Oversteer on trail brake

[omx500]

Hi. I went along to a driver training day at my local track (Baskerville near Hobart) yesterday. My instructor said I was pretty 'loose' on trailing brake into corners especially downhill ones. He took the wheel and we had a couple of 'moments' and he said that it would be worth looking into dialling some of that trait out of the chassis, as well as driving better to compensate. My NA has the same gear front and rear, as far as suspension goes, so I had a bit of a go with the following:
- lowering rear tire pressure went in two lots of 2 psi increments from 34 down to 30 with no obvious improvement
- reduced low speed damping by two lots of 2 clicks of 14 total on rear shocks with no obvious improvement in trail brake oversteer but noticeable worsening of transition weight transfer (I was all over the place at 4 clicks back - that is softer than I run on the road)
- reduced rear ride height by about 10mm which seemed to improve it slightly, but then my front brakes melted and I went home

Am I on the right track with the ride height? Should I try adjusting rear camber and if so, which way (can't get much more negative with standard adjusters). Rear spring too hard? Rear sway bar too big? Finally, it has just occurred to me that I cannot rule out a brake bias issue, though by the time I am turning in most of my braking is done and I really am just easing off.

Any thoughts or advice appreciated.

Cheers, Omar

[manga_blue]

I only use trail braking to correct understeer in cars for that are prone to it. I never use trail braking in my own car cos it's balanced and doesn't need it.

If you do have too much oversteer you'd just need to fix it in the usual ways: smaller rear bar (or just remove it), softer rear springs, stiffer front bar or springs, etc.

[chrons_rotary]

+1 on softer rear end, start with the anti-roll bar, and see how you go.
I've gone back to standard 12mm NA rear bar with larger front bar. Seems to help with the same issue.

[madjak]

Try setting up the car with some rear rake (6mm higher at back of chassis rails compared to front) and set your rear toe to 0.

Mx5s typically have excellent trail braking so you generally have something wrong if its happening everytime. You can easily lose the rear on a bumpy track if your suspension isnt that great.

[greenMachine]

Wheel alignment? Especially rear toe?



ETA: Madjack beat me to it, I'd even add a little toe in. And bushes - wornout bushes will allow movement which undermines your alignment.

[omx500]

Thank you for the rapid input folks

I surmize that I have got something wrong to introduce this trait. Pretty sure I am lower in the back than the front, so if going the other way is the done thing I can give that a go. Will hunt around for a standard rear sway bar as this is something easy to try as well. Current rear toe is about 1.5mm with camber just under 2.5 degrees neg at regular ride height before I lowered it looking for an improvement. I will see if I can get closer to zero toe. I have all new poly bushes front and rear - what a fun weekend that was. After that I can look at springs as I just took what was suggested when my local shop made enquiries to the spring manufacturers on my behalf. On the road this issue is not noticeable, but I know it is hurting my lap times at the circuit. That and my gearbox which seems to completely lose gears, just when you would really like one. But that is a story for another place and time

Thanks again.

[pepejesus]

Hi. I went along to a driver training day at my local track (Baskerville near Hobart) yesterday. My instructor said I was pretty 'loose' on trailing brake into corners especially downhill ones. He took the wheel and we had a couple of 'moments' and he said that it would be worth looking into dialling some of that trait out of the chassis, as well as driving better to compensate. My NA has the same gear front and rear, as far as suspension goes, so I had a bit of a go with the following:
- lowering rear tire pressure went in two lots of 2 psi increments from 34 down to 30 with no obvious improvement
- reduced low speed damping by two lots of 2 clicks of 14 total on rear shocks with no obvious improvement in trail brake oversteer but noticeable worsening of transition weight transfer (I was all over the place at 4 clicks back - that is softer than I run on the road)
- reduced rear ride height by about 10mm which seemed to improve it slightly, but then my front brakes melted and I went home

Am I on the right track with the ride height? Should I try adjusting rear camber and if so, which way (can't get much more negative with standard adjusters). Rear spring too hard? Rear sway bar too big? Finally, it has just occurred to me that I cannot rule out a brake bias issue, though by the time I am turning in most of my braking is done and I really am just easing off.

Any thoughts or advice appreciated.

Cheers, Omar

What are your:

Spring rates front and rear
Ride heights front and rear, measured at the pinch welds
Sway bars front and rear
Wheel size
Tyre make and size
Alignment settings (camber, castor, toe - front and rear)

Is it an NA6? How much power are you making?

[93_Clubman]

gone back to standard 12mm NA rear bar

If you're ever looking for an 11mm rear bar, our NA8s used them, as opposed to the 12mm used on our NA6s.

[omx500]

What are your:

Spring rates front and rear
Ride heights front and rear, measured at the pinch welds
Sway bars front and rear
Wheel size
Tyre make and size
Alignment settings (camber, castor, toe - front and rear)

Is it an NA6? How much power are you making?

Thanks for your interest. I didn't put in all detail as I thought it was more of a balance thing. I had to go and look at my springs to get actual rates. They are apparently 200lb/inch front and 150lb/inch rear which sounds a lot softer than numbers I have seen around, but nonetheless that is what they are. They are mounted to Drummond Motorsport "50mm" coilover bodies. I have FM extended coilover top mounts in the rear to maintain a bit more stroke. Can't measure ride height as wheels and brakes are all off atm. It is lower in the back than front, but not sure how much. Bars are 22mm front and 16mm rear. Wheels are 15x9 +35 with Hankook Ventus RS3 in 225/45. Current rear toe is about 1.5mm with camber just under 2.5 degrees neg at regular ride height before I lowered it looking for an improvement. Front has 1.2 degrees negative camber with zero toe. Poly bushes in everything except diff mount. It is NA8 1996 and makes 210kW.

Does this give you anything to go on?

[pepejesus]

The biggest problem is the rear bar. It is waaaaaaay too big. You will never need a 16mm rear bar.

If you only make one change, go back to a stock rear bar, which should be 11 or 12mm.

Other low hanging fruit would be your rake (ie difference in height front to rear) and wheel alignment.

As others have mentioned the MX5 generally likes a bit of positive rake (ie front lower than rear). Try about 5mm. DON'T do this unless you have changed the rear bar as it will likely worsen you overseer.

After you replace the rear bar and add some take you may also find some improvement by adding more negative front camber. I wouldn't do this until you've both replaced the rear bar and sorted your ride height and added some rake.

Once you've got it fairly stable and consistent, I'd recommend just getting as much track time as you can and improving your driving. Once you're consistent and quick, I'd suggest new shocks/springs with much higher rates (550/300 - 800/500lbs/in or 10/6 - 14/9kg/mm) and a bigger front sway (1.125" x 0.188" hollow) and a 14mm rear.

This is a good cheat sheet for setting up a quick, consistent MX5: http://949racing.com/miata-race-alignment-info.aspx

[omx500]

The biggest problem is the rear bar. It is waaaaaaay too big. You will never need a 16mm rear bar.

If you only make one change, go back to a stock rear bar, which should be 11 or 12mm.

Other low hanging fruit would be your rake (ie difference in height front to rear) and wheel alignment.

As others have mentioned the MX5 generally likes a bit of positive rake (ie front lower than rear). Try about 5mm. DON'T do this unless you have changed the rear bar as it will likely worsen you overseer.

After you replace the rear bar and add some take you may also find some improvement by adding more negative front camber. I wouldn't do this until you've both replaced the rear bar and sorted your ride height and added some rake.

Once you've got it fairly stable and consistent, I'd recommend just getting as much track time as you can and improving your driving. Once you're consistent and quick, I'd suggest new shocks/springs with much higher rates (550/300 - 800/500lbs/in or 10/6 - 14/9kg/mm) and a bigger front sway (1.125" x 0.188" hollow) and a 14mm rear.

This is a good cheat sheet for setting up a quick, consistent MX5: http://949racing.com/miata-race-alignment-info.aspx

Thanks for the specific advice. Link is great too (I'm still reading it). Now, where did I put that standard rear bar.......the tip methinks
More time setting up and actually driving to get some consistency is a real goal. Extra power has made me a loose cannon.

Cheers

[93_Clubman]

To give you an idea of NA & NB anti-roll bar sizes & combinations over the years:

MX5 ADM OEM anti-roll bar sizes:
NA6: 19mm Front & 12mm Rear (USDM 1993 NA6 LE with Bilsteins: 19 F & 11mm R)
NA8: 19mm F & 11mm R (USDM standard Packages: 19mm F & 11mm R)
NA8 Clubman: with Bilsteins & TorSen: 20mm F & 11mm R (USDM R Package with Bilsteins & TorSen: 20mm F & 12mm R)
NB8A: 22mm F & 11mm R
NB8A: 10AE with Bilsteins & TorSen: 21mm F & 12mm R
NB8B: 22mm F & 12mm R
SP: 22mm F & 12mm R
SE: with Bilsteins & TorSen: 23mm F & 14mm R

[omx500]

Just to provide a bit of follow up, on getting onto a properly flat floor and measuring things, I found that I have too much rake (car too high in rear). So, lowered rear to be only about 5mm higher than front. Still have 16mm rear bar due to not finding another but have increased front bar to 25mm, up from 22mm. I haven't done full laps at my local circuit (did a hill climb there on the weekend), but the car feels significantly more planted and I seem to have better traction accelerating out of corners as indicated by both total G and corner exit speed. There is a practice day this weekend so I will go and test through the main corner I was having issues with to see what improvement has been made. Also keen to try my A050Rs to see if they are dead from time/lack of use or if they can be revived. If you have 60 seconds to waste you can check out a run from the hill climb on YouTube https://www.youtube.com/watch?v=R9U_k3KmTTA.

Thanks for the help with this

[lightyear]

If you have too much travel on the rear under droop it can cause the back to get loose.

[SnaveSnavely]

I'm now running a 14mm adjustable rear bar with my 24mm solid front... As covered, obviously the whole thing is a system, spring rate, tires, camber, etc but my car feels much better for the 14mm bar, even set on soft which is the equivalent of about 13mm. I have the one from 5X racing with 3 holes, 949 racing also make an adjustable bar with 2 holes. I have seen documentation where Emilio mentioned they tried the 5X racing bar before he put his own into production and found it was binding on the stiffest setting, not something I need to worry about right now but keep it in mind...
I think you would also find with that much tire and that much power you would benefit from much higher spring rates...