheel, and I'd go with a 245/40 in the rear. That should be a good combo.


I looked over your setup again, and I think you have far too much roll stiffness, especially for street tires. There is no reason to have 9/8 kg/mm rates AND larger swaybars. I actually think the swaybars are the reason you're lifting up your rear wheel. They "push down" on the outside wheel and "pull up" on the inside wheel. I'd say go back to the stock sways and see how you like the balance of the car. It should be fine with your springrates, and I bet you go faster.

Don't pick a different diff that'll work when you're picking up a tire in a corner. 4 tires on the ground is faster than having 3 tires on the ground - it's that simple. Change your suspension setup, not the diff.

I'm not a big fan of big swaybars, especially for cars that can run enough springrate for their roll stiffness. You get nasty problems like picking up wheels, and you generally reduce total traction at the end of the car where you really "jump up" in bar size.

019
08-24-2005, 09:11 AM
I looked over your setup again, and I think you have far too much roll stiffness, especially for street tires. There is no reason to have 9/8 kg/mm rates AND larger swaybars. I actually think the swaybars are the reason you're lifting up your rear wheel. They "push down" on the outside wheel and "pull up" on the inside wheel. I'd say go back to the stock sways and see how you like the balance of the car. It should be fine with your springrates, and I bet you go faster.



he's already tried going back to a stock sway (according to his first post) and apparently it still lifts, even with the rear sway off. i was thinking the same thing tho. i def think the rear spring rate is too high.

for the camber thing, i've run 2.5*/2.0* for a couple years now daily driven and my tires are fine, just like def said.

i'm curious about one thing. what class do you run in? judging from your mods, i would say SM.

J_Rho
08-24-2005, 11:57 AM
This car is only legal in a Prepared class or EM, because of the fender brace. Without it, the car would be Streed Mod legal. If the adjustable control arms and tension rods used poly ends instead of spherical bearings, it would be legal for STU. With that and 8" wheels all around, it would be legal for STX.

Def, the whole "push up" and "pull down" function of swaybars is a bit of a misnomer. Springs and bars act together to produce a certain amount of roll stiffness. Let's say 2 cars (A and B) have EQUAL roll stiffness front and rear, with car A having very stiff springs/soft bars, and car B having soft springs/stiff bars.

In a corner, they will spin the inside rear EQUALLY!

The OP's problem is that he is unloading the inside rear wheel sufficiently to produce wheelspin. It may not be picking the wheel up completely, but instead of the 700+ pounds of load it has at rest, dynamically it may have less than 150 pounds of load, enough to break the inside rear loose.

Spring and sway bar stiffness work together at one end of a car to produce a given roll stiffness. The relationship of the front and rear roll stiffnesses produce a car's "Lateral Load Transfer Distribution" (LLTD), which describes how, as a car rolls and weight is transferred to the outside tires, what percentage of that total load transfer is borne by the front and rear axles. Generally, the end that gets the most of the load transfer will break away first. Stiffening springs and/or bars at one end of a car will make that end take more of the LLTD.

It gets complicated from here with tire efficiency curves but to skip ahead, generally the end that gets more of the LLTD will break away first - if that's the front, you get understeer, if it's the rear, you get oversteer. So to some extent, yes, you are reducing total traction at one end of the car by going with a stiffer swaybar (or stiffer springs), but really, only in comparison to the other axle, which you're helping, by reducing how much of the LLTD it must take.

"Stiff springs/soft bars" vs. "soft springs/stiff bars" is one of the big debates throughout racerdom. The argument against stiff springs is that as spring rate goes up, the ability of the tire to follow bumps and inconsistencies in the road surface goes down. The argument against big bars is they reduce suspension "independentness", as single-wheel bumps get transmitted across the axle and upset the tire on the smooth surface. Soft springs also increase the vehicle's pitching movements under braking and acceleration.

So, to sum up:
*He is lifting the wheel not because of the overall sway bar size, but because the rear is taking too much of the LLTD.
*Reducing LLTD in the rear can be accomplished by softening the rear springs/bars and/or stiffening the front springs/bars
*To combat the understeer bias as the result of this LLTD change, better front alignment and/or more front wheel/tire will be needed

As an aside, I agree more front camber is needed. On a strut car, something north of 3 degrees negative is probably ideal. As the car rolls and the outside suspension compresses, it's only losing more and more camber. Slick-shod E36 BMW M3's run over 5 degrees of static negative camber on their stut front suspensions, even with extremely stiff springs and bars.

I also don't think the OP's spring rates are too high from a racing perspective, as long as they're well damped. I'm starting with 550/450 (~10k/8k) on my '92, along with big swaybars, good Konis, and I'll only be on 7.5" wheels and 225 section street tires.

019
08-24-2005, 04:37 PM
^^^^
very informative post. basically saying the same thing that has been posted: he needs a softer rear and better tire setup.

i forgot that the fender braces were not SM legal. i can't imagine running a 240 in a Prepared or EM....

from the sounds of your project car, can it be safe to assume you're building an STS car? http://forums.freshalloy.com/images/graemlins/laugh.gif

J_Rho
08-24-2005, 05:10 PM
from the sounds of your project car, can it be safe to assume you're building an STS car? http://forums.freshalloy.com/images/graemlins/laugh.gif

Shhhh, not too loud, I don't want any of the Civic drivers to hear and get scared! http://forums.freshalloy.com/images/graemlins/cool.gif

SequenceGarage
08-24-2005, 05:10 PM
how much preload do you have on your springs? Add preload to the front to effectively make the front stiffer, and just a tad of preload on teh rear to hold the spring in place (like 2mm)

Good luck!

_Def_
08-24-2005, 07:45 PM
Def, the whole "push up" and "pull down" function of swaybars is a bit of a misnomer. Springs and bars act together to produce a certain amount of roll stiffness. Let's say 2 cars (A and B) have EQUAL roll stiffness front and rear, with car A having very stiff springs/soft bars, and car B having soft springs/stiff bars.

In a corner, they will spin the inside rear EQUALLY!




Just doing a freebody diagram, yes the normal force differential on the two tires will go into countering the lateral weight transfer of the car. Yet in reallife you rarely have steady state cornering, and losing the independent nature of the suspension definitely causes more inside wheel lift. The swaybar will also have a different wheelrate than the springs, so you get a non-linear response, which can also make abrupt lateral g's serve to help "pick up" the inside rear wheel.

One of the biggest problems I have with "suspension design" books is that they treat too many things as static. Sure, that is important and helps, but there is a complex response to a system as complicated as a suspension. Throwing a big rear bar on a FR car has been emperically shown to cause rear traction problems when trying to put the power down out of a tight corner. It feels great on a high speed sweeper because you can get the car set and have ample roll stiffness without the "unpleasant" side effects of a stiff spring making you lose compliance on bumps and undulations, but as soon as you get to a low speed corner that will typically have the lateral G's peak right before you're trying to put down the power out of a corner you run into problems.

Even if the car is still spinning a wheel with the stock rear swaybar, I think that it is a step in the right direction.


I'll tell you the diff can easily be setup to not spin the wheels coming out of low speed, high G corners. I run one on my car, and my car has way more power and torque than an NA KA.

motonagam
08-24-2005, 07:49 PM
Wow J Rho, that was indeed a very informative post... as well as everyone else who posted with suggestions to help me out. Thank you!

I will reduce my rear LLTD with suggestions you have provided. For my next autocross this weekend, I will try something easy first, like disconnecting my rear sway bars and stiffen the dampening in the front (currently, all four corners are set to full soft). And yes, I have already tried disconnecting my rear sway bars, but not in conjunction with other adjustments to the suspension. If that does not help, I will go further by adding more negative camber in the front, and possibly also adding more spring preload in the front. I don't know how you measure the spring preload (when someone asks me how much preload I put on my springs, I'm not sure how I would answer that), but I preloaded it by turning it tight as I can with my hands (no tools). When I first installed my Kei Office coilovers, I didn't put any preload on them. I just turned the spring perch until the top of the spring touched the upper plate, but I found that to be too loose because my springs were making popping noises as I turned the steering wheel left and right, so I had to tighten it.

I'm thinking about purchasing another set of wheels for autocross, because I want to become competitive in autocross and run Kumho V710s. Would getting racing tires make my problem worse, because V710s has a lot more grip? I'm thinking of getting 8" wheels with maybe 235/40 for all four corners. Although the V710s may have more grip, would it actually be better because of the equal tire width all around?

In autocross, I run in the DSP class. As for the fender brace, are they really not legal for SM? If it's not legal for SM, I assume they must not be legal for SP? Oops, I didn't know they were not legal... I better reread my rule book again. haha. I installed my fender brace just recently (a week or two ago), so I only ran with them in one autocross event.

I will try to apply everyone's suggestions on the autocross course and see how my car does. Again, thank you for enlightening me about suspension setup.

Masashi

motonagam
08-24-2005, 08:45 PM
oh, and another thing... it is very weird that I can spin the inside rear tire very easily on left turns, and not so easily on right turns. I'm actually having more problem with the inside rear wheel unloading on left turns. I checked my ride height, and they are near identical left to right. I set my ride height lower at the front, because I wanted my car to understeer less. Could my ride height also contribute to my HLSD "problem?" I have not corner balanced my car, but do you think it is a good investment to do so? I don't have the scales to do it myself, so I am going to have to take it to a race shop to do it. But it is quite a mystery to me why left turns are worse than right turns (and right turns better than left turns).

Masashi

_Def_
08-24-2005, 09:26 PM
My S13 has drastically different rear tire weight distribution, so that's probably a good bit of the problem.

Go to a race shop and have the car corner weighted after you arrive at a setup you like. That should help balance things out.

019
08-25-2005, 10:13 AM
that's a good point. corner balancing is always recommended with coilovers. the ride height might look the same all around, but you'll be surprised how off your corner weights are when you do so.

the RUCA and the TC rods actually put you out of DSP as well, but depending on how strict your region is, they might just let you run DSP anyhow. you're not allows heim joints in DSP.

SequenceGarage
08-25-2005, 12:35 PM
sounds like your coils aren't setup the same, the main problem here is that this suspension system is very aggressive and has very short travel. The only way to get rid of the problem is to disallow the car to lean forward and roll over to the outside - unfortuantly there is no way to make the shock extend even further out (as stock suspension for example would). I think that would be the ideal for your situation, where IMO you want a slightly softer suspension for autocrossing.

I measure preload by how many MM i add from the time the coller touches the spring.

1mm = 9 or 8kg of preload. Try out 10mm for now, that should totally disallow any forward lean under braking and turn in, hopefully the inside rear tracking.

The problem is that now your suspension is totally solid for the first 90kg of force on the spring, thats 180kg on the front of the car.

Overall i think you need a softer suspension system with more travel, such as TEIN FLEX.

motonagam
08-25-2005, 11:52 PM
I will look into corner balancing my car. I think you're correct, 019, about the ride height might look the same all around, but the actual weight at each corner are way off. I noticed that my RF coilover body has about 3-4 more threads sticking out of the bottom strut bracket (meaning the RF is probably set lower) compared to the LF, eventhough my ride height looks perfect. When I get my car corner balanced, I have the gut feeling that the RF is heavy and the LR is light, which explains the problem I'm having with the inside rear wheel lifting more during left turns.

Since I spent quite a bit of money on my Kei Office XT-alpha coilovers (about $2,300), buying another coilover set to replace it is not going to happen. I want to tweak the suspension with what I already have. By reducing LLTD in the rear to keep the inside rear wheel from unloading, the car will naturally understeer. I will combat the understeer bias by putting more aggressive camber, and with a better tire/wheel combo like what everyone has suggested. An idea that just came across my mind is to maybe put stickier tires in the front, like putting Kumho V710s up front, and leaving my Neovas in the rear.

Thank you SequenceGarage about the explanation of spring preload. I will test out what you have suggested. Thanks!

J_Rho
08-26-2005, 12:17 PM
2 last thoughts:
Don't mix tire types, especially those as dramatically different as 710's and Neovas.

Hyprocritical as it may sound, don't trust most of what you read on the Internet. Most people don't know what they're talking about. I may be one of them. http://forums.freshalloy.com/images/graemlins/ooo.gif

In your particular case, I'd try to find somebody in your region with a proven record in car setup, and have them help guide you in getting your car dialed in.

motonagam
08-27-2005, 06:36 PM
I had second thoughts about putting 710s in the front, and leaving the Neovas in the rear, because it would now probably oversteer too much. I'm going to stick with the same type of tire, and lessen the tire stagger to like 235/245. For the next autocross event, I'll let someone I know in my region co-drive my car, and see what he thinks. I do agree with you that it is the best way to get my car dialed in.