In a FWD car, an LSD seems to make your car pull in the direction the road is cambered to when you get on the gas. The harder you accelerate, the harder the car pulls to one side. In skiing lingo, the car wants to follow the fall line - the line water would travel if it were running down the road. The LSD also increases the tendency to tramline (follow the ruts in the road) when you're on the gas. Normally, this is fairly manageable, and I thought I had gotten used to my Mini's tendency to do this, until...

I was passing another car on a 2 lane road the other day. The road had a real steep crown to it, and as I moved left over the centerline to pass, I could feel my car start to pull to the left as I got on the gas. I was only at about 1/2 throttle and corrected easily, but as I completed the pass and pulled back over the centerline, now at 3/4+ throttle, my Mini suddenly went from pulling hard left to pulling hard to the right and nearly started to fishtail. Even with both hands on the wheel, I nearly had a moment. Things could have been nasty if I had been shifting. Just something to keep in mind if you are LSD-equipped - keep an eye on the camber of the road.

Anyone else had this happen?

 

You'll find that LSD seems to make a lot of strange things happen. That's kind of the point.

 

Aaah - fond memories of the 70s....

 

That happens without the LSD just as bad.

 

Hmmmm... I just assumed it was the LSD because I haven't experienced anything like this in FWD cars I've driven before.

 

You may want to get your alignment checked by a quality alignment shop. If your toe setting isn't right it might cause this.

 

That would be my guess also, it seems like a "toe out" problem...

 

Picture yourself in a boat on a river, with tangerine trees and marmalade skies.

 

Classic post!

 

If i hear ONE more LSD joke on NAM....

 

Unless you had tried this same manuver in the same section of road before you had an LSD then I would have to agree with the others on here. Get your alignment checked. You mentioned that you have had numerous suspension modifications done. If one of these was to lower your car then that can significantly alter the alignment settings and have detremental effects on the handling.

You are also running 225 wide tires which would make tramlining and sensitivity to road camber much more severe, amplifying any mis-alignment issues.

Oh, if you had been shifting it definitely would not be effected by the LSD because the MINI LSD is a torque sensitive/activated unit and when you are shifting there is no torque being applied, so no worries.

 

Thanks for the suggestions. It's quite possible this problem is just amplified by the suspension mods I have - wide tires, front (and rear) swaybars, and M7 control arms - that basically dial out any compliance in the front suspension. The alignment was done by a reputable shop 1 month after the installation, but that's not to say it couldn't be out again.

BTW, shifting would have been a problem in this instance because I would have had only one hand on the wheel.
l

 

Suspension geometry is a touchy thing.

You're running 225s...I'll assume that you have installed these on larger rims??? If so, did the offset change? If the offset has been changed (say 45mm to 35mm as an example of magnitude), so has the SAI - Steering Axis of Inclination or King Pin Axis. In other words, the point at which the SAI inersects with the ground has been moved closer to the tires center line. This will begin to introduce some straight line instability problems when one of the front tires generates more grip than the other. In essence, you've change the scrub radius, you've made it less negative if you've reduced the offset number. Negative scrub radius causes wheels forces to toe in. These toe in forces cancel each other out, in other words stabilize one another. As the scrub radius moves more towards positive, these force begin to seperate or toe out. This is very bad when considering that front wheel drive cars have much more force generated at the front wheels than do rear wheel drive cars. The typical example; one wheel on a front wheel drive car loses all its braking while the other retains all its braking. Negative scrub radius helps to keep things straight - forces are toed in. Positive scrub radius under the brake lose example in a front wheel drive car...you're in the weeds deep. For those really thinking, off throttle positions exert a torque on the front wheels - negative vacuum. This causes funny little things to happen up front too.

In your example; off camber road with bumps??? The down hill wheel grabs until the uphill wheel hits a bump then it grabs and vise versa. How many times a second do these forces fight one another while traveling at what speed on this road?

When I bought my BBS RGRs, I knew I would have to install a camber kit to get my SAI or King Pin axis back in line so that I retained the stock negative scrub radius.

You can correct this by adding some additional negative camber; this will in effect kick the SAI back out a bit...doing this will also affect roll center in a more or less positive way if your car has been lowered.

Just keep in mind that too much negative camber can cause instability problems too. Everything within reason, you're not driving a Ferrari.

Or, get wheels that have the exact stock off set.

If your new wheels and tires are heavy, and if the offset has been altered, and if you have tires prone to tram-lining, (some are and some aren't) then you've got a big stability problem. An LSD cannot compensate for these changes...it may in fact exaserbate instability.

There are many things that can be changed on or in a production car. Most, should be left alone.

 

Wow, meb, that hurt my brain. I have the same sort of problem, especially with the autocross alignment. I think that it's all liveble, though. At the end of the day it all come down to "there ain't no such thing as a free lunch". There are no suspension changes (lsd, too) that will not have an effect on other aspects of your ride.

 

Veni Vedi Vici,

Yes, there is no free lunch...although, it is more free if you're starting from scratch.

You can very easily determine your SAI; Determine the angle of your Macphearson strut on a sheet of paper. Determine its location - height and proximity to the wheel you have now(simply a front end view of your set up)

Draw a line from the top of the strut tower thru the center line of the strut to the ground. Draw a vertical line thru the tire's center line to the ground. The distance between the place or point where these two lines intersect with the ground is the scrub radius. The extension of the strut line for the Mini will fall outside the centerline of the tire at the ground - this is negative scrub radius. When this line falls inside the tire's center line the scrub radius is positive.

If your new wheels have a smaller numeric offset (38 istead of 42 for example), the point that depicts the tire's center line at the ground will be closer to the point running from the strut tower to the ground when compared with stock wheels.

Now, if you add more negative camber, the point at the ground running from the strut tower moves out farther because the angle is sharper; the strut tops move closer to the center of the car. If you determine your current condition, and then find the stock condition, you can dial in enough negative camber to bring everything back to stock. If you're lucky, this camber value will be beneficial enough to aid cornering force too. See, adding too much negative camber does lots of things if you think about what I just wrote; with a stock offset, the scrub radius becomes more negative than stock creating a condition that is potentially too stable; or heavy and unresponsive steering. I added this in response to your autoXing comment.

Also, because this new strut angle is sharper, the role center is lowered - a good thing in a lowered car, not a stock height car. I don't have time to explain this one, but it is central to roll couple - the relationship between the center of gravity and role center.

Scrub radius is very very important if you want high speed stability anyhwere, but especially on off-camber bumpy roads. This is one area where Lotus has traditionally shined brighter than everyone else.

Of course, improper damping and spring rates will upset any really good geometry. So as you wrote, no free lunch.

If I know I'm going to 'play' with a new car, I try to buy wheels with a slightly smaller offset - numericically smaller...because, changing camber above and beyond many things, affects many things...especailly scrub radius.

EDIT - the last paragraph is true if purchasing a front wheel drive car.

 

Just turn the steering full lock to both ends, and back the car up six inches. The difference between the center of the resulting pattern on the tread and the centerline of the tire is your scrub radius. Then try it with the stock wheels to see if it is significantly changed.

Manufacturers like Audi use a negative scrub so that bumps steer the car away from the side the bump is on, and as Meb pointed out, braking results in toe-in (lets them use nice cushy soft bushings). Positive scrub tends to pull the car toward the bump, and braking (including engine braking) results in destabilizing toe-out. It is simple leverage from the outer edge of the tire being farther from the turning axis. Wider tires tramline more because both the outer and inner edges are farther from this axis.

 

http://puffnstuff.ytmnd.com/

 

A great way of depicting the difference to the driver;

Negative scrub radius - the car essentially takes care of itself

Positive scrub radius - leaves all the correction and involvement to the driver

...dunno why, but I never thought of the lock to lock comparison method. We learn every day If there's an easy or hard way to figure something out, I always naturaly gravitate towards the hard way.

 

meb: you beat me to it!

 

...maybe my wheelchair is faster than yours

How've you been jlm?

 

!!!As I re-read my procedure above I found that I mis-wrote something important. The SAI is an imaginary line that runs from the center of the strut tower bearing thru the center of the lower ball joint. Above I indicated that this lines runs thru the center of the strut. The strut and ball jooint may line up on some applications, but use the ball joint.


Another thing...I was just reading some notes from about 22 years ago. tire side wall deflection will change scrub radius. The scrub radius will become more negative. If you can determine the amount of deflection - its affect on scrub - you can compensate for it to a point. This is important when considering what static scrub radius is. Bringing things close to stock is always safe, but perhaps will not reveal the true potentiall this car has. It's the inter-relationships between things like scrub, mechanical trail, caster and camber that typically screw us up.

 

Damn it Meb, I'm a Doctor, not an alignment specialist!

 

the steering axis is a line between the upper strut bearing center and the lower ball joint, but on the Mini the strut axis is outboard by about an inch or two on the bottom. what this means is that as the steering travels lock to lock, the strut axis travels around the surface of a cone (precesses). that is what Meb was pointing out. here is my contribution: the precessing strut rod attached to the upper bearing in the rubber mount causes that bearing mount to flex...a lot...when steering. (not much when the suspension compresses though).

you can see all this by jacking up the front of car, pulling one wheel and turning the steering lock to lock.

 

meb - thanks! That makes a lot of sense. Here's some more data to help with the theorizing:

I'm running SSR Comps 16x7.5 ET 42. Don't know off the top of my head what the stock offset if for the standard Cooper S 16" wheels. Anyone know?

I'm running -2 degrees in the front, -1.5 in the rear. Factory camber settings (according to my alignment shop) are -0.5 front, -1.5 rear. See attached pic.

Wheels and tires are quite light at 11.5 and 19lbs respectively. Tires (225/45/16 Toyo Proxes T1S) do seem to be prone to tramlining, but it was much less noticeable prior to the coilovers, sway bars, control arms, and other suspension mods.

My thoughts exactly.

This particular size of T1S is "extra load rated" with a load rating of 93. I could run these same tires on my Audi A6 (4200lbs with me in it) with no problems! In my experience, as the load rating of the tire increases, the stiffness of the sidewall increases. I noticed little difference in sidewall stiffness vs. the factory runflats. Consequently, handling seems a little sharper than it would normally be for a 45 series tire, and the extra load rating means that, on a Mini, the tires are operating at much less than their designed capacity. I would presume that sidewall deflection is minimal here.

I noticed that, on my car, caster is 0.4 degrees different from right to left. Is that relevant to the discussion?

 

Pure&Simple,

jlm developed a camber/caster kit a while back so that folks could make sure caster is equal left to right. This is quite important; caster and its counter part measured on the ground plane - mechanical trail - help to raise the front end as the steering wheel is turned in either direction. When this happens, the weight of the car makes the steering want to return to center. If caster is substantially unequal, this returnability will feel fast in one direction and slow in another...low effort and high effort respectively. The leading tire will also grab at undulation and bumps in the road before the trailing tire giving the driver a very unstable ride.

I would make sure that your caster is spot on, and that, your scrub radius is reasonably close to stock - this means running very proper wheel and tire sizes...light weight if possible. I would have to imagine, I've never read this however, that if the offset is changed, returnability is also changed; if the offset become extremely negative, the car will not raise as far as was designed and will have poorer returnability. If it becomes too positive, it will raise and lower with very little action on the steering wheel creating an unstable feeling. Think about the cone example jlm pointed out above. By changing offset, the base of the cone becomes wider with more negative offset, and smaller with more positive offset. And, I would also imagine, that the cone is eliptical. the implication here is that as offset becomes more negative, the effects of caster may change exponentially...

There are many theories regarding suspension set-up. There are also many compromises inherent in every set-up. There are a few no, no set-ups as well. If you "get lost" in your set-up, always return everything to some baseline - stock???

In the process of setting-up your ride, keep all ten of your fingers on the stock parameters designed into the car, AND, the relationships that exist BETWEEN these.

DEVELOPMENT is where you get to experiment with your ideas. Make small chnges in one area at a time. Make sure that where 'things' need to be equal, they are - caster & camber etc (assuming we're writing about a road going car as opposed to a circle track car).

What you should not do in any case is destroy the fidelity built into this car by trying to make it something it is not. Cornering at 1G might sound great, but if the car doesn't talk to you - send to you clear and stable messages, then you will not get to 1G consistently. This is why I try to pay attention to some of the stock relationships. I'm not a suspension engineer. I know enough to be very dangerous as they say, so I don't veer too far.

I'll respond to some of your other stuff a little later today or tomorrow. Take care.

jlm, I'm sometimes at a loss for describing what I mean, so thanks for pointing the above out