Wider Track Understear

[The Zork]

Car- SE with mild tune
Rubber- AO50 225/50/15
Ohlin dampers set 3/4 hard with same number of clicks front and back

I was trying to dial out the natural understear on corner entry by adding 10 mm spacers at the front at PI. The opposite to what I expected happened with increased understear. Took the spacers off resulted in sharper turn in.

Is this because of front/back mismatch? Is it worthwhile adding 10mm spacers to the back at the same time as the fronts for a wider track all round?

[project.r.racing]

I would look at other suspension components before resorting to spacers and having your wheels fly off the hubs.

[ManiacLachy]

Everything I've read says MX-5s are best with a square wheel/tyre setup. So, I wouldn't run spacers.

Try adjusting your rear shocks to be a bit softer than the fronts, or conversely the fronts to be harder then the rears.

Another consideration is the rake in your ride height, try raising the rear higher than the front a little.

Also, what kind of sway bars are you running, and if they're adjustable, how are they set?

Basically you want the rear to be looser than the front if you're after less understeer, more oversteer.

[NitroDann]

The correct fix is alignment, followed by spring rate front/rear bias.

In cornering the springrate is a combination of springs and swaybars. Damping is not springrate.

If you already have an appropriate alignment (post specs so we can tell you) then you need a stiffer rear or a softer front, or its a driving style issue.

Its likely a combination of driving style and too much front (or too little rear) combined coil/swaybar springrate.

Dann

[manga_blue]

X2 for what Dann said but it also indicates you have too much front toe-in. Toe should be zero for a track car

[Magpie]

Understeer is caused by lack of grip at the front. The theory suggests that a wider track will have less weight transfer and hence more grip. Therefore your assumptions were correct, but if it did not fix the issue then the problem is elsewhere. Corner entry understeer is also caused by going in too fast and or hard braking and turning at the same time (not trail braking).

As Dann suggested, set a known baseline, that is an alignment and suspension setup. Do you have any tyre temp/pressures? Are the fronts on average hotter than the rears? Is one side on average hotter than the other side?

With respect to the NA MX5's there was a 25mm difference between front/track width from factory, so adding more front track width will be a significant change from factory. Not sure if the NB have different front/rear track width.

[greenMachine]

Gotta agree with what has been said above.

However, regarding shock settings, changing shock settings will affect under/oversteer, at least on turn-in. The effect may be minor, and I would only suggest it for minor tweaking of setup. It sounds like you need more than minor tweaking.

One other thing not mentioned above that will make a difference is playing with tyre pressures.

My 2c would be to get a track-oriented alignment, and do a track day. Play with whatever adjustability you can - tyre pressures, f/R shock settings, moving settings on ARBs. BUT, only make the changes that will address your specific handling issue. If you don't know what to do, take along a friend who does. And take a note book and write down what settings you start with, each change (make one, test it - don't make lots of changes all at once) and the effect it had. Note the weather and track conditions too. Of course, if a good alignment fixes the problem, you can then experiment with minor changes to fine tune the handling.

At the end of such a day of testing you will have a much better understanding of your car and what (if anything) is required to fix any remaining handling issues.

[Magpie]

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[lightyear]

Take your car to Nick at Lux Racing, he has all the specs from what worked on my car, I didn't get much understeer except going over Lukey. Full corner weight and wheel alignment will be about $350.

[StillIC]

Adding wheel spacers does not automatically reduce under/oversteer, as far as I can see or understand. Why would it? The spring rates haven't changed, or more specifically, the wheel rates haven't changed as the relationship between the movement of the uprights and the springs / sway bars hasn't changed.

But what might change is the distortion of the suspension bushes due to extra leverage on the suspension by having the wheels mounted further outboard, potentially changing aspects such as camber and castor. Without thinking too much, the alignment may also be upset by differing the distortion of the suspension bushes.

Further, don't discount potential aerodynamic affects of having more tyre exposed to the air. It was common knowledge in Indy cars that increasing toe out on the inside wheel in a turn helped the turning action, even though that tyre was barely, if at all, touching the ground.

[manga_blue]

I'll just re-state the need to get a good track alignment first.

I played around a lot with offsets a few years ago. Using spacers and differents wheels I tried +42 down to +10 at the front with a whole swag of steps in between, until I worked out the sweet spot was +28 to +32 on my car. At the same time I played around with toe. What I found was that if the toe was right for the car and driver, and on mine it was zero, then changing offsets made virtually no difference. If toe was not right then increasing offset exacerbated the problems caused by toe.

For example if there was too much toe-out and the car was a bit too twitchy at, say, +35 offset then it became crazy twitchy at +20. If there was to much toe-in and the car turned in too slowly then dropping the offset by 20mm turned the car into a Toyota.

What I was thinking about the OP's problem was that slow turn-in can feel like understeer. I suppose really that slow turn-in and initial understeer are pretty much the same thing. If the car is understeering at turn-in it's pretty easy to keep it understeering for the rest of the corner. So if the OP's car was suffering slow turn-in from excess toe-in then adding spaces would make it worse. In any case it's easy enough to fix that yourself in the shed with a couple of spanners and some stringline. (Just search yutube for stringline alignment)

[The Zork]

Everything I've read says MX-5s are best with a square wheel/tyre setup. ManiacLachy
I am tending to agree about square setup. I might try spacers all round and keep the relationship the same. See what happens. I am looking for speed increases beyond, just fast. The alignment has been done for the track, camber, caster, ride height, toe (out), rake (up 4mm rear). Tyre pressures started at 24 lb cold and all tyres came in at 31lb after 15 minutes 8 lap sessions, all day including with and without spacers. The factory did a fine job originally but it is a compromise, they did not setup the car for racing speeds and more. So I'm trying to stretch the development of the car.

you need a stiffer rear or a softer front, NitroDann
This is what is confusing Dan, by removing the front spacers, the front spring rate increases (effect of moment torque), making the front relatively stiffer. Yet the car behaved as though the spring rate was softer.

The theory suggests that a wider track will have less weight transfer and hence more grip. Therefore your assumptions were correct Magpie

Adding wheel spacers does not automatically reduce under/oversteer, as far as I can see or understand. Why would it? The spring rates haven't changed, StillIC
But this is what did happen. Removing the front spacers did make the turn in sharper.

There are benefits in widening the track, both front and rear in that lateral weight transfer in cornering and braking is reduced without stiffer sway bars and stiffer springs. Question is.. will a wider track front and rear be better than only a wider front track or rear?

[StillIC]

...
you need a stiffer rear or a softer front, NitroDann
This is what is confusing Dan, by removing the front spacers, the front spring rate increases (effect of moment torque), making the front relatively stiffer. Yet the car behaved as though the spring rate was softer....

This is incorrect. The wheel rate does not change with a spacer. The rate at the upright is the rate at the wheel, whatever the offset of the wheel. The suspension is not a swingarm (which would do this), it is a double wishbone arrangement.

Also, you have misused the terms moment and torque. It is one or the other. Torque implies rotation, moment implies no movement but both are turning forces. There is not more of either acting to decrease the wheel/spring rate as the upright still rotates (in front view) around the roll centre of the suspension. That is, the virtual swingarm length has not changed. The extra leverage of the spaced out wheel only acts at the upright to put more force into the control arms and bushes, not the springing system.

What is different, and I neglected in my previous post, is that as the front wheels are now further apart with a spacer, the load transfer on to the outside wheel is ever so slightly less. This then explains what you felt, as now you have a situation where the spring rate doesn't change, and the load is lower on the tyre. That is, less grip, not more when a spacer is added as spring rate to wheel load ratio has increased. I think I have solved it.