Hmmmm....

Something to think about.

 

I haven't tried anything dramatic with the stiffest setting, I'll see at the weekend (if it doesn't rain).

I didn't notice much difference between the two settings. I did do my fastest lap on the stiffer setting. It might be about 0.5s faster at the stiffer setting, or I might be seeing patterns in noise.

 

After todays event we have at least found some oversteer. With the bar at full stiff we were getting some lively handling. I'll leave it like that for a few events so we can get used to it. Then I'll see if we need more or less. Its currently just a bit more of a handfull than we're used to.

Also we just about managed to get a test of the ZII vs Rivals. The ZIIs were 225/45-16 on 16x7 and the Rivals 225/45-15 on 15x6.5. Not exactly conclusive, but its either a draw or the ZIIs are slightly better and cope with slightly damp conditions very well. The ZIIs also worked despite being only 75-85°F.

 

Interesting on the tires. Nice to hear about the ZIIs. Many reviews I've read have the Rivals slightly ahead. But some speculate that the difference and edge going to the Rivals has a lot to do with the "shaved" nature of the tire vs the ZIIs.

BTW - I assume the Rivals were on 15"x6.5 wheels (vs 16"x6.5)?

Take care with the new found oversteer. It can be a "bite" that doesn't let go on the track.

 

Not sure what you mean by "shaved". The Rival's tread wraps around the side of the tire, the ZII tread doesn't go to the edge of the tire. I'm worried I won't know if I'm wearing the corner as there's nothing to measure there. With the RE-11, the corner was the limiting factor in tread wear, the Rivals are showing most wear on the corner as well.

I did, I've fixed the post.

 

I am referring to cutting down ("shaving") the tread that the racers do. This is done in part to not over heat a new tire and to give them less tread squirm/better handling. http://www.tirerack.com/tires/tirete....jsp?techid=67

The Rival come with 7.2/32 inch tread which is the absolute minimum that is allowed for a new tire for autocross (hope I got that right). The ZIIs on the other hand come with 8/32 inch tread depth. Some think having less tread on the Rivals gives it a slight advantage over the ZIIs.

As for the tread not going to the edge of the tire, are you going over the tread edge markers on tire? These are triangles or something similar that are located around the tire at the edge of the tread and the start of the sidewall. These are something that you need to watch. However, with a camber-challenged car like the MINI, you will almost always be rolling over onto the sidewall if pushed hard.

 

if you get a test n tune of some sort in, I'm definitely very very interested to see time comparisons.

any thoughts on getting a data acquisition system?

 

I got both sets shaved to 5/32 anyway. The Rivals had slightly less tread as we'd run about 35 runs on them, but they're both pretty similar 3.2vs 3.6mm.

 

Currently the iPhone, GPS and OBD are doing fine with Harry's lap timer. The problem is finding a system which can give the immediate results and do better.

 

gotcha. we've got something on the android side that seems to work pretty well, but i've yet to test it out personally.

 

SoloStorm is the Android version. It works with the Race Technology DL1.

 

^ right, that's the one. i've yet to test it out though

 

I made another attempt to measure the spring rates today. The idea is simple, now I've got the corner weight scales. Measure the corner weights, and the ride heigh. Add driver, and measure them again. That should give you a reasonable approximation of the spring rates. Only it came up with numbers which made little sense. I thought the sway bar was getting in the way, transferring load so exaggerating the side to side differences. So today I did it again with the sway bars disconnected. The rear had both end links disconnected, the front had one side disconnected, the other side shouldn't do anything on its own.

The numbers I come up with make no sense still. Like this (in lb/in):
197 176
273 72

The fronts don't look too bad, but the rears don't. I can't believe the rears have such different springs side to side. If anyone wants to check my work and workout how I screwed up, the corner weghts are here:

before: http://btwyx.com/Pictures/BertieE.jpg
after: http://btwyx.com/Pictures/BertieD.jpg

And the ride heigh measurements here: http://btwyx.com/Pictures/RideH.jpg

The ride heights are in mm, I made two measurements, one with a ruler from bottom of the rim to wheel arch. One with calipers from the top of tire to the wheel arch.

There is a theory that the springs are progressive, which might complicate matters, but I don't think that would account for the difference. The spring would have to multiply its rate over a few mm.

 

^ interesting. Apparently the springs are linear, so it really shouldn't be very difficult for me to put something on the spring and see how much it compresses. I don't really have a weight handy though

 

I examined the bits of the rear bar today. I worked out where some of the clunks I'd heard came from. One of the endlinks had a dent it in, about the shape of a spring, and one of the springs had its coating rubbed off at one place. There was also a dent in the lever arm where the wheel had hit it. The wheels had bits of the rim chipped off as well.

 

so there are clearance issues with the bar you purchased - sounds pretty bad at that. pics?

 

I didn't take any pictures.

Not necessarily a clearance issue, there are a lot of adjustments which can be made.

The endlink-spring problem only happened when the bar was at full stiff, the endlink is closest to the spring at that point. I backed off the stiffness by a few mm and hopefully that will fix that issue.

The lever to rim issue may be solved by changing the link length. I had to change the link length to get the bar to full stiff, it may need a tweek.

 

Btwyx...
I have thought about doing the same thing. Haven't acted on my idea, though. Since you have, you have definitely come across a vexing question. Is it possible that if you are going from fully extended to slightly or a little more compressed, you are dealing with that first coil compressing into the spring seat? That will likely be non-linear.

Do you have 2 jacks that you can use to lower or raise the car while it is on the scales instead of sitting in the car to get the deflection? You could put the car on the scales and weigh the car and measure the spring length or a set of references associated with the springs (like the top spring mount to the lower suspension arm. Then jack the car on both sides using the front jack points so the front scales read 200#s less then measure the spring travel. You can't read the change in car body height as springs will move more because of the way the suspension travels. Then do the same for the rear. Also, don't forget that the tires are another spring. So you need to measure that change in their heights also and subtract it from the spring height measurement. Just a thought.

 

I later though that I should have done the measurements with the driver also sitting in the passenger seat. That would have got a few more measurements, as it is I'm comparing 24lb with 112lb.

The jack idea is interesting, I'll think about this if I revisit the subject. I only have one jack, but I can measure the force/distance from normal load to no load like that. I'll just have to do it one side at a time. If I can find some suitable ballast (which wouldn't object to sitting in the car for a long time), I could do the measurements from heavy load to no load. I can't think of anything suitable to use as ballast though.

 

I decided to get the required decal pack from SoloPerformance. While I was there I got them magnetic backed and then decided to get the MINI contingency decals. Only one "wheel" decal, is that all you need?



There's also the Tire Rack window banner, but I haven't installed that yet.



(And they also threw in some of their own stickers as well.)

 

I did that, I used one jack and did one side at a time. Cathy's out of town, so I had to improvise driver weight with the set of OEM wheels, a case of water and a couple of spare 15" wheels. I came up with the graphs as below. I also incorporated the previous measurements for the front, they fit the line. The previous measurements for the rears didn't fit, so I didn't add them. I still don't know what went wrong with the previous measurements.

The springs do seem to be progressive. The working range of the springs is towards the tips of the graphs (828, 818, 556, 481 lb). Out there the spring rates seem to be roughly (in lb/in):

205 192
333 369

I'm surprised the rears are greater than the front. Given how the springs get harder as the car gets heavier, I wonder if leaving the spare tire in would be an advantage. Not only would it move the rear springs into a higher range, it evens out the corner weights very well. The question is whether the improvement in suspension would overcome the extra weight.

 

I made another attempt at plugging the numbers in the suspension calculator at Autocross To Win ( http://farnorthracing.com/autocross_secrets16.html ). This time the numbers seem to make sense. The shock curves aren't too far from the shock curves I actually got, except they're upside down. I'm wondering if the shocks were measured upside down. The fronts especially as its the shock body at the top so is installed upside down.

Some of the inputs like roll centre heigh and CG height are just guesses. Anyone got better numbers? I tried measuring the motion ratios, that's the numbers I used. Unsprung weights are also a guess.

The rear springs seem much too stiff. The model never switches to oversteer, I'm not sure why.

 

nice data. i'm pretty sure the motion ratio for the rear suspension is very low. not sure exactly what the number is so i don't want to give a number

 

aha:

To optimize this situation for stock or lowering springs (especially for Stock class autocross), we consider the ratio of the motion ratios (0.89 / 0.65 front/rear).

http://fatcatmotorsports.com/FCM_Products_Mini.htm

 

I worked out why the Autocross to Win suspension calculator never showed oversteer, I didn't have reasonable values for the CG hight and roll centre height. I'd just plugged in 12" for both. If a car has its CG at the same height as its roll centre its never going to roll. I plugged in 12" and 24" instead and it now shows oversteer.

If that's anything like reality, the new bar increases the overall roll stiffness by something like 40%. That may be why as I was driving Algy (the JCW with unmodified sport suspension) today, he felt distinctly like he was rolling somewhat disconcertingly. Its like suddenly undoing all the suspension mods we've made to Bertie, that got my attention. I didn't notice the incremental tightening up, but a sudden undoing is noticeable.