Increasing rear grip...options?
 

Coming out of a corner getting on the gas my car feels loose. 28 front bar and 12 rear 550/325 springs. Otherwise the car feels pretty good. I could add 3mm packers to the front shocks...would that improve weight transfer to the rear? I could add the 3mm packers to the rear which would increase spring rate as the rear springs compress but I think that would increase oversteer. I could add a bit toe in at the rear at next alignment and I could increase the rear negative camber. Does increasing the rear negative camber increase rear grip? On my street car I've got .5 degree more negative camber at the rear vs the front and the car feels more stable on track.

Try removing your rear bar.

remove rear bar winner!

suspension tuning 101:
softer = more traction
harder = less traction

you can either make the grippy end less grippy or make the slippery end less slippery.

Note that in more advanced lessons, this does not always apply.

Differential. OMFG my green car hooks on exit.

Less rear ride height, less front rebound. Don't bandaid it with toe changes. When was the last time the car was cornerweighted and/or aligned? What does the tire pyrometer say?

Ride height at the pinch welds is pretty much even front and rear about 5 1/4 inches, I know lower is better but I have to be able to get it in my trailer. Shocks are SD Bilsteins so no rebound adjustability.

I'm not sure it's a differential issue as the 1.8 track car has a VLSD and the street car has a 1.6 with an open, granted, the 1.8 has gobs more torque.

With those spring rates you might actually be on the bump stop with the outside rear in the corner, giving you an artificially high spring rate at the rear. Check the travel with the zip tie method before changing a bunch of things.

Bump the power down.

:ban?:

The Viscous LSD is the 1.6 LSD. Are you sure you have two failboat diffs in your car? Or do you have a torsen in the 1.8 race car?

There's a little table I posted long ago from a book I read even longer ago about oversteer and understeer. Basically what the others have said, but it includes tire pressure, tire grip, spring rates, sway bar rates, downforce, and other things, and whether or not they increase oversteer or understeer when you increase/decrease them. Hopefully that sentence made sense.

This idea occurred to me as the track car has NB mounts and the street car has 1 inch ISC mounts.

The track car has a 99 1.8 swap with the stock 4.3 VLSD and the street car has the open diff.

Better throttle control. Oh, and no rear bar.


INB4 datalogs.

After crawling under the cars it's obvious that the street car with ISC top hats has more shock travel before contacting the bumpstops, probably 3/4 of an inch more. Since the setup is basically the same between both cars I'm going to try different top hats in the rear of the track car and see if that solves the issue.

You can effectively get the feel of no rear bar by disconnecting one end of one end link and tying it out of the way. I would do this before anything else as it is a very easy experiment.

Check the sag like you said. If the bump stops are hitting on corner exit (and the fronts are not hitting at the same time) the rear will get loose, maybe very loose depending on the stiffness of the bump stops. If this is what is going on, you need to fix the sag before you try anything else. Removing the rear sway will not help.

If it is the bump stops, I have seen this before and you will go nuts trying to correct this until you increase the sag.

^He's right. Less rear bar would make it much worse if you are hitting the stops. More rear spring or more rear bar is the answer if you can't readily add travel.

My car is pretty neutral with 550/350 and FM sways (25mm full stiff/17?mm full soft) and ISC hats. You have more front bar, less rear bar, less rear spring, and less travel. I could see where you would get enough body roll to get hard against the stops.

The bumpstops came with the shocks. They were cut down to 36mm. I'm guessing that they ramp up much faster than the 46mm ones on the street car.

Also, I crawled under the car and with NA Bilsteins I can slip one finger between the top of the front shock and the bumpstop so the amount of lowering left at the front is very little. Since I'm running at a very rough track any more lowering at the front probably isn't a good idea.

It's like he's not even listening.
:winner:

More rear bar won't work as I already had a 15mm rear swaybar and it was even looser than with the 12mm swaybar.

The front bumpstops should be engaging before the rear bumpstops as I have a 3mm spacer up front.

Pretty sure most of the people posting in this thread are telling you LESS rear bar, not more.

I already have a 12mm rear bar on the car and 28mm on the front.

Disconnecting an end link is the least rear bar of all.

Every time someone suggests removing the rear bar on a track car, I cry myself to sleep.

Me too.

I still don't think he has checked into whether he is running out of rear suspension travel mid-corner.

Unfortunately I can't as it's a track only car so I'm going to make a change before I go to the track again. Most likely I'm going to switch rear shocks between the 2 cars and replace the NB top hats with ISC top hats for a bit more travel. Just bouncing on the back of the car the setup currently on the track car is softer than the one on the street car.

Shouldn't that be *more* front rebound? The reasoning being - on corner exit you're unwinding the wheel and adding more power... the car derolls and squats, so nose comes up, and more low speed front rebound will reduce front grip and thus reduce oversteer?

I swapped the shocks today. The 325# springs went on the street car and the 350# went on the track car. I also added the ISC top hats. I took the street car for a ride and it's apparent that the shocks that were on the track car are valved softer than the ones that were on the street car. After the switch the street car now kinda glides over rough pavement. Hopefully the increased rebound will help keep the rear planted better coming out of corners on the track car.

More front rebound will keep the nose down on exit and increase front grip. Let the nose rise and weight shifts to the rear tires on exit as it should. Hence less front rebound (at least IMO).

Doesnt weight shift irregardless of body roll.

In talking with a friend he said that in dirt track racing that they'd use a soft rebound shock on the right front and a stiff rebound shock on the left rear to increase rear traction coming out of a corner. Since we have to turn both ways on a road course you'd tune both shocks on an axle the same.

You should race dirt track. Problem solved for us.

A dirt track Miata would be cool.

So do you know if you were on the bumpstops exiting the corner or did you just guess you were?

Just a guess, I'm headed to the track Sunday so I'll find out if I've solved the issue.

I was talking to a guy today who told me that the local dirt track was having a race this weekend with a $50,000 purse. No wonder dirt track racing is so popular.

Mine is very well balanced with the fm front barand stock rear bar. Much better than with no rear bar. The car it's extremely predictable.

I've got the nb tophats, fcm bumpstops. I think the rear bumpstops may be cut in half, id have to check.

Maybe slightly less front bar?

https://www.miataturbo.net/attachmen...ine=1381454550



The balance of any car between oversteer and understeer is a function of weight balance front to rear versus spring rates front to rear (and the sway bars are springs). If a setup has so much rear spring that a rear sway bar is unnecessary to achieve balance, then you have too much rear spring or not enough front spring (including sway bar). This is a bit of a simplification for the sake of illustration as there are a few other mitigating factors involved as well. Such as, the rear suspension bottoms out causing the car to behave as if the rear spring rate has suddenly skyrocketed (thus causing an abrupt onset oversteer condition).

Update, took both cars to Nelson Ledges. If you aren't aware, this is probably the bumpiest track in North America. The track car felt better, I didn't feel as if the rear was going to slide out at the touch of the throttle. So the changes that I made by switching shocks and springs and tophats seemed to improve grip. Incidentally, switching shock and springs to the street car didn't seem to have a negative impact. The 1.6 is definitely lacking power but the car is able to put that power down quite effectively. For some reason it seems to hook a little better than the track car(maybe the cage plays a role). My friend took me for a few laps and about scared the crap outta me. I was seriously impressed that despite throwing it into the corners it didn't break traction despite the tires howling in protest. That being said, after driving both cars, I'm coming to the impression that the Yoke AD08's are a more forgiving tire than the Star Specs on track.

I would be willing to bet your problem is shock travel, shit bump stops, greasy tires, bad driving, and unpredictable diff engagement combined.

Yeah, but if you take all of that out of the equation there wouldn't be a problem.

Oh, I'll readily admit that I'm not the best driver, I'm fairly slow but I am focusing on being smooth and increasing speed. My friend who is a good driver has said that the street car seems to stick better as well.

One thing I noticed last time I was at the track, we swapped the Hoosier R6s on the track car and the improved grip helped. I was actually able to get the car to understeer by turning the steering wheel quickly.

Maybe the VLSD is the difference between the 2 cars.

Read up on how a miata VLSD works, then go wtf was Mazda thinking. I currently am DD one now and it literally works some days and some corners or not at all. Try to get a 4.30 Torsten out of a VVT car.

Have you considered the possibility that the track car is on the rear bumpstops when cornering under power? :party:

Yes, that is why I traded shocks between the 2 cars and added the ISC top hats on the rear of the track car for more travel. It seems to have helped. I decided to add another 3mm packer on the front shocks to see if that makes any difference in feel. I plan on hitting a trackcross event at Pitt Race in a couple weeks.

One difference between the two cars is that the street car seems to have a bit more suspension travel.

Yeah, I guess I was being a little sarcastic. I think your entire problem is that you're hitting the rear bump stop under power. It's worse with the more powerful car because the rearward weight transfer is greater so you're hitting the stop sooner/harder.

I know that people riding around on the street with soft springs take it for granted that they'll be on the bumpstops when they get frisky so they tune them front/rear to balance things. I can't help much with that, but adding the packers to the front will cause the front bumpstops to engage earlier and supposedly bring some balance. I'm not qualified to tell anyone the best way to set up a car for track, but I'm pretty sure that's not it.

The ISC top hats helped increase rear wheel travel. Great. I would try raising the rear ride height to increase available travel even more. I know it goes against what Andrew suggested, but I'd give it a try anyway. He won't know if you don't tell him.

Well, I'm currently using the 1 inch top hats in the rear, I have a set of 1.5 inch top hats in the garage, I could swap them over winter.

I figured I'd update this again. Since the last track day I added 3mm packers up front, lowered the rear of the car slightly and took off the Star Specs and installed the Yoke AD08's. I went to Pittrace to do a trackcross. In September the temp was mid 70's and the car was really loose. This weekend the temp was 45 and cloudy with gusty wind and the track lacked grip. Surprisingly after a couple laps the Yokes warmed up enough to provide adequate grip. The car seemed easy to control and wasn't prone to over rotation as in the past. Overall I was pleased with the feel of the car and was able to drop my times by 3 seconds compared to September. Even though I made more changes over the last month than just tires, I was impressed by the grip the AD08's held in the cold.

I think you touched on a good point about knowing when your tires are warmed up. My RE-11s took three laps at barber before everything came together.

Just so I got this straight......you've ignored every suggestion from qualified persons, made your own changes based upon anecdotal evidence suggested by local dirt track racers....and finally noted that tires up to "temp" made the car predictable......yet no use of tire temp gauge......on a short course where speed and cornering forces are limited...

You must be a colossal idiot.

I've tuned the car based on advice setup from the 949 page and from the advice of some members here who have contributed. As for you asshole, If you don't have something of substance to add then STFU.

Based on what I've read of Emilio's Xida setups he doesn't have much shock travel until the front bumpstops start to engage. That would suck on the street but it seems to work quite well on the track. Looking at the Xida photos you can see the rear bumpstop is quite a bit shorter than the front one. That caused me to conclude that adding a second 3mm packer up front might be beneficial. Adding the ISC tophats in the rear increased travel and seems to have made a difference as well.

Savington, your suggestions seem to have helped, thanks.