mx5-kiwi

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?

k24madness

iTrader: (6)

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

Leafy

iTrader: (1)

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.

mx5-kiwi

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?

Leafy

iTrader: (1)

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.

guttedmiata

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.

Seefo

iTrader: (5)

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.

oregonmon

Camber numbers?

mx5-kiwi

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

mx5-kiwi

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....

Seefo

iTrader: (5)

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.

wannafbody

iTrader: (1)

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

UrbanSoot

iTrader: (24)

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

oregonmon

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.

Rennkafer

iTrader: (3)

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.

Seefo

iTrader: (5)

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.

oregonmon

Ewww, your right.

Savington

iTrader: (31)

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

jacob300zx

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.

emilio700

iTrader: (3)

This.