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New tires, rear end feels loose

5K views 26 replies 10 participants last post by  freewayflyer 
#1 ·
My mods are in my sig. When I'm running my daily tires, DWS06 275/40-19, the car feels reasonably well balanced. But, when I put on the RE-11s, 285/30-18, the backend feels like it wants to just break loose. The frontend sticks well and braking feels good.

I've tried running a lower air pressure, 36f/35r and it didn't seem to make a difference. I also tried 36f/36r, no noticeable change. This was measured with a digital tire gauge, tires cold.

Any suggestions?

Thanks.
 
#2 ·
I dunno, just a SWAG to keep the conversation going --

Since you posted here, I assume you are talking about road track performance . . .

Those cold pressures could be way too high. What are your hot tire pressures? Most tires have small roll-over marks on the sidewall; often they look like a little triangle. Are the scuff marks short of those symbols, or going past?

If the scuff marks from the tire rolling over in corners is short of the symbols (meaning toward the trear / away from the sidewall) you should (generally) reduce the tire pressure. If the scuff marks are going across the symbols, then add tire pressure.

My guess is you might need to be down around 32 psi HOT; but like I said that is just a silly wild assed guess.
 
#5 ·
I think noldevin hit the nail on the head.

275/40R19 is 10.83" wide and 27.66" tall. That's 86.9" per rotation.

285/30R18 is 11.22" wide and 24.73" tall. That's 77.7" per rotation.

That means your usual tires are 2.93" taller and 9.2" farther with EACH rotation.

That's a huge difference.

Stopping won't change much because you're running a hair wider tire; however, you practically just put steep gears in your car expecting it to plant the same as your regular tires. I'd throw on a set of 295/35R18s
 
#4 ·
JBert - I put them on and drove to/from work for a week to make sure they were balanced and any manufacturing lubricants were worn off. When I get to a track, I'll follow your tips. Thanks.
 
#7 ·
The "steeper gear" I knew would happen with the smaller diameter. I'm thinking it's also exacerbated by my "ham fisted" throttle technique. I've gotten a lot better since buying the car, but I still have a tendency to just "stab the throttle".

Hopefully, I can learn to work with these tires. They only have about a 200 wear rating, so I'm guessing they'll be ready for replacement next year.

One suspension upgrade I haven't done is swaybars. Would a new set of front/rear swaybars help at all?

Thanks for all the comments/input. I think this is a classic case of, "more is not necessarily better". reality.gif
 
#8 ·
A stiffer front swaybar should theoretically help push the car more towards understeer, but that still won't fix a very short rear with a very thin sidewall and punchy throttle application. Smooth is fast :) One thing that has helped me a lot with smoothness is the "string theory"
Basically pretend your steering wheel is attached to your foot, so you can only apply gas or brake proportional to how straight the steering wheel is.
Also any time you stiffen up the suspension, the car tends to react quicker. Which is not always a good thing, especially when you're having oversteer issues.
 
#9 ·
This ↑↑↑


flyer - Your track tires have about an inch less sidewall, meaning that the treads can't flex as far and that they will develop lateral force more quickly. If your 275/40's are on 9" wide wheels, there's a little more cornering stiffness coming from your track tires' 10" wides and things will happen more suddenly from that effect as well.


Tire flex ↓↓↓ . . . compare the left rear against the right rear.




The softer-responding 275/40's have been covering you for greater "errors" in the speed of adding throttle (and probably steering as well). Since it's easier for most people to keep up with slower-responding setups, you haven't had to get particularly smoother/slower with your throttle inputs until now. It'll probably take time, and definitely conscious effort.

Ideally you want to be able to feel when the rear tires are just starting to drift soon enough to breathe off the throttle and let the tires stay stuck instead of blowing clear through this zone so fast that you don't feel a thing until "oh, crap, gone and done it again". Running wet autocrosses is as good and as safe a way as I can think of for first discovering this.


Norm
 
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#11 ·
With the OE springs and the Strano 35/22 bar combination I was running the rear shocks anywhere from 1/4 to 3/8 turn softer than the front struts. That was with all three sets of settings (track, street me only, street with wife along, listed in order of decreasing firmness).


I've recently swapped BMR's GT500 handling springs on, along with the Strano 25 rear bar. Haven't been able to play with it long enough or hard enough to call anything for that setup yet.


Norm
 
#12 ·
On the track, I crank the front shocks all the way firm and the rears are 1/2 turn from full firm. On the street, I leave the front/rear turned to the full "softest" setting.

I've also been paying closer attention to how I drive. The flexing sidewalls are covering-up my exuberance with the throttle. I've started applying "string theory" while driving. Hopefully, some discipline in my daily driving will translate to the track.
 
#13 ·
I've also been paying closer attention to how I drive. The flexing sidewalls are covering-up my exuberance with the throttle. I've started applying "string theory" while driving. Hopefully, some discipline in my daily driving will translate to the track.
Hold onto that thought . . . and consider the idea that most everything you do in your street driving ends up teaching you the way you'll naturally tend to drive on the track because it'll be habit. Both the good things and the bad.

The good stuff is things like 'smoothness' and this 'string analogy', thinking about corners in terms of turn-in, apex, and track-out points even while still staying in your one lane, and not riding the brakes even lightly as you go around corners.


Norm
 
#15 ·
This ↑↑↑

And if there's even a remote possibility that you'd ever run a track day session in the wet, have some wet autocross experience under your belt first.


Norm
 
#18 · (Edited)
You asked about anti-roll bars. I think adjustable ones can be a useful tuning tool, but here are a few things to keep in mind if you go that way. All anti-roll bars tend to lift the inside wheel. If you have a car with good torque and marginal traction [ ie. your smaller diameter tires ] it will be difficult to put the power down as early and as forcefully as you would normally wish to do. I see that you have the standard 3.31 gears so you probably have a standard clutch type limited slip diff. I think the torsion type would be better in this situation, but I suspect even a torsion will be hard pressed when the inside wheel is trying to lift. With adjustable bars a softer rear setting may help keep the inside planted.
 
#19 ·
I installed a Ford Racing suspension package and Nitto NT 05s and this was amazing on my 2010 GT. Broke this in at Ciruit Mont Tremblant and at Watkins Glen. Amazing. Then, on a hot day, I ran out of brakes. Now I need racing brakes (Brembo Kit) and probably cooling ducts. Where does it end? Or does it?
 
#23 ·
Well, I finally had another trackday. Actually, 3 days in a row up here at Road America (NW Shelby Club). After two days, I will say this, "It was just too cold for those tires. It also seems they need to get one really good heat cycle." After the first session, the car felt 100x better. I'm now playing with tire pressure to optimize grip/feel. I'm having a hard time figuring out the car's limits with these tires.

On a scary note, I almost blew through Canada corner. I hit the brakes a little too late. I made it through the turn, but it was not something I want to repeat.
 
#26 ·
Tire pressures are probably the hardest to figure out, track to track, month to month. When I run RA, I run 30 psi cold in all 4 and after 1st lap, they're ready. For me, the Toyo RR's need to be around 38-39 hot, that what I shoot for. I go to Mid Ohio and I run 29 front, 28 back cold. Any more, I don't get the grip I need. My tires range between needing 31 to 28 cold, depending. I keep notes of what psi I start off at and what I end up at for each track and what the air temp is, etc...so I have a base line. I'm now incorporating the ambient air temp changes into my psi's. At Gingerman a few weeks ago, started off at 69* in the morning, 1st run, but by after noon 3rd & 4th runs, it was 89*, so my psi rose 2 psi just from air temp. Lots to keep track of, but does pay off in the long run. Now I'm running Hoosier R7's and have to start al over with my note taking. IT NEVER ENDS!! But its a blast.
 
#27 ·
Blazin72 - I ended-up with 36F/35.6R. And like you, I'm still not sure of the limits with these tires.

These tires plus Carbotech XP12s equals brake ducts in my future.

A big thanks to everyone for all the comments/suggestions.
 
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