Understeer problem after NT05 install

[MustASH]

Hi folks,

Short story :

I've replaced my NT555 for a set of used NT05, but now I got a big understeer problem in 2 turns of my local track. I never had any understeer with the NT555.

I'm Wondering what I should look at first, considering I tried different lines without any change. I join a short clip to let you see and listen the car understeer and pulling as f... into the exterior wall.

Your help/hints/suggestions/questions would be appreciated !

But first, here's the actual tires/suspension setup :

'05 GT Vert
4x 275/40-18 NT05 - 36 psi @hot
H&R Race on Koni sport at 1/2 turn from full stiff in front, 1 turn from full stiff at rear
-2.5 negative camber
Front swaybar ajusted at full stiff
No rear sway (Should reinstall it would give more load on the rear ?)

Here the short video (sorry for the poor sound quality...) :


https://www.youtube.com/watch?v=eUBjiM6hFbU

 

[BMR Tech 2]

The stiffer your front bar or the softer your rear bar, the more prone to understeer the car will be. Soften the front bar up or add a rear sway bar.

 

[Norm Peterson]

Agreed ↑↑↑

I'd start with an adjustable rear bar on the softest setting and go up until it gets too "loose" (and dial it back to the previous setting).

I'd also try 2 or 3 psi lower tire pressure in the rear than up front, up to maybe as much as 4 or 5 after some experimentation to see what you do like. That can lighten the understeer a bit all by itself, while giving you a little extra cushion against power-induced oversteer.


On edit - how wide are the wheels that those NT05's are mounted on? Do you know how many events were on them when you got them?


Norm

 

[Sky Render]

My guess is the (slightly) stickier rubber is letting you push your car harder and find the car's limits.

I second the idea of adding an adjustable rear bar. BMR and Whitelline both make excellent adjustable rear bars.

 

[Speedboosted]

Get a Whiteline rear bar, and go from there. Who's front sway bar do you have? H&R Race springs have a lot of rate in the rear (I have them as well) but I still think they need a sway bar out there to accompany them. You could also try to drop some PSI from the front tires, thus giving them slightly more grip.

For my setup with those springs I have Bilstein's all around with camber plates at -3* on the track, BMR 35mm at the stiffest setting, Ford Racing 22mm rear bar, Whiteline watts, and 275/35 BFG Rival on 18x9.5 F14's. I just recently borrowed the 22mm rear bar from a friend to try out after I had too much understeer with the factory 20mm bar. The FR 22mm bar was a nice improvement, but as it was non adjustable I didn't care for it that much so I just purchased a Whiteline rear bar that will be going on soon. Tons of adjustability and it's incredibly easy to adjust as well.

 

[CCS86]

My guess is the (slightly) stickier rubber is letting you push your car harder and find the car's limits.

I second the idea of adding an adjustable rear bar. BMR and Whitelline both make excellent adjustable rear bars.

Yup. More grip = more force = more body roll and suspension deflection = changes in toe, more positive camber, etc

 

[MustASH]

Wow, thanks guys for your inputs. It seems you all point into the same direction, so I'll reinstall my rear sway first, then play with tire pressures and testing around that.

I have the Eibach 35mm ajustable front sway, and their 22mm non-ajustable rear.

For Norm : NT05 are mounted on 18x9 wheels (I dream on 18x10 upgrade...) and I have 25mm hubcentric adapter in front to clear Brembo. That's not the best setup but for now it's doing job.

I should be able to test it on next monday, I'll keep you posted for sure. Until there, feel free to post if you have some other good idea.

Thanks !

 

[fast Ed]

If it was me, I'd start with lowering the rear tire pressure first ... easier than getting underneath the car to put the bar back on! The way I look at it, it's a front-heavy car, with the same tire size all around 3 to 4 more psi on the front makes sense. Works on every Mustang I've ever tracked.


cheers
Ed

 

[Tri-bar]

Before you put the rear bar back on, just soften up the front bar , one hole at a time.. With those spring rate. a 18MM rear bar may be all you need if you leave the front on full stiff. my $.02, never been a fan of big rear sway bars.

 

[noldevin]

Like Tri-bar said, I'd loosen up the front bar before adding a rear bar. Adding the rear bar will just take away grip from the rear to even things up. I'd rather add grip back to the front by loosening.

 

[Norm Peterson]

Loosening the front bar is an easy test, but while you improve the front vs rear distribution of lateral load transfer, you're paying for that with slightly more roll (and camber loss up front). Indirectly, you still lose a little rear grip.

On a stick axle car, the front bar is your primary means of limiting roll once you've chosen your springs (and perhaps settled on the amount of camber that you're willing to run).

The rear bar is only for fine tuning the handling after that, and that's where I think OP is at this point.


Norm

 

[MustASH]

I love this forum ! your comments are helpful, I appreciate it. Not much guys up there track their Mustang, so you guys are my reference. For now I'll reinstall my 22mm rear bar, lower the rear tire pressure and test it on track tomorrow in the evening. If i'm not satisfied by the way I feel the car or my lap timer, I'll soften the front bar directly at the track and see the results. If I'm not satisfied at all with the rear bar, I'll get if off and play with the front bar and tires pressure only, until I am.

I'll let keep you posted.

A+ and thanks !

 

[MustASH]

Should I trim the rear bump stop ? According to H&R they told me not, but when I look to this picture Im wondering if the frame hits them when I had the OEM springs ??

Btw, rear bar installed.


Sent from my iPhone using Tapatalk

 

[Speedboosted]

Take the first nub off that bracket and see where it gets you.

 

[Tri-bar]

http://www.maximummotorsports.com/Rear-Bumpstop-2005-14-Mustang-pair-P1447.aspx

 

[Norm Peterson]

Maybe try removing half a nub if your lowering is minimal. You can always cut more off if it's not enough, but the more you can leave the softer the hit when the body does contact it. That's a good thing.


This almost matched the amount of lowering I was shooting for and eventually managed to end up with.

Norm

 

[DevGittinJr]

http://www.maximummotorsports.com/Rear-Bumpstop-2005-14-Mustang-pair-P1447.aspx

This.

 

[MustASH]

Live at the track...after 1 session, WAYYYYYY too stiff at the rear. Oversteer as f....


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[Vorshlag-Fair]

We always tell people to NOT base any testing / issues / problems on old junk tires. At least with a new fresh set of NT05s it sounds like you have 4 new tires now, so the understeer might have been there all along. And while I'm not the biggest fan of the NT-05 tire, at least its a 275mm and fresh - which is better than most "random folks" in S197s we see at the track.

Definitely run a rear anti swaybar bar... the S197 cars will almost always push without a rear bar (every single one I've driven w/o a rear bar was an understeering pig). Yes, there's a dozen internet experts that will pop up saying "no bar is best" but as soon as you ask them "what have they won?", they tend to shrink away.

I like larger, adjustable swaybar at both ends, which I use for tuning and some roll control. Then proper spring rates and real dampers to get ride of the rest of the roll and all of the squat, dive & heave.

There is no magic swaybar or "formula" that works for everyone, but a set of bigger-than-stock bars with several adjustment holes tends to work for most folks. Lots of variables go into picking the spring rates, camber settings, tire pressures (500psi!), and other variables (like shocks and spring rates) - which depend on the tires, competition use/level, roads/tracks you drive on, etc.

 

[Tri-bar]

MustAsh, grab one. then set your front bar to stiff.
https://www.partrequest.com/auto-pa...6-18mm-rear-sway-bar-6r335a771bb-251643157067
http://www.ebay.com/itm/05-06-07-08...ash=item3a9719da4b:g:O2sAAOSwDk5UEgQN&vxp=mtr