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DaewooOfDeath
DaewooOfDeath Dork
4/27/12 11:04 a.m.
fast_eddie_72 wrote: Holy cow, did I ever miss a few important posts here! Thanks everyone, especially DOD. Man. Lot of reading to do here. Hope I understand it all.

If you get lost ask. If I know I'll help you out. If I don't know, I'm sure we can find somebody who does.

mazdeuce
mazdeuce Reader
4/27/12 1:28 p.m.

Ok, after reading through the long post 3 or 4 times, I'll ask. At the very beginning it says that an MR2 is like a FWD backwards so the theory will still work on a FWD if you turn it around. Fundamentally though, a MR2 will always have more weight on the rear and this will have a significant effect under braking and acceleration. This makes me question a LOT of his math when trying to apply it. He mentions in passing that fast FWD cars are softer on the front to aid traction and generate more of their roll stiffness in the rear by having super stiff springs. I'm.......not sure where to go with all of this. I believe in his total spring rate calculations for the car, but can't quite figure out how I should split them up with FWD. Help?

kevlarcorolla
kevlarcorolla Reader
4/27/12 3:15 p.m.
DaewooOfDeath wrote:
kevlarcorolla wrote: I played around alot with bars/no bars on my composite bodied AE86 autox car,for sure bars were slower.The scratch built car that followed it was designed with no bars and it works pretty well,no intention of adding the weight of a bar to it. Friend of mine went barless on his gze mr2 converted to turbo car with off the shelf koni's,my suggestion would be to spend the extra loot for valved dampers to match.His was pretty bouncy.
I read your stuff over on the MR2 forums. Didn't know you went barless as well. I guess that makes three known cases, Steve's mid engined car, your FR car, my FF car.

You forgot about my mid engined rwd car,I'm struggling a little with it in that it oversteers at the beginning of a run and transfers to understeer thru the run as the rear tires come up to temp.I can't see sways adding any improvements,unless I could make them remote adjustable via electric motors controlled by a pre programmed lap top-I'm way to busy to think about a manual adjustment. I initially picked wheel rates based on a targeted frequency,way too soft even though it was theoritically "stiff" being just under 3hz.I haven't redone the math with the current springs but it should be over 5 hz now and still rides very well over bumps even at slow speeds thru the paddock.If I softened the springs and went to bars I'd be in trouble trying to keep the car off the ground at speed with its 2" ride height. I have some minor changes to try for its 1st event on the 13th,hopefully go the right way with them.

Curmudgeon
Curmudgeon MegaDork
4/27/12 3:54 p.m.

In reply to mazdeuce:

If you look at the picture of Hoeschler's X 1/9, you'll notice the inside front tire is off the ground. BTW, I saw that car up close at a regional and it was a very well put together and detailed car. Beautiful work, hundreds of hours went into it.

That happens a lot in the rear of VW's with the torsion beam suspension and it's not even noticeable. It also means the outside front tire on his car and the outside rear of the VW's is not being overloaded because the tire is not sliding. The suspension in both cases has very little 'droop' travel.

The thing to keep in mind during suspension tuning is that good ol' circle of friction, if the tire can develop x amount of grip and the force applied to it does not exceed that amount then the tire will not slide, doesn't matter which direction or combination of directions it's being applied. Example: say the tire can develop 1000 pounds of grip. Let's say in a turn you apply a total of 800 pounds, doesn't matter in what direction. The tire will not break loose.

Now if you develop 1050 pounds of force, again in any direction, the tire will lose traction. Keep this in mind always. It's very easy to overload the front tires on a front driver because they are asked to accelerate, brake and steer the car, quite often two things at once so managing tire load becomes very critical. You can either reduce the force applied by slowing down (not good) or spreading some of the load to its inside brother.

Now back to the FWD setup: you have to control the body roll somehow in order to keep some of that weight on the inside front tire, otherwise you overload the outside front and thus slide. If you go with super stiff front springs, it's easy to lose traction, the tires will just hop on any decent acceleration. So what do you do?

Stiffen hell out of the rear; this will help resist body roll and its subsequent effect on weight transfer at the front, thus helping put some of that load on the inside front tire. Odd as it sounds, if you stiffen the right rear of the car it adds more weight to the left front because the car is like a four legged stool with two short legs: the weight all goes on the long legs. That's why corner weighting is important but that's another whole discussion. But what that means is that as the suspension in the rear tries to compress in, say, a left turn the super stiff right rear spring helps to push down on the left front.

BTW: Ordinarily I'd say don't let body roll concern you as long as the car grips and steers well, but this is one situation where yes I'd be concerned and take steps to correct it.

Curmudgeon
Curmudgeon MegaDork
4/27/12 4:46 p.m.

fasteddie: About strut suspension camber gain, there is none to speak of. Where you start is where you end, at least in any reasonable amount of suspension travel. I learned that on my '79 RX7 CSP car (which I still miss terribly sometimes).

So that means that it is possible to have body roll alter your camber setting but there is no real way to have a 'camber curve' as you would on a SLA car.

One thing which will drastically alter your camber setting is to merely turn the steering wheel. That's because the stub axle that the hub goes on is mounted to the strut at a fixed angle. As you turn the steering wheel and watch from the side, the tip of the stub axle will go down as it moves away from the straight ahead position, it describes an arc as you crank it from end to end of the steering.

As you turn the steering left, for instance, the right front tire loses camber as the tip of the stub axle goes down and to the front. The left front tire gains camber as the tip goes down and to the rear. So the fix: crank in a snotload of camber (my RX7 was at 3 1/2 deg negative, it looked like something was busted) and also a whole E36 M3load of caster, IIRC on my car it was 4 1/2 deg which is a BUNCH on a RX7, it involved pretty much mangling the upper strut mounting. Adding caster will cause the outside front tire to lose less camber than at more sedate settings. It will cause the inside front to add more, but that can't be helped and in practice doesn't make much, if any, difference.

DaewooOfDeath
DaewooOfDeath Dork
4/27/12 7:13 p.m.
mazdeuce wrote: Ok, after reading through the long post 3 or 4 times, I'll ask. At the very beginning it says that an MR2 is like a FWD backwards so the theory will still work on a FWD if you turn it around. Fundamentally though, a MR2 will always have more weight on the rear and this will have a significant effect under braking and acceleration. This makes me question a LOT of his math when trying to apply it. He mentions in passing that fast FWD cars are softer on the front to aid traction and generate more of their roll stiffness in the rear by having super stiff springs. I'm.......not sure where to go with all of this. I believe in his total spring rate calculations for the car, but can't quite figure out how I should split them up with FWD. Help?

Turn them backwards. If you have a 2200 lb car with a 56/44 weight distribution (ie the exact opposite of the MR2), you should be in the neighborhood running 400 lb front springs and 500 lb rear springs.

A good general rule of thumb is that, all else equal, the softer end of a car, whether front or rear, will usually generate more cornering grip than the harder end. This has to do with weight transfer.

Take your Mazda for instance. If you run a really stiff rear suspension in concert with a soft front, you will lift your inside rear wheel. This has two effects:

1st, it will make your outside rear tire do all of the cornering for the rear axle. This tends to overload said rear tire and cancel your car's natural understeer.

2nd, it will transfer the maximum amount of weight from the rear inside to the front axle. This improves traction to the front for braking, cornering and acceleration. This is because the stiff rear suspension is increasing the load the front tires see without increasing the amount of mass they have to turn. Any time you are turning, that stiff rear will increase the front grip, whether you are on the gas, brakes or just coasting.

So, with a naturally understeering, FF car, you want a relatively stiffer rear suspension and a relatively softer front. On the other hand, in a naturally oversteering car like an MR2, that stiff front is always transferring weight to the rear wheels, also increasing grip in cornering and acceleration.

Here's a graphic illustraiton.

This GTI has a ton of roll stiffness in the rear. As you can see, it is not understeering.

Compare and contrast with this stock Mazda 2. This car has a lot of stiffness in the front and is therefore understeering.

DaewooOfDeath
DaewooOfDeath Dork
4/27/12 7:15 p.m.
kevlarcorolla wrote:
DaewooOfDeath wrote:
kevlarcorolla wrote: I played around alot with bars/no bars on my composite bodied AE86 autox car,for sure bars were slower.The scratch built car that followed it was designed with no bars and it works pretty well,no intention of adding the weight of a bar to it. Friend of mine went barless on his gze mr2 converted to turbo car with off the shelf koni's,my suggestion would be to spend the extra loot for valved dampers to match.His was pretty bouncy.
I read your stuff over on the MR2 forums. Didn't know you went barless as well. I guess that makes three known cases, Steve's mid engined car, your FR car, my FF car.
You forgot about my mid engined rwd car,I'm struggling a little with it in that it oversteers at the beginning of a run and transfers to understeer thru the run as the rear tires come up to temp.I can't see sways adding any improvements,unless I could make them remote adjustable via electric motors controlled by a pre programmed lap top-I'm way to busy to think about a manual adjustment. I initially picked wheel rates based on a targeted frequency,way too soft even though it was theoritically "stiff" being just under 3hz.I haven't redone the math with the current springs but it should be over 5 hz now and still rides very well over bumps even at slow speeds thru the paddock.If I softened the springs and went to bars I'd be in trouble trying to keep the car off the ground at speed with its 2" ride height. I have some minor changes to try for its 1st event on the 13th,hopefully go the right way with them.

That orange thing with the bike engine? That's nuts.

Have you considered doing big, smokey burnouts at the start line?

mazdeuce
mazdeuce Reader
4/27/12 7:54 p.m.

Thanks a bunch for that. The pictures were excellent and illustrated things well. One last question. The author expressed a pretty strong opinion for springs over bars in generating wheel rate. Looking at the rabbit above with a wheel in the air (and my 2 is already getting a bit of air stock) does it particularly matter if the roll resistance comes from bars or not? His complaint was that bars reduce the mechanical grip of the opposing wheel and that doesn't seem to be much of a concern when the wheel is already off of the ground.

DaewooOfDeath
DaewooOfDeath Dork
4/27/12 8:13 p.m.

I haven't experimented enough to tell you for sure but I do have these thoughts.

  1. Once the wheel is off the ground, the ratio spring/bar rates doesn't matter at all.

  2. The rear wheel in your car will not immediately lift off the ground.

  3. The front wheels will, in any case, grip better without bars if you accept Hoelscher's theory about bars decreasing grip.

I have never played with a Mazda2 so I can't tell you what will happen, but I tend to think a spring only setup that lifts the rear wheel will have identical maximum grip to a springs and bars setup. However, I would also tend to think that the spring/bar setup would be less predictable on the way to that maximum grip level. On the front, I would stay away from a swaybar in any case.

mazdeuce
mazdeuce Reader
4/27/12 8:38 p.m.

Gracias.
I know from the data acquisition that I already have no traction on the inside rear tire on corner entry. I still think I can make the rear a lot stiffer before it becomes an issue. First and foremost this is a car that I drive my kids to school in every day. As much fun at 250 lb springs and no bar on the front sounds, I probably have to make a few compromises. I think that for the time being I've been convinced to stay away from a bigger front bar, and maybe think about the smaller one on the fiesta. Front spring rate needs to increase, somehow. The rear needs to increase as much as I can stand it.
Now I've got a direction.
I can do this.
Thanks.

kevlarcorolla
kevlarcorolla Reader
4/27/12 10:26 p.m.

That orange thing with the bike engine? That's nuts.

Have you considered doing big, smokey burnouts at the start line?

Yeah the orange thing,I try and spin the tires fairly hard at the launch.I'm not smart enough to know for sure what the cause of the oversteer cold and understeer hot is,I'm going to try and work the front for a little more grip untill I think thats all it has to give and then reduce the rear grip to suit the latter half of the run.One thing Steve also liked that hasn't been mentioned yet is rear toe out,I've set the orange thing up with a touch of toe in to be safe but one of the little changes this weekend will be the move to a little rear toe out.

Also for the guys with street cars reading this it should be stressed that springs only should really only be tried with dedicated cars,as much as I'm not a fan of bars I believe the spring rate needed is too high for a street driven car to go sans bars.My suggestion for dual purpose cars would be to use the stock bar on the drive wheel end and upgrade the opposite end in conjuction with stiffer springs. Another thing Steve did to make his barless MR2 work was relocated inner pivot locations,he kept those locations to himself(as he's completely entiltled too)so following his set-up will have differing levels of success due to the factory geometry.

DaewooOfDeath
DaewooOfDeath Dork
4/28/12 1:53 a.m.

@ MazdaDuece,

I agree with Kevlar. If you have a street car, you probably don't want to go barless. I did, but I enjoy shooting my rear seat passengers into the ceiling with every speed bump.

My car is about the same weight as yours, maybe a little heavier, and I run 500 lb (9kg/mm) springs at all four corners. Were I to go completely racecar, I would probably move to 12 kg front and 14 kg rears.

Even with the 9kg springs, the ride at the heavier front of my car is bearable, it's just the lighter rear that gets punishing.

To summarize, I think you'd be surprised how little comfort 250 lb springs sacrifice. Might try in the neighborhood of 300-350 lbs, actually.

That with a stock bar in the front and a big rear bar would likely be a good place to start.

DaewooOfDeath
DaewooOfDeath Dork
4/28/12 1:55 a.m.
kevlarcorolla wrote: One thing Steve also liked that hasn't been mentioned yet is rear toe out,I've set the orange thing up with a touch of toe in to be safe but one of the little changes this weekend will be the move to a little rear toe out.

Yah, I saw that. Something about aiming the toe at the center of mass? I wonder if he'd also support toe in on a FF or FR car's front axle, by that logic.

kevlarcorolla
kevlarcorolla Reader
4/28/12 7:24 a.m.
DaewooOfDeath wrote:
kevlarcorolla wrote: One thing Steve also liked that hasn't been mentioned yet is rear toe out,I've set the orange thing up with a touch of toe in to be safe but one of the little changes this weekend will be the move to a little rear toe out.
Yah, I saw that. Something about aiming the toe at the center of mass? I wonder if he'd also support toe in on a FF or FR car's front axle, by that logic.

I haven't been on the MR2 site nor read Steves write up in at least 3 years but I think the rear toe was to simply get the car to pivot around its center.My mid engined awd geo metro ice racer uses basically alot of bumpsteer to move the rears into a rear steer mode,it works really well and isn't too unpredictable to drive.

Toe in on a fwd wouldn't point towards the center of mass(in my head anyways)and toe out within reason seems to do nothing but good things(unless your a street tire getting worn out of course).If it was easier to change adding ackerman so 0 toe and reduced scrub in a straight line could be run would be a good way to go imo.Adding toe out is really only faking ackerman anyways so...

Curmudgeon
Curmudgeon MegaDork
4/28/12 7:50 a.m.

Oversteer cold/understeer hot: I'd look at the tires to see if they are really coming up to temp. Cold oversteer to me says the rear tires are too cold to bite initially, but understeer hot says the rears are now up to temp but the fronts are not. IIRC you were running Kumho V710's. That sub 1k lb car is probably just not heavy enough to generate a lot of heat in a stiff carcass tire made for a ~2500 pound (or heavier) car. Might want to goop the tires up with traction compound... or go to a softer tire, like maybe a Hoosier A6 or similar.

kevlarcorolla
kevlarcorolla Reader
4/28/12 11:30 a.m.

Your right I did some intital testing and autox on the 710's,right again that they are the wrong tire for this car.I knew that but thats what I had on hand untill I sourced some lightweight 13's and slicks,I bought a set of used hoosier non dot slicks for some light 13x8 keizers and the difference is amazing. 710's,excuse the loose panel rattling; http://youtu.be/r_opHgKNnfU

hoosiers; http://youtu.be/8r8nhAWhCgs

I have a set of new Avons on the way,now the real chassis sorting can begin.With regards to tire temps its pretty clear that my rear heavy(57%) rwd car will heat the rears quicker than the front-can't really see a way around that.The areo engineer that designed my wings also worked out the loads based on COP location relative to wheel location,the way its set up is to apply more load to the front at slower speeds and gradually shift to more load in the rear for safety at higher speeds.

mazdeuce
mazdeuce Reader
4/28/12 12:57 p.m.

I was thinking about it some more. It seems that once you're lifting the inside rear wheel, no amount of rear bar will increase weight transfer, you've gotten as much as you can get. At that point, all additional wheel rate has to come from increasing spring rate. Or maybe thats not right because as you continue to load the outside rear wheel you have to push up on the inside rear wheel as well because they're connected with the bar. The combined spring rate is still there, but not any actual transfer of grip with the wheel in the air. In fact you're transferring grip to the front inside wheel as you prevent roll. This works so long as you can keep traction on that lone rear wheel. Is this right?

kevlarcorolla
kevlarcorolla Reader
4/28/12 2:33 p.m.
mazdeuce wrote: I was thinking about it some more. It seems that once you're lifting the inside rear wheel, no amount of rear bar will increase weight transfer, you've gotten as much as you can get. At that point, all additional wheel rate has to come from increasing spring rate. Or maybe thats not right because as you continue to load the outside rear wheel you have to push up on the inside rear wheel as well because they're connected with the bar. The combined spring rate is still there, but not any actual transfer of grip with the wheel in the air. In fact you're transferring grip to the front inside wheel as you prevent roll. This works so long as you can keep traction on that lone rear wheel. Is this right?

Your initial gut feeling was right,once the tires lifted thats the end of a larger bar providing benefit.At that point if there's still too much total body roll than the front rate should be increased.

DaewooOfDeath
DaewooOfDeath Dork
4/28/12 3:26 p.m.

Exactly, once the rear goes airborne, there's no point in making that end stiffer until you address the front.

mazdeuce
mazdeuce Reader
4/28/12 3:48 p.m.

Got it.
Thanks.

Curmudgeon
Curmudgeon MegaDork
4/28/12 5:35 p.m.
kevlarcorolla wrote: Your right I did some intital testing and autox on the 710's,right again that they are the wrong tire for this car.I knew that but thats what I had on hand untill I sourced some lightweight 13's and slicks,I bought a set of used hoosier non dot slicks for some light 13x8 keizers and the difference is amazing. 710's,excuse the loose panel rattling; http://youtu.be/r_opHgKNnfU hoosiers; http://youtu.be/8r8nhAWhCgs I have a set of new Avons on the way,now the real chassis sorting can begin.With regards to tire temps its pretty clear that my rear heavy(57%) rwd car will heat the rears quicker than the front-can't really see a way around that.The areo engineer that designed my wings also worked out the loads based on COP location relative to wheel location,the way its set up is to apply more load to the front at slower speeds and gradually shift to more load in the rear for safety at higher speeds.

Hoosier non-DOT compounds tend to take some heating. I have some 13x10 Bassets and R45's on the Jensenator and there is just no way to get them hot enough to stick during an AX run. Might work on a hillclimb but not till the halfway point, by then it's too late. So I am going to 15x8's and Hankook C51's (their AX compound) and see how that does.

mazdeuce, you are right on the money. But here's the thing: if the car lifts the inside rear wheel but does not understeer, there is no point in messing with the front just to lessen the roll angle. Anything you do to lessen the roll angle will involve stiffer front springs and now you are headed in the loss of traction direction.

fast_eddie_72
fast_eddie_72 SuperDork
4/28/12 5:46 p.m.
DaewooOfDeath wrote:
fast_eddie_72 wrote: Holy cow, did I ever miss a few important posts here! Thanks everyone, especially DOD. Man. Lot of reading to do here. Hope I understand it all.
If you get lost ask. If I know I'll help you out. If I don't know, I'm sure we can find somebody who does.

I will get lost, at least some. Thanks in advance. What I need now is a little time to think on it and work on the car.

I think I have everything loaded for tomorrow's autocross. With my handy new pyrometer, at least I hope to come back with some information. I really want to try to do this smart, not just throwing parts at the car and hoping they work.

pres589
pres589 Dork
4/28/12 7:56 p.m.

In reply to Curmudgeon:

"if the car lifts the inside rear wheel but does not understeer, there is no point in messing with the front just to lessen the roll angle. Anything you do to lessen the roll angle will involve stiffer front springs and now you are headed in the loss of traction direction."

If you're on a course in an FWD car and it isn't pushing in the corners and it's got a tire in the air... aren't you "done" with the suspension? More rear roll stiffness isn't going to do anything positive for you (which has already been stated) and more roll stiffness in the front can only unsettle what sounds like a settled situation. Right?

Seems like the thing to focus on at this point would be more power, more brake, or more driver ability to improve things. Please correct me if I'm wrong, I'm just watching and trying to learn concepts through discussion here.

kevlarcorolla
kevlarcorolla Reader
4/28/12 11:28 p.m.

R45 hoosiers are too hard for a light car,I would only use 25b's which heat quickly but in typical hoosier fashion they cycle out just as quick hence the switch to avons for this year.

In reply to press589:

No such thing as being "done" with suspension tuning,yes conventional wisdom says stiffening the front will reduce grip but only if eveything else was optimised to perfection.Nearly all cars can be improved apon,you will likely suffer in other ways from a stiff ride etc however.My usual approuch to tuning a production based chassis is to set a reasonable ride height for decent wheel travel than set camber for a max of -2(any more than that and you start to reduce braking,transition repsonse,corner exit traction of the drive wheels etc etc)and then work spring/bar rates to control roll and maintain good tire temps across the tire and then once thats narrowed down fine tune the toe/castor for good turn in/sweeper grip. Simply follow that and the car will be very nearly as good as can be without a pro team engineer calling the shots and revamping the complete suspension geometry.

Curmudgeon
Curmudgeon MegaDork
4/29/12 8:28 a.m.
pres589 wrote: In reply to Curmudgeon: "if the car lifts the inside rear wheel but does not understeer, there is no point in messing with the front just to lessen the roll angle. Anything you do to lessen the roll angle will involve stiffer front springs and now you are headed in the loss of traction direction." If you're on a course in an FWD car and it isn't pushing in the corners and it's got a tire in the air... aren't you "done" with the suspension? More rear roll stiffness isn't going to do anything positive for you (which has already been stated) and more roll stiffness in the front can only unsettle what sounds like a settled situation. Right? Seems like the thing to focus on at this point would be more power, more brake, or more driver ability to improve things. Please correct me if I'm wrong, I'm just watching and trying to learn concepts through discussion here.

Exactly. Chasing the body roll would do nothing to add to the car's performance. There is no point in chasing further down that road. Anything done to 'correct' the body roll would likely make things worse.

A case in point: back in my R/C racing daze a LOT of guys would get all torqued up because the car would bottom off a jump. This was something they chose to battle at pretty much any cost. Now here's the problem: by the time the car was modified to the point that it would not bottom over the one or two jumps constituting maybe 5% of the course, it was so stiff that little bumps would make it dance like a flea on a hot griddle. IMHO, better to tune the car for the 95% and tolerate (or better yet, drive around) the 5% problem.

So if 95% of the AX car's actions are where they need to be but there's a 5% so called 'problem' remaining which constitutes only body roll, why battle that 5% and screw up the 95%?

Now, if the more power/better brakes/better driver ability change brings the understeer back, then it's time to start looking into further modifications.

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