Suspension Loose rear end at high speed braking
It was Meb that was having issues, ... he always has issue. 
Rear control arms have no part of the equation here, unless of course their bushings are blown out.
I think it might help if you describe the problem a bit more. When does the car start to over rotate? Is this happening in the straight line braking, or during trail braking. The Mini is normally well under control during trail braking due to it's excellent ABS system. Do you have DSC (DCS? I forget the right order here
) Even when off that usually still works with the ABS in trail braking.
Have you had any issues with any of the above, or any issue with low tire pressure warning?
How are your engine and trany mounts looking. You'd be surprised what these can do to effect the car's geometry during braking.
How are those coilovers? Old? What are your spring rates front and rear?
Sounds like you may have the car pretty slammed (real low). You may be bottoming out your front struts under braking. This will really upset the car.
Also, if you have been running slammed for a while you just may have destroyed the shocks by now.
It's always so hard to offer advice on these things when you can't actually feel what's going on.
Rear control arms have no part of the equation here, unless of course their bushings are blown out.
I think it might help if you describe the problem a bit more. When does the car start to over rotate? Is this happening in the straight line braking, or during trail braking. The Mini is normally well under control during trail braking due to it's excellent ABS system. Do you have DSC (DCS? I forget the right order here
) Even when off that usually still works with the ABS in trail braking. Have you had any issues with any of the above, or any issue with low tire pressure warning?
How are your engine and trany mounts looking. You'd be surprised what these can do to effect the car's geometry during braking.
How are those coilovers? Old? What are your spring rates front and rear?
Sounds like you may have the car pretty slammed (real low). You may be bottoming out your front struts under braking. This will really upset the car.
Also, if you have been running slammed for a while you just may have destroyed the shocks by now.
It's always so hard to offer advice on these things when you can't actually feel what's going on.
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I think onasled asked the best set of questions that must first be answered before developing a more thorough understanding of the situation at hand. Only then may we truly begin to analyze and address the problem.
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Joined: Dec 2002
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From: Sammamish, WA
Likely 'bump stoping' or oversteer or a bit of both
Yes Beecher and Sucubus,
After reading the profile of nikoamok's car, especially the aggressive front and rear anti-roll bars, I would worry about massive oversteer with heavy braking. The load shift forward with braking will unload the rear end of an already 'loose', or oversteering suspension set up. In that circumstance, the direction which the rear end rotates is anybody's guess. Won't be straight ahead, that's for sure.
And, very aggressive braking with a significantly lowered car, assuming stock struts, is the formula for potential 'bump stopping'. The load shift forward with braking will compress the front struts and springs and could max out the strut travel. The bumpstops will compress more and more, and the effective front spring rate goes way up toward infinity. At that point, front end compliance with the road drops toward zero and the car starts having a life of it's own, independent of the driver. 'Bump stopping' can be difficult for a driver to separate from a horrid oversteer problem because the car behavior seems the same to me.
Also, I wrote Beecher about my suspicions that the rear strut monoball upper strut bushings (read 'zero compliant' bushings) in my MINI were contributing to instability under heavy, heavy braking on uneven track surfaces. I've settled my MINI down by increasing the rear suspension compliance a bit and tuning out a little of the rear roll stiffness, just a 'little'.
Hope this helps,
John Petrich in Seattle
After reading the profile of nikoamok's car, especially the aggressive front and rear anti-roll bars, I would worry about massive oversteer with heavy braking. The load shift forward with braking will unload the rear end of an already 'loose', or oversteering suspension set up. In that circumstance, the direction which the rear end rotates is anybody's guess. Won't be straight ahead, that's for sure.
And, very aggressive braking with a significantly lowered car, assuming stock struts, is the formula for potential 'bump stopping'. The load shift forward with braking will compress the front struts and springs and could max out the strut travel. The bumpstops will compress more and more, and the effective front spring rate goes way up toward infinity. At that point, front end compliance with the road drops toward zero and the car starts having a life of it's own, independent of the driver. 'Bump stopping' can be difficult for a driver to separate from a horrid oversteer problem because the car behavior seems the same to me.
Also, I wrote Beecher about my suspicions that the rear strut monoball upper strut bushings (read 'zero compliant' bushings) in my MINI were contributing to instability under heavy, heavy braking on uneven track surfaces. I've settled my MINI down by increasing the rear suspension compliance a bit and tuning out a little of the rear roll stiffness, just a 'little'.
Hope this helps,
John Petrich in Seattle
Last edited by Petrich; Feb 14, 2008 at 09:16 AM.
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Yes, I always have issues...but the car is a stocker again and all is well.
Do not discount rear brake bias. This and poor bump control can contribute to looseness. Even in stock guise, my mini is not an inspiring car to trail brake. So, remembering the friction circle, I don't trail brake. But at some level, if you are racing competitively, I think you need to be able to trust the car's trail braking ability because protecting your corner requires this technique at times.
Check these
Rear toe
Front to rear spring balance
Sway bar balance
Damping balance
binding anywhere?
Track width
Ride height
Brake bias
Front to rear frcition balance - brakes
...my car as a stocker with 100K miles on the stock struts will definately rear steer if one side hits a bump while hard into a turn. This just happened thru a fast sweeper. I won't do that again.
Do not discount rear brake bias. This and poor bump control can contribute to looseness. Even in stock guise, my mini is not an inspiring car to trail brake. So, remembering the friction circle, I don't trail brake. But at some level, if you are racing competitively, I think you need to be able to trust the car's trail braking ability because protecting your corner requires this technique at times.
Check these
Rear toe
Front to rear spring balance
Sway bar balance
Damping balance
binding anywhere?
Track width
Ride height
Brake bias
Front to rear frcition balance - brakes
...my car as a stocker with 100K miles on the stock struts will definately rear steer if one side hits a bump while hard into a turn. This just happened thru a fast sweeper. I won't do that again.
New York scrcasim at work. hehe
been there done that.....but still haven't solved the problem. i've discussed this w/ meb and tried several (if not all of his suggestions). my solution to date is "large ***** and soft hands" during threshold braking. it is not comfortable, but i never felt that the car was going to swap ends. the Alta PSRS helped, but did not eliminate the problem. my car will end up being track only, so i haven't ruled out any solution......good luck.
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Joined: Dec 2002
Posts: 314
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From: Sammamish, WA
If it walks and talks like a duck.............
Muladesign and Michael (meb),
I'm laughing but trying to be a bit serious as well.
Good point; of course, the driver's braking skills will have a significant on the behavior of the car. Not a NY sarcastic point at all; a much needed perspective. That said, "it is easier to go fast in a car that is easy to drive than it is in a car that is difficult to drive". The set up described in the original post would be difficult to manage by the most expert of drivers. So, yes, driver skill will make a significant difference, but with this particular setup mere mortals will have the same handling challanges.
Michael, give me a break, ha ha. Assuming all the chassis modifications the owner's MINI front roll center is probably subterranean, the rear roll stiffness has got to be huge with that world class rear anti-roll bar and the result is massive oversteer, "if it walks and talks like a duck.....". Let's call a spade a spade.
John Petrich in Seattle
I'm laughing but trying to be a bit serious as well.
Good point; of course, the driver's braking skills will have a significant on the behavior of the car. Not a NY sarcastic point at all; a much needed perspective. That said, "it is easier to go fast in a car that is easy to drive than it is in a car that is difficult to drive". The set up described in the original post would be difficult to manage by the most expert of drivers. So, yes, driver skill will make a significant difference, but with this particular setup mere mortals will have the same handling challanges.
Michael, give me a break, ha ha. Assuming all the chassis modifications the owner's MINI front roll center is probably subterranean, the rear roll stiffness has got to be huge with that world class rear anti-roll bar and the result is massive oversteer, "if it walks and talks like a duck.....". Let's call a spade a spade.
John Petrich in Seattle
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Joined: Dec 2002
Posts: 314
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From: Sammamish, WA
How are your rear control arm bushings?
Bean old buddy,
We've PM'd but I have forgotten your name.
Have you considered your rear control arm rubber bushings as a potential source of rear end instability under braking? Hope that you are not 'bump stopping' or have a crazy over steer set up.
Regards,
John Petrich in Seattle
We've PM'd but I have forgotten your name.
Have you considered your rear control arm rubber bushings as a potential source of rear end instability under braking? Hope that you are not 'bump stopping' or have a crazy over steer set up.
Regards,
John Petrich in Seattle
Bean old buddy,
We've PM'd but I have forgotten your name.
Have you considered your rear control arm rubber bushings as a potential source of rear end instability under braking? Hope that you are not 'bump stopping' or have a crazy over steer set up.
Regards,
John Petrich in Seattle
We've PM'd but I have forgotten your name.
Have you considered your rear control arm rubber bushings as a potential source of rear end instability under braking? Hope that you are not 'bump stopping' or have a crazy over steer set up.
Regards,
John Petrich in Seattle
greg
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Joined: Dec 2002
Posts: 314
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From: Sammamish, WA
Greg, I meant your rear trailing arm bushing
Greg,
I'm sorry, but the terminology is confusing. I was referring to what Bentley terms the rear 'trailing arm bushing': that big black bushing located at the front end of your rear trailing arm mount to the chassis. I was not referring to your upper and lower rear control arms. That large bushing can distort under significant suspension loads and cause some unpredictable rear end behavior.
As an aside, Bentley refers to the equivalent component in the front suspension as the 'control arm'. I just got mixed up when I posted to you.
Regards,
John Petrich in Seattle
I'm sorry, but the terminology is confusing. I was referring to what Bentley terms the rear 'trailing arm bushing': that big black bushing located at the front end of your rear trailing arm mount to the chassis. I was not referring to your upper and lower rear control arms. That large bushing can distort under significant suspension loads and cause some unpredictable rear end behavior.
As an aside, Bentley refers to the equivalent component in the front suspension as the 'control arm'. I just got mixed up when I posted to you.
Regards,
John Petrich in Seattle
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I'll throw this in the mix...I have a 99 Si that is pretty well set up - actually being set up as we write. This is a car that I can take very deep into a turn while on the brakes. I'm not suggesting this is good or bad or correct.
The weight distribution of the Si and the Mini is about equal, but the Si's wheel base is longer by about 6". So if the Si's CG is lower, and its wheel base is longer, all else equal, less weight will move forward under braking. The Si also won the dubious longest braking distance award when it came out - since fixed - and still very stable under braking.
Does wheel base factor into the Mini's sligtly untidy back end under braking? I think so; weight transfer under braking does affect balance. There might be other factors such as toe compliance under braking, camber compliance/compensation - affected by swing arm movements. Remember the VW Beetle's literal swing arm suspension? Not exactly an inspired piece trail braking at speed.
If I were to pick one glaring fault with the mini, braking stability would be it.
I worked with Todd to get the proper brake material balance front and rear with my wilwood set up and the R compound tires were factored in as were a few other details. The mini was still a little spooky.
In an HPDE environment you can brake in a stright line and not worry about someone stealing your corner. In an all out race you have to be able to protect your corner and at times that means braking and turning deep into a turn.
"big *****, soft hands" Nice! That should be a quote that lives on.
The weight distribution of the Si and the Mini is about equal, but the Si's wheel base is longer by about 6". So if the Si's CG is lower, and its wheel base is longer, all else equal, less weight will move forward under braking. The Si also won the dubious longest braking distance award when it came out - since fixed - and still very stable under braking.
Does wheel base factor into the Mini's sligtly untidy back end under braking? I think so; weight transfer under braking does affect balance. There might be other factors such as toe compliance under braking, camber compliance/compensation - affected by swing arm movements. Remember the VW Beetle's literal swing arm suspension? Not exactly an inspired piece trail braking at speed.
If I were to pick one glaring fault with the mini, braking stability would be it.
I worked with Todd to get the proper brake material balance front and rear with my wilwood set up and the R compound tires were factored in as were a few other details. The mini was still a little spooky.
In an HPDE environment you can brake in a stright line and not worry about someone stealing your corner. In an all out race you have to be able to protect your corner and at times that means braking and turning deep into a turn.
"big *****, soft hands" Nice! That should be a quote that lives on.
Last edited by meb; Feb 16, 2008 at 06:54 AM.
4th Gear
Joined: Dec 2002
Posts: 314
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From: Sammamish, WA
I think that these bushings might be an option
Greg,
I was told of these bushings by SteveS on NAM: https://www.northamericanmotoring.com...coil+rmw+4 00 Steve has thought about them more than I have and maybe he'll post and share his knowledge with all of us. (I hope that this is the correct link)
It would be wise to be cautious about the Alta front monoball bushing product, at least, in their stock configuration. It seems that those bushings are a great product, but people, including Matt (Dr O), have reported the set screws can loosen up on the bushing. There's got to be a simple fix for that, though.
In the rear, I am using a monoball bushing that I obtained from the BMW 3 series and Z-3 racers; Turner MS and BimmerWorld. They sell, or once sold, a track oriented monoball replacement for the BMW 3 series rear 'trailing arm' rubber bushing that fits MINI. Expensive to purchase and install, but zero compliance.
Michael,
As always, you are right on. Want to comment on your Honda and brake thoughts.
You are correct, the 95" (I think) wheel base of the MINI does materially contribute to the longitudinal load transfer with braking. Yes, yes, yes. Wish I could complain about the same problem with acceleration. Ha, ha. I too note instability with straight line threshold braking in the MINI, especially on uneven track surfaces, a distinguishing feature of our local tracks. But, eliminating most of the suspension compliance, except for the upper rear strut bushing, settled just about all that down. So, at present, I'm happy.
About your brake bias observations and problem solving: that is one of the biggest criticisms of those big brake kits. Those kits, typically, gain a small increment of reduced braking distance by redistributing the longitudinal brake bias toward the rear a few percentage points. That's it. Yes, their thermal mass does help reduce fading in a track environment. So does high temp track pads, air ducts, and driving technique. For what it is worth, I retained my stock MINI brake system, and use high temp pads, bronze bushings, and air cooling. I don't have to worry about fine tuning the brake system for my level of driving. The stock MINI onboard brake biasing system seems to have enough bandwidth of manage the balance for me with the modified stock system. Will be interesting to see you your Honda does on the track this season. Best of luck.
Have a good weekend everyone. I'm off to the opera with the wife.
John Petrich in Seattle
I was told of these bushings by SteveS on NAM: https://www.northamericanmotoring.com...coil+rmw+4 00 Steve has thought about them more than I have and maybe he'll post and share his knowledge with all of us. (I hope that this is the correct link)
It would be wise to be cautious about the Alta front monoball bushing product, at least, in their stock configuration. It seems that those bushings are a great product, but people, including Matt (Dr O), have reported the set screws can loosen up on the bushing. There's got to be a simple fix for that, though.
In the rear, I am using a monoball bushing that I obtained from the BMW 3 series and Z-3 racers; Turner MS and BimmerWorld. They sell, or once sold, a track oriented monoball replacement for the BMW 3 series rear 'trailing arm' rubber bushing that fits MINI. Expensive to purchase and install, but zero compliance.
Michael,
As always, you are right on. Want to comment on your Honda and brake thoughts.
You are correct, the 95" (I think) wheel base of the MINI does materially contribute to the longitudinal load transfer with braking. Yes, yes, yes. Wish I could complain about the same problem with acceleration. Ha, ha. I too note instability with straight line threshold braking in the MINI, especially on uneven track surfaces, a distinguishing feature of our local tracks. But, eliminating most of the suspension compliance, except for the upper rear strut bushing, settled just about all that down. So, at present, I'm happy.
About your brake bias observations and problem solving: that is one of the biggest criticisms of those big brake kits. Those kits, typically, gain a small increment of reduced braking distance by redistributing the longitudinal brake bias toward the rear a few percentage points. That's it. Yes, their thermal mass does help reduce fading in a track environment. So does high temp track pads, air ducts, and driving technique. For what it is worth, I retained my stock MINI brake system, and use high temp pads, bronze bushings, and air cooling. I don't have to worry about fine tuning the brake system for my level of driving. The stock MINI onboard brake biasing system seems to have enough bandwidth of manage the balance for me with the modified stock system. Will be interesting to see you your Honda does on the track this season. Best of luck.
Have a good weekend everyone. I'm off to the opera with the wife.
John Petrich in Seattle
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From: Somewhere that no longer exists
wow, we are getting everyone who is anyone in on this one. This is the NAM i know and love. Anyway, i dont know how my reading between the lines is, but is the consensus is that our brake bias is a tad to high on the front end? I would like a bbk, but mostely for looks, and was going to enlarge the rears as well (probably a 10.5 rear and 11.75 front, but this is a discusion for elsewhere and another time). But should we bias more to the back? I keep meaning topull the fuse out of the the abs to see what locks up first but havnt yet, and am considering pulling the abs all together and replumbing the car and putting in a brake bias valve. Sounds like fun no? Thanks for the help guys. Beecher
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Joined: Dec 2002
Posts: 314
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From: Sammamish, WA
Beecher, you are a creative one, aren't you !
Beecher,
Wonderful ideas, but to what end? At least leave the ABS alone and drive the car. But then again, you may discover something wonderful. Just ignore me.
John Petrich in Seattle
Wonderful ideas, but to what end? At least leave the ABS alone and drive the car. But then again, you may discover something wonderful. Just ignore me.
John Petrich in Seattle
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From: Somewhere that no longer exists
hahah, i just love building cars and tinkering with them. As far as im concerned, no car is ever finished, or perfect. You guys will be the first to know if i find anything. the main reason i want to pull the abs is i prefer the feel of non abs brakes, which may or may not actually be a result of the tiny brake lines, or maybe just disc brakes, but only one way to find out. I have driven cars without abs (mind you they are also drum brake), one of which is a 1940 ford (not mine tho, the brake in mine still leak like a sieve)) And you could mash on those brakes with bias ply tires mind you, and that car would come to a grinding halt, and the brakes felt good the whole way down. With the mini, you climb on the brakes, and i dont find you feel the brakes. Of course, i need the brass bushing and stainless lines, and then maybe my opinion will change, ive driven a car with that stuff, but not hard, and only around town. Im just always looking towards something else to work on and play with, and hope i stumble into something good. haha Beecher
Couple things...
If you make a tri-angle between the front contact patch, the rear contact patch and the center of mass as seen from the side, then the ratio of the height of the Cm to the wheel base tells you your weight transfer, pretty much.
Drum brakes have a very, very different feel due to something called "brake gain" where the friction of the pad in the drum actually increases the pad-drum force. This is a function of how far inboard the shoe pivot is compared to the radius of the drum. But there is no brake gain in disk brakes. That's why they feel so different, and why unboosted disc brake set-ups are few and far between.
If you want to disable ABS/DSC, you can just pull a wheel sensor connector as well. This will kill the system (as it won't know what to due) and it will just shut-up and stay out of the way....
Matt
Drum brakes have a very, very different feel due to something called "brake gain" where the friction of the pad in the drum actually increases the pad-drum force. This is a function of how far inboard the shoe pivot is compared to the radius of the drum. But there is no brake gain in disk brakes. That's why they feel so different, and why unboosted disc brake set-ups are few and far between.
If you want to disable ABS/DSC, you can just pull a wheel sensor connector as well. This will kill the system (as it won't know what to due) and it will just shut-up and stay out of the way....
Matt
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From: Somewhere that no longer exists
is they why they feel so different? thanks. I had just assumed it was large brake lines and no complicated hardware to flow threw. I guess i dont have to worry about that now. I still want to know what locks first tho, so i know the best route to upgrade brakes. Either that, or im putting drums on the mini !!!!