Suspension Science, Setup, and Tuning
 

This thread is a result of a discussion that started in the Gen2 Modifications-> Suspension forum about sway bars and why changing sway bar stiffness affects the "balance" of the car - understeer vs. oversteer, etc. I thought it would be interesting to start a thread that was not model and generation specific so that there would be broader participation and input.

Topics for discussion:
spring rates
sway bar stiffness front/rear
what makes a car tend to understeer vs. oversteer
camber and toe
wider lower aspect ratio tires vs. narrower, higher aspect ratio tires, the resulting contact patch, and the effect on handling/acceleration/braking
anything else related to the general topic

The overall goal is understanding why certain changes have the effect they do - for example why increasing rear sway bar stiffness reduces understeer.

Some links to videos and articles that I have found interesting and informative:
"The Physics of Racing" Articles: http://www.optimumg.com/technical/technical-papers/

SquawSkiBum - is there any way to just make a copy of that swaybar thread that we were on, put it in here and rename that copy? There were a lot of good posts in that thread that were relevant here and it would be be good to carry on that discussion here. Maybe delete out the start of that thread from the copy so only the relevant posts are left in that copy? Sorry I don’t know if this can be easily done.

Following

I am going to see if I can populate this thread with the discussion we have already had. I have an idea of how to do it and retain the connection to the original person who made the original post. It may take some time, so bear with me. SquawSkiBum - let me know if this works for you and the blog...

Eddie - PM me with the post numbers from the original thread that you think are worthy and I think I can move them.

I should be able to do that this evening. :thumbsup:

One of the things that needs to be considered when setting up a MINI it handle well is the benefit that increased negative front camber will have. First off, from the factory the MINI is delivered with 0 to -0.3 deg of camber. This means that with any amount of body roll the car will have positive camber and the tire will roll over onto its sidewall. This will greatly limit the turning forces that the tire can generate and exasperates the understeer that most people are trying get rid of.

Charlie Thompson posted this great picture of this exact condition:
https://www.northamericanmotoring.co...ml#post4461677

This car is setup with -2 deg camber and has a 22 mm RSB.
Note that, in this case, if the car just had the 22 mm RSB and factory camber, the front camber would about +1.7 deg and there would be a lot of sidewall scrubbing and understeer.

This situation becomes worse with sticky tires. As g loading goes, the more the tire was to roll over. This is counteracted by increasing negative camber.

Thanks, Charlie, for the great picture post. :thumbsup:

Nice! I believe this line of reasoning is why a front sway bar should be on the list of modifications when you are serious about handling. Limit the amount of front roll, and you can maximize the front grip without having to add too much front camber.

No, a stiffer front sway bar will increase weight transfer to the outside front tire during cornering, which will increase understeer despite the decrease in overall body roll.
It will also tend to lift the front inside tire somewhat during cornering, resulting in more likelihood of wheel spin and less cornering in hard turns unless you have a good LSD.
Increasing the stiffness of the rear sway bar will limit total body roll without overloading the front outside wheel. Increasing negative front camber will also help as noted before.

I was saying to add a bigger front bar in addition to a bigger rear bar. Need to limit vehicle roll all around.

Beefing up both bars won't really help in certain situations. For autocross, a beefier front sway bar will just increase the amount of understeer you have to deal with and hamper the car's rotation on tight sections. From what I've heard over the eyars, some guys who track MINIs will do both bars but many (if not most) only do the rear one.

In SCCA Street classes one may put in either a stiffer front bar, or a stiffer rear bar, but not both.

Now that Gollum is in STU I could add more roll resistance to the front end as well, but I hesitate.

The problem is, as previously stated, that the car already needs a mechanical limited slip, and a stiffer front bar with the electronic differential still in play may not be helpful.

I expect it would trade some mid corner performance gain for a compromised exit and increased brake wear.

Your mileage may differ.

Cheers,

Charlie

Here's another picture of front camber loss while cornering, leaning really hard on the front right tire in turn 5 at Laguna Seca. This is before all my suspension upgrades, the only mod at this point was a 22mm RSB. Still on the stock runflats even. Unfortunately the angle of this picture isn't good for drawing the lines and angles on, but it is obvious the front right wheel has positive camber due to the body roll.

https://cimg4.ibsrv.net/gimg/www.nor...5fbd5df454.jpg

I need find a comparable picture after my suspension upgrades and -2 degree camber setting.

Sway bars came into prominent use in the late 50’s when cars gain more power and the cars were struggling to get power down and would spin the inside rear drive wheel when in a turn. To help with this, they put in large front bars, no rear bars. What that front sway bar does is add weight to that inside rear wheel to add traction to it. It causes understeer because the two rear wheel now have increased grip (traction) and the two front wheels have less traction with the weight being moved rearward and off that inside front wheel. So, inherently a front engine, rear drive car with this setup will understeer. But, it is a compromise with having traction for getting power down. You see the effect of that large front sway bar all the time with sports cars racing and the inside front tire is off the ground in a corner (and I am not just talking about when it jumps over the curbing). To overcome this understeer, a rear swaybar is added to balance the need for rear wheel traction with the need to reduces the understeer. It does this by moving weight off that inside rear wheel (which the front bar just was used to add weight to) and back to the front inside wheel. The net affect is greater roll stiffness and better use of the tires, so the balance achieved by the sum of the 2 bars is greater than either bar independently. And yes, I am ignoring the addition of LSD and power on oversteer (which eats up tires) for this discussion; but they, too, are tools to design a good handing car.

The same thing happens when you just add a larger front bar to a MINI. It added weight to that inside rear wheel. That is it causes understeer because the two rear wheel now have increased grip (traction) and the two front wheels have less traction with the weight being moved rearward and off that inside front wheel. Hence the understeer. But, as with the RWD car discussion above, adding a larger rear bar moves weight off the inside rear wheel and adds it back to the inside front wheel. Traction increases in the front and decrease in the rear, reducing understeer. But (again), same as the RWD car, the balance achieved with the addition of both bars will make better use of all 4 tires and the result will be greater than just the addition of a single bar.

Part of this is tied up in the grip of a tire (as discussed in the video above). That is, a more lightly loaded tire (as in foot print psi) will develop a higher level of grip than a more heavily loaded tires; 4 tires will have higher grip than 3 (classic Mini 3 wheeling it)...

The principles are the same with sway bars whether it is in the setup of a RWD car or a FWD car. That is, you can add a larger front bar to RWD as long as it is appropriately balanced with a larger rear bar. Same is true for a FWD car. You can add a larger front bar as long as you add an appropriately larger rear bar. The stiffness ration between the front and rear bar will provide an approximation of the amount of understeer/oversteer the car will have in comparison to the bars that you are replacing. That is if you are replacing both bars on a JCW with larger bars that have the same front to rear stiffness ration, the understeer according to that ratio will be no worse than the JCW setup. However, because of the increased roll stiffness and better use of all 4 tires, the understeer will actually be less than what you started with (I have experience with that).

The gist of this is that it all interconnected. There is usually no “all or nothing” (excluding the sudden loss of rear wheel traction and the car spins). So, you can add just a rear sway bar, say, a 19mm and you will see a nice improvement. With a 22 mm RSB could now take advantage of a larger front bar and see an even greater improvement in handling overall.

I autocross for fun and by no means an expert...
With that said, I have autocrossed with a Cooper S with a 20mm RSB and my current Cooper S with the 27mm bar up front and the 25mm hollow bar in the back and, for me, my current Cooper is a far better autocross car. Now I will say it is not ideal. I have the RSB set on the softest setting for the track and in tight autocross turns, it will not swing the tail around (partly my lack of talent is surely to blame) and it now has very little ability to throttle steer it. However, my other S was also problematic as it would plow the front end, which seems to be worse.

I do understand the advantage that having just a really stiff RSD gives a talented driver on an autocross course. That certainly helps with throttle steering the car.

Most of this discussion has to with what the driver is trying to achieve. My thought with my discussion is just to say that there are tools that may be usable if given a chance. And I will reiterate that the above is only my experience, which by no means may be the best.

Charlie,
I feel your pain on this one. I have your same issues with the DTC on my current S. Before I put in the Quaife, the Toe at WGI was miserably hard on my brakes because the eLSD had to work so hard to keep the inside wheel from spinning. The eLSD may be wonderful for street use but its limitation do show up when pushed to its limits. I will say that the Quaife is compatible with the eLSD. I will also say that I don’t know what the 27mm front bar and having the rear bar on the softest setting is doing to the eLSD. With the Quaife and out on the track, the brake wear seems to be reasonable and even on both sides.

Duly noted and thanks for the input. I love that so many people are providing input here :thumbsup: Oh and my the way, I'm not a very talented autocross driver either :lol:

I think this thread is a great idea. Over the years, since my first MINI purchase, I have read and talked to people about what it takes to setup a good handling MINI. I have also had some “issues” as result of trying some of these ideas. This has caused me to look even deeper into this subject.

Because I have posted here and plan to follow this thread, I thought it would be a good idea to provide a bit background. I think this is especially important as I have found that suspension setups tends to target specific handling objectives or are suggestions for setup come from people’s experience with one objective or another, but there may be a lack of clarity as to what type of driving the suggestion maybe best applied. The suggestion may or may not apply to what you want. But the general questions that I read are “how do I make it handle better?” or “how do I get rid of the understeer?” Those questions should include what the “application” will be. That is, will it be a road car that is more fun; a car doing DEs; racing; autocross? I think, knowing the background of the person responding to either of these questions may help the reader to understand the application of the suggestions.

As with most people, I very much enjoy driving MINIs. As I said, I enjoy autocross for fun and I enjoy driving on the track. From the factory, I find MINIs in general (Base and on up, but have not driven a JCW with the optional JCW sports suspension or GP2) to be a great road car, but found frustration with the handling when pushed to their limits. More frustration with a base Cooper and successively less frustration with each level up in MINI suspension line. I am not a suspension engineer, but I do enjoy figuring out what makes something work. I also believe that BMW/MINI, who spend Millions of dollars and has suspension engineers designing suspensions, actually do know what they are doing and know how to design a good handling MINI. My experience comes mostly from taking my car to the track and working to resolve the issues I have had on the track. My objects for my car’s handling is for those DE events. I consider DE events to be different from racing or autocross and the suspension setup equally different. My objective is a suspension that has no drama and will give me the best chance of driving the car back home after an event, but will still be rewarding and fun to drive on the track. With my 2012 S and the suspension it has, I survived hitting antifreeze at 100+ mph at the apex of a turn, with guardrails 2-3 feet off the sides of the track. The car has a slight amount of understeer at the limits, which I am sure helped. The rewards and fun come from being able to perform well on the track and, on occasion, out perform others. By that measure, I have achieve my objective.

When I first got into taking my MINIs to the track I didn’t know the magnitude of the differences between the 4 suspension options MINI had available for these cars (5 if you count the GP2. It wasn’t until I had my second S that I bought with the optional sports suspension that I realized how much the BMW/MINI engineers targeted each successive suspension for better handling and make it more neutral handling. I also got an appreciation for how much little changes make for a big change in handling. With just a millimeter difference in the size of the sway bars and 10s of lbs difference in spring rates, the S with the its base suspension went from a car that wallowed at its limits to one that was pretty nimble when fitted with MINI’s optional sports suspension. My conclusion from that experience was that the base S suspension, while an upgrade from the base Cooper suspension, was not particularly good for handling when pushing the limits. The sports suspension was far superior. When I looked to further upgrade the car, I looked to what the BMW/MINI engineers had design and I targeted my changes to maintain the stability of that sports suspension, while further improving upon its track worthiness. For this I looked at the front to rear sway bar stiffness ratio of the sports suspension that my car had and that of the optional JCW sports suspension. For this the suspension with a lower ratio being in the direction of reducing understeer (eg: increasing the rear bar stiffness relative to the front bar stiffness). I found that the 27mm front bar and the 22mm solid/25mm hollow rear bar would give me a slightly lower ratio than the sports suspension with the rear bar in the softest setting and slightly lower than the optional JCW sports suspension with the rear bar in the stiffest setting. I think this gives me a comfortable range of adjustment without going too wild. And, no, the understeer is not oppressive and, in most cases, it is manageable with the gas and brake pedals. I am also a believer in the Randy Pobst school of driving - late braking and trail braking deep into most corners. This does a lot to remove understeer and my car responds really well to it.

I will admit, my suspension targets stability. This is probably not what you want for a top level, winning autocross car. It is also probably not what you want in a top level, winning race car. But it makes for a great DE car. One that I can learn driving skills on without too much fear of not being able to drive it home. It also makes for a really fun canyon carver and probably would make for a really good setup for someone learning to race.

I will also say that this is also not the only way to change the handling of a MINI. I have seen huge changes with adjustable shocks/coilovers and front to rear spring rates. Increased spring rates and/or stiffening the shocks up front will have the same effect as adding a stiffer front sway bar. That is, A large RSB can be balanced with stiffer rear springs. However, Each of these options has their own pros and cons. Each of these pros needs to be weighed against the objectives. It is possible that a mix and match of all of these is what is needed for your needs. If I was to do my car over again, I might do it a bit differently. I might have gone with a slightly less stiff FSB and increased the front spring stiffness. This would increase the independence between the front wheels, but I might loose some of the camber improvement under hard braking.... ‘Tis to think about.

Hope this help to explain my perspective on this subject.

Then again, my point of view on this post

Best handling Classic MINIs, EVER!
 

I can only guess what has gone into making these Classic Minis handle like they do:


Enjoy!

PS: And clearly driven by some very skilled drivers. :nod:

Charlie,
Your comment poses an interesting question.

You are correct that a larger sway bar helps with getting power down at the opposite end of the car. That is, a larger front bar will help a RWD car and a larger rear bar will help a FWD car. And vice versa - a larger RSB will hurt a RWD car and a larger FSB will hurt a FWD car (just discussing drive traction here)

So, you have come up with a dilemma - how does one increase the roll stiffness of the front of a FWD car without loosing the drive traction that the RSB is providing?

Does anyone know the answer to this question? I don’t...

But, I will take a guess (go figure). Stiffer springs up front. Here is my logic for this guess...
I have had discussion about larger FSB vs stiffer springs with MrBlah, who prefers stiffer springs. He raises what I believe is a valid point. That is, the sway bar removes some of the independence between the left and right wheels. He doesn’t like the way a larger sway bar causes the car to be upset when going over curbing on a track. For him, the stiffer springs provide the independence he is looking for that the sway bar doesn’t have. Clearly, a large FSB will play into drive traction. Will stiffer springs, which will increase roll stiffness, also not have a significant impact on drive traction because they retain the independence between those two wheels? I believe the answer should be “yes”. But, likely the operative words here are that they should have no significant impact. There always seem to be a Ying and a Yang with everything we touch in a suspension.

Stiffer shocks up front may also be a possibility for the same reason I believe stiffer springs would work in this situation. Especially shocks like the adjustable Ohlins or Bilstein coilovers, which adjust both the jounce and rebound versus just adjusting the rebound as in the Koni yellows (although, they should work too).

Again, just my guess.

Hmmmm - what fun would it be to have a MINI Cooper Factory race car driver’s licenses, unlimited access to a track, unlimited funds and a shop to try out all of these ideas? :nod:

I've found stiffer springs is the way to go. Stiff springs up front and No front sway bar.

Alan, this is an unusual arrangement; a bit of old school, 1950s race car setup. What do you find about this setup that suits you? And, if you don’t mind, what experience are you coming from or what type of driving do you do with this setup?

It keeps the fronts planted, and we can actually use the rear bar to balance the car.

I got this car February of '03. Did autocrossing for a few years, then switched to HPDE in '05. Did a season of TT in '12, and started racing with NASA & BMW in '13.

MSRC 17 CCW NASA Race 3 March 2019

We take our racing super seriously, and I've got the google eyes to prove it!

https://cimg4.ibsrv.net/gimg/www.nor...0af611da52.jpg

With the install of the TVS900, the car will be back in GTS2 for the race at CotA this weekend although still about 15 HP shy of what it should be. That should be corrected when we swap the 70mm pulley for the 65mm pulley. Didn't have time to get that done before this race.

Love the googly eyes... :thumbsup:

OK - serious race car here and, just a guess, you have tried various suspension parts to get to where you are now. Great to have you share your experience here.

I have heard racers talk about removing the front sway bar on FWD cars, but didn’t understand how that could be. It would have little roll stiffness. But what they were not saying, which you said, is you have to replace the front sway bar with stiff springs to get that roll stiffness back.

And when you say “keep the fronts planted”, do I assume correctly that you are referring to the independence of the front wheels and not having the one side bounce the other side? Also, would it be correct to assume that the shocks play into this as well? And lastly, would you say this might be a little too stiff in the front to make a good road car? Sorry if these question seem a bit basic, just looking to learn a little.

Also, not looking for you to give any setup secrets here and not looking to take you away from getting ready for this weekend. It would be great if you could share what you can, when you have time. Thanks for that.

Hope this weekend goes well for you. Do you have another thread of your car build and/or race adventures that you could link to?

We are on at least the 6th iteration of this suspension. I have consulted with pro drivers that ran "very similar" cars in a major national racing series. I have had a pro driver in the car several times. (Which is quite humbling - He beat my best time by 4 seconds on his first flying lap on a about 2:10 second lap - I did get 2 of the 4 back during the day)

You are correct that keeping the fronts planted means making them 100% independent. And you are correct that stiff is a big number - I hesitate to state it here because it is dependent on shock valving and tire selection. The more grip the more spring you need to run. I will say DO NOT disconnect the FSB unless you have at least 500lbs/in springs - I have broken a front spring.

I have run greater than 500 lb/in front springs on the street without any issue. You must have the shocks valved to support the spring rate. Most of the rough ride on these cars comes from limiting shock travel from lowering, and very low aspect ratio tires.

I have my Youtube channel referenced above, and Instagram

You can follow the fun this weekend via Race Hero There is test and tune on Friday, so the event may show up then. Otherwise it should be available first thing Saturday.