9" 6UL's - Setup issue?
 

Hi Guys

This is for the guys who have race/autocross experience with the 9" 6UL or similar....

In swtiching to the 9" rim from the 7" rim my lap times have improved by 1-2 seconds at various tracks. So overall I am very pleased.

However, I believe a consequence of this change is a strange/new handling problem....

On corner entry, If i turn the wheel beyond a mild turn in (i.e for a slower/tighter corner) I get a really nice hard turn in but as the steering wheel gets to a certain point the rear end just lets go and the car starts to spin....

It "feels" to me that it is somethting like when you turn the wheel in a Kart how the chassis jacks up...by design in a Kart.

I can drive around the problem but wonder if there is a different caster/camber setting you guys use for the wider wheels...i have pretty much tried to adhere to the suggested 949racing specs at this point.

One of more experienced guys here wonders if it is related to the wheel extending in beyond the lower ball joint and it changing the geometry as you turn in more and more....along the lines of my kart analogy above..

Can anyone help, make suggestions based on experience?

What are your caster numbers? Increasing castor also increases the jacking effect. Try backing it down some and see what happens.

Making more grip, getting more roll, running out of rear droop travel either because of the rear sway bar binding or the shocks not having enough droop travel. If these koni race shocks are more than a couple years old, they're the old too short in the rear body koni race's and that could be the problem.

Aha, thanks guys. Good places too start.

I'll try and find my last alignment chart for the Caster. Is there a different setup generally used with the wider wheel?

These Konis are at least 4-5 years old....can you elaborate a bit on what you said Leafy?

The common koni setup is race fronts and sport rears, not entirely because of the damping curves, but that the race rears used to be much too short of shocks and didnt have enough droop travel. A couple years someone brought this up to Koni Lee at solo nationals and they fixed it. I cant say for certain that its your problem.

You said nothing about a tire change, so I assume you are stretching the same tires onto a 2" wider wheel. You now have less radial roll. In other words, your side walls cannot flex as much to be forgiving of either over driving or a less than optimal set up. The sidewall flex you had before was masking one or both of these problems. Now with less "forgiveness" you need to make appropriate adjustments. In additional to sharing alignment specs, please also share sway bar and spring info.

sounds more like a bumpstop/stiffness/traction issue to me.

I think most suspension geometry is calculated with reference to the contact patch centerline. I think there is slightly difference between offset from the 7" and 9", but probably not enough to have a significant issue.

Camber numbers?

Oddly, just got back from a track day at a different track (home base) and it is less of a problem...in fact it is almost a benefit as it helps the car turn in a bit better (almost karting style)...at this track..Hampton Downs. Some off camber corners and down hill off camber etc..Long sweeper can use the "kick" to get the rear to come out a bit when required....as well as throttle. :)

At the A1 Taupo track the corners must be slightly slower/tighter and it was a hinderence...

Re the tyres, I have only used the 225 on the 9" previously I had 205 on a 7"...

Okay found the last wheel alignment sheet...

Toe front Ttl +0.3mm Lft Fr 0.00 Rt Fr +0.3

Camber Frt Lft Fr -02.30' Rt Fr -02.47'

Caster Frt Lft Fr +03.26' Rt Fr +5.44'

Toe rear Ttl +5.2mm Lft Rr +2.70mm Rt Rr +2.4mm

Camber Rear Lft Rr -02.37 rt rr -02.29'

One of my alignment cam bolts is worn/screwed so he had problems with it...have been meaning to order new ones but keep forgetting...not sure which one now...

Sway bars are Whiteline.

24mm front mid setting of 3
16mm Rear soft setting of 2

Just to clarify, the 9" with 225 was/is worth 1.5 - 2 seconds per lap at most tracks...so overall there is a huge improvement from the 7" 205 combo....

in bold either looks suspect, or awful. not sure how balanced the sway bars would be with 10k/6k springs... I would put that in a suspension calculator and aim for 60% FRC range. that should put you close and do the fine tuning with the sway bars, shock adjust and possibly tire PSI if you have to.

fixing the difference in caster probably won't fix your problem, but caster is important as it has an effect on dynamic camber.

Rear Toe...I don't know, I am not used to seeing alignment in mm, but that is just too much. 5.2mm ~ 1/5"...and that is in toe out? probably the problem right there.

I'd drop down to the 14mm Mazdaspeed rear swaybar or jump up to the Racing Beat Large Race front swaybar.

Disconnect your rear sway bar for 1 session and see if it improves anything.

That's total toe though. So it sounds like about .10"-11" each side. Which is around 1/16" give or take. So that should be fine right? The caster needs to be equalized, was this with you or ballast in the car?
I think you should probably shoot for more front camber by maybe a 1/2 degree than the rear from what I've seen recommended.
Also your front toe should be even as well.

1/16" = .0625"... he's closer to 1/8" per side which is way too much. 1/8" total is the max 949 recommends. IMO he needs to have a shop do the alignment a little more carefully, and with him (or his weight) in the drivers seat (if he wasn't). The numbers should be quite a bit closer than that side to side.

1/5 divided out is 1/10 per side really. I think 1mm on each side is probably just fine. toe out is nice to have, but definitely not in large amounts...especially if you like your tires. At least he can turn it down to see if it has an effect. Just a suggestion.

Ewww, your right. :noob:

Your front sway is too small, rear sway is too large, and you have too much rear camber/not enough front camber.

I think the first thing I would try would be the stiffest hole on the front sway and more front camber. Then if you want less tail happy then that switch to the stock rear bar.

This.

righto...thank you all, will work on those issues raised.

In regards to the front camber, from talking to the alignment guy and "assisting" him do it the last few times, we are struggling to get any more camber with out blowing out the other settings...is this common or are we not on the right track?

Question about the sway bars, what is the preferred front and rear (in mm if you can...)...these are/were the only aftermarket items available locally at the time but I will try and see what I can get in the sizes suggested......freight is a killer to here, probably more so with sway bars being an odd shape.

The ideal Miata setup, IMO:
700/400 or 800/500 springs (700/400 is more supple over rough tracks, 800/500 is ultimately faster on smooth tracks)
1.125" tubular (28 or 29mm) front sway or 1" (26mm) solid front sway (tubular peferred)
12-14mm rear sway to taste (OEM is 12mm, MSM is 14mm, nothing larger)

Street camber: 2.2F, 2.0R, 3.75 caster, 0 toe
Race camber: Start at 3.0F, 2.5R, 3.75 caster, 0 toe, tweak camber according to pyrometer data and/or tire wear needs.

In order to get 3.0 in front without offset upper bushings, you need to run the car low. 4.0-4.1" at the pinch weld. If you are only at 4.4-4.5", you'll only see ~2.5* on camber. Essentially, run the car as low as you can in the front without it constantly bottoming everywhere.

Is that caster recommendation due to the depowered rack or is there another reason for keeping it around 3.75 degrees?

Depowered rack. I like a pretty light wheel, so I keep caster under 4 degrees and use a 340 or 350mm steering wheel.

Would you say more caster is better if you can handle it? My very basic understand is that more caster will mean more dynamic camber, which 'generally' seems good?

robert

Could be better, but unless you're winning races and looking for those last few tenths, I wouldn't bother.

I run max caster on all our cars. I have had team drivers complain about the steering effort so I switched from 320mm to 350mm wheels. I didn't reduce caster. I like the additional cross weight jacking effect that helps the car initiate rotation into turns.

Emilio, this cross weight jacking is part of what i was trying to say/understand at the beginning.

Obviousley we have centred on some bigger issues for me to get sorted for now but in the mx5 / miata.. is cross weight jacking a progressive thing or does it happen/take effect at a certain point on the turn of the steering wheel...I mean in your actual practice/experience.

Becasue that is what it feels like...if my full turn in on the steering wheel was rated at 1-12 I would say it starts to have effect at around position 5 of the turn. Any corners with less turn in or more gentle turn in (1-4) it doesn't do it but from pos 5 onwards it does...

Does this make any sense?

I may have just entered the realms of La La Land... :facepalm: but in the interest of (hopefully) learning something new I am up to SOME ridicule...maybe not MT level burn but i'll try and tough it out....:fawk:

^is it doing it more when turning one direction than the other?

Dont think so, havn't noticed it as such, I can and do use it to advantage often. Just that at Taupo it is beyond the use as a tool and is more of a hinderence. At the Hampton Downs track I just went too it is actually a benefit on some of the corners (IF you are ready for it...) and doesn't come in to play on the rest as they are less hard turn ins (1-4).

It has less to do with how much you turn the wheel and more to do with how fast the corner is. Slower corner = more wheel, faster corner = less wheel.

It's all irrelevant though, since you have too much rear sway, not enough front sway, too much rear camber, and not enough front camber. Nothing you do with the alignment is going to fix that :)

Thanks Savington. Yeah I understand, have started looking for better sways and will reddress the alignment numbers. Looking forward to dropping times further when sorted.

Was just trying to understand how the caster/weight jacking works...or feels like.

Speed of turn makes sense.

My plan at this immediate stage is to set front bar to it's stiffest setting and get the camber figures closer to what has been suggested.....hopefully that will show improvement for the race meeting next week.

Then I can work on replacing the rear sway bar and finding a bigger front.

Thanks for all the insight guys, much appreciated.

Linear, progressive. It is not an on/off switch.

Post good in car video with the symptom. And fix what Andrew suggested you fix.

okay, temp solution.

rear sway was on hardest setting have set it too softest setting (1/2 options)

front sway was on mid setting (2/3) will set too hardest.

Going to reduce the rear camber and try for more front camber, going to reduce the rear toe to 1mm. Will probably leave caster at 3.75 or more but try to get more even/balanced with me in the car.....

Race day on Sunday, will be interesting to feel any differences....will refit the OEM sway (12mm?) on the rear after this race during end/off season period.

thanks for all the advice in the meantime.

But why would his balance change with a wheel change? (Offset changed too?)

Doesn't more cross-jacking from caster produce a stronger wedge/dewedge in the direction of more understeer?

Do you find that adding caster requires other alignment changes to maintain balance?

No
No

Okay, quick update.

After qualifying second.

Race 1. Started 2nd on grind (Standing start) raced with the two other top guys and came first of a field of 18.

Race 2 Handicap start. From Pit Lane over a lap behind 1st car. Got stuck in every one elses battles, went too slow on the first 2 laps (worried about cold tyres) and lost the draught of the guy who started 2 secs in front of me. 9th place (6th through 11th all bunched up).

Race 3. Same Handicap start. Focused on sticking with guy who started 2 seconds ahead of me and we both fought through to the front. He JUST got the last guy on line by .2 sec and i came home in third 2 secs back.

I can now turn in much later and harder than ever before. I do miss the rear swinging out on one of the sweepers (p;oitns me out better) but the trade off is worth it.

I believe this car is now the fastest track car Mx5 actively racing in New Zealand (Hampton Downs 1.15.511 and hopefully .5 - 1 sec more to come off with fine tuning).

I stand to be corrected though and I am really pleased to have a a car that is in for win contention now.

So thank you for the suggestions in this thread, as usual you guys were spot on.

Will continue working on the suspension setup and balance but it is getting there.

Now i just need to stop my turbo from falling off at every meeting, via studs/nuts coming loose (TSE kit has improved this, although 2 of the 4 WERE loose again) down pipe cracking or my new manifold cracking 200 degrees around the collector...the latest issue :facepalm:

But that is a whole new thread soon to be appearing..:vash:

In car video?

Hopefully have the first race available in a few days...

this is the before and the reason for the original post...


The spin IS related but is also caused by cold rears and a heavy handed downchange, turn in and brake release.

If you go to high res you will see a couple of really fast corrections, usually on turn in or mid corner.

Watching you climb the curbs and seeing how they upset the car brought back memories of my Koni Race shocks. Now with AST/XIDA's that's a thing of the past. I am amazed at how big of a difference it was.

Not sure what to say about the handling otherwise. Are you really running 225/50/15's? Why no 225/45/15?

Sure am, We just cant get them in to stock in the country..will try again soon when this set wear and/or when Emilio has a new supply of wheels, i want to run at least 1 spare set ready to go.

Here is the race footage from the weekend.


Still really thrilled at the result, depressed at the broken manifold, turbo and down pipe...yet again. Will need some advice and clarification relating to the " how to race a turbo Miata " thread soon...