Strong Understeer in tight low speed corners and tire pressure question

[darock]

Hello Guys.

I read a lot about suspension, sway bars and alignment the last few weeks. I have an "issue" with the car going through tight low speed corners. It understeers. Higher speed corners do work fine and the car feels very stable and balanced even at really high speed cornering.
We have some small roundabouts around where I live and I tried circling one of them at night just for testing. It turns in without power but feels really slow. The more steering angle, the more understeer. When I add a little throttle it starts to push the front wheels even more. Sadly the cops stopped my testing after the third round, but they were very cool and just gave me warning not to do it again :biggrin:

Setup of the car as follows:
OEM 91 springs
Bilstein shocks on lower perch
stock 91 sway bars
Front tires: Federal RSR 595 215/40 R17
Rear Tires: Federal RSR 595 255/35 R18
Rims: Advan T7

Alignment (this setup is actually from the NSXprime wiki):
Front total toe out: 2mm
Front camber: 1°
Caster: 9.5° (did not touch the adjuster yet)
Rear total toe in: 3mm
Rear camber: 2.2° (can't get less due to lowering)

Before the alignment, the car felt way better in lower speed corners but unstable at higher speed corners. It had less than 0.5° front camber and toe in (don't remember how much). It would push sideways a bit when exiting a corner on throttle.

I have two options I may want to try.

1) NSX-R sway bars.
I would just install the front one as the common opinion here seams to be, that the rear R bar is too stiff for a street driven car.
I understand that there is a chance of reducing the front grip with this but also a chance of improving turn in and mid corner grip.
Is it beneficial to install the front R bar?

2) change the alignment

Any thoughts?

The second thing I want to check is the tire pressure. I actually did not look at the tire pressure and I thing it might be a bit too high. What is the tire pressure rating of a stock NA2 with the 215/255 tires?

EDIT: Found it: 33psi(2.3bar)/40psi(2.8bar)

Cheers,
Bernhard

 

[Hapa88]

Installing the front NSX-R bar will actually make Understeer even worse.
I would definitely look into your tire pressures and adjust accordingly.

 

[darock]

Thanks for the comment.

Just got back from the gas station.

The front actually wasn't too bad. It had 2.2 and 2.45 bar. The rear was 2.4/2.45 so quite a bit low.

I made it 2.3 front and 2.6 rear and the car is a lot more tail happy now. More control over the balance on throttle. I did not think that just this minor adjustments would make a noticeable difference. Adding more pressure in the rear may make it too loose in the back.

Still, the NSX R sway bar is not off the list. Swapping the sway bar is a pretty easy job so I might just try it. Why do people love this modification so much if it makes understeer worse?

what do you think about the alignment? especially the caster? I was thinking about going back to 0.5° initial camber which will result in reduced caster as well. Right now it makes just over 4° camber at full steering angle which is too much for anything else than race tires. I have a feeling that it does loose grip in the front due to too much camber at higher steering angles.

Bernhard

 

[darock]

Just for fun I swapped in the R sway bar today. It started raining so the way home was perfect for testing.

The car feels more balanced. Understeer is still there but the car feels like it improved a lot in terms of mid corner grip.
Steering on center is still light and not really precise but I believe that's due to the toe out.
What really changed is what happens when the steering loads up. The car turns in like it has never done before. The steering effort is higher though.

I think I will try adding a little more tire pressure in the rear because I could not get it to oversteer even on slightly wet streets.

The whole front end feels more planted.

I thought everything will get worse with the R sway bar but not at all.

Bernhard

 

[goldNSX]

You have pretty much caster with 9.5. You could go back to 8 which reduces camber in sharp turns.

 

[darock]

That's definitely on my todo list

 

[darock]

Maybe someone is interesting in my experiences with the front NSX R sway bar.

After driving the car for nearly a week now I can say the following.

1) The steering effort definitely increased. Even at lower speeds. Probably more apparent on cars without power steering like mine.

2) Initial turn in is a lot sharper.

3) The front end feels more stable and predictable. I could not test tight lower speed corners yet but will do on the weekend.

4) At really high speed cornering (100+mph) the car feels save. Not that it didn't before but it feels more solid to me now. It gradually starts to understeer when entering the corner with too much entry speed.

5) there is no way making the car oversteer on public roads unless you do something totally irresponsible.

6) Body roll is still there but noticeable less.

So over all, unless the slow and tight corner behavior gets a lot worse, I will keep the R sway bar.
I think it enhances the over all balance of the car without negative side effects.
But that's my personal view, somebody my have different desires for how the car should behave.

Bernhard

 

[RYU]

Tire pressures matter more than most people think. So glad you're sorting out what pressures work for you.

In terms of alignment. My initial reaction is to adjust two things. Your front camber and rear toe, but then I realized you probably on the stock rear bushings and not have any of the non-compliance parts installed so i'd stick with near stock rear toe. Let's move on to front camber. It's simple... max out the neg camber in the front. If you're concerned about tire wear... you need to run a lot of rear toe anyway which will eat up your rear tires so might as well max out the front neg camber and gain all the benefits.

 

[stuntman]

Try trailbraking. When you get on the throttle, it unloads the front tires and makes the car understeer worse -as it should.

If you want a neutral car that doesn't understeer when going around in circles on a roundabout (when you're slightly on throttle) the car will probably be too neutral when trailbraking or in higher speed corners. If your roundabout example has excessive understeer, it would try adding a little more front camber, closer to -1.5* but on top of that, your stock (soft) rear springs, and front springs, are probably making it even more pronounced.

 

[RYU]

Try trailbraking. When you get on the throttle, it unloads the front tires and makes the car understeer worse -as it should.

If you want a neutral car that doesn't understeer when going around in circles on a roundabout (when you're slightly on throttle) the car will probably be too neutral when trailbraking or in higher speed corners. If your roundabout example has excessive understeer, it would try adding a little more front camber, closer to -1.5* but on top of that, your stock (soft) rear springs, and front springs, are probably making it even more pronounced.

^^ listen to this man.

 

[darock]

Thanks for all the information!

I have to practice trailbraking. I always try to finish braking before entering the corner to minimize the risk of oversteer. I am definitely not a good enough driver. Planning to do a driver training anytime soon.

I will try adding camber. This will get me even more caster. Should I still reduce caster like goldnsx mentioned?
I don't get why adding camber will make it better. The car makes 4degrees of camber at high steering angle. That's already quite a lot for street tires.

How about rear toe. 3mm toe in is half what honda suggested. Shouldn't this make the rear end more alive? Going back to 6mm theoretically generates more unter steer or am I wrong on that?

bernhard

 

[stuntman]

I would add -0.5* front camber and maybe take out some caster to make the steering lighter.

I'd leave the toe at 3mm and work on your driving. Try reading these:


DRIVER DEVELOPMENT: Car Control

http://www.motoiq.com/MagazineArticles/ID/3519/DRIVER-DEVELOPMENT-Car-Control.aspx

"Braking late, hitting apexes, and powering out of corners: The goal of driving a car quickly requires keeping the tires at their limit at all times. Having good car control is a crucial skill in the development of a driver to not only drive a car fast, but also to have the ability to get up to speed quickly in a new car and on new tracks. However, without self control and discipline, having good car control can actually work against you."


DRIVER DEVELOPMENT: Learning Processes

http://www.motoiq.com/MagazineArticles/ID/3686/DRIVER-DEVELOPMENT-Learning-Processes.aspx

"How do we become better at driving? Practice makes perfect, right? Well not necessarily. Without feedback and direction, additional practice won’t always improve your ability and can possibly lead to the development of bad habits. So in reality, perfect practice makes perfect. In Part 2 of the Driver Development series, we discuss how drivers learn to improve their craft through various methods from coaching to data analysis to simulators which all aid in the advancement of the skill of driving."

 

[RYU]

At your ride height i'd be surprised if you get much beyond -1.5* in camber anyway which is why I suggested to just max it out. Please report back how much neg cam this ride height could get you. THanks!

 

[darock]

Thanks

1.5degree camber and reduce caster back to 8 sounds like a plan.

I will report back as soon as I find time to do the adjustments.

Bernhard

 

[darock]

I thought you may like that

Seriously, I have no idea what made the difference. Was it the tire pressure or the sway bar? no idea ... The car felt amazing the whole weekend around the alps.

No understeer like it had before but stable. No oversteer when entering a corner on the brakes, even downhill. No oversteer when powering out of a hairpin in first gear.

Only the higher steering effort needed is a bit of a draw back when driving tight mountain roads half a day. I will follow your advice on camber/caster adjustment in near future.

Bernhard