MM K-Member upper or lower control arm holes - Ford Mustang Forums : Corral.net Mustang Forum
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post #1 of 24 Old 01-16-2017, 02:34 PM Thread Starter
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MM K-Member upper or lower control arm holes

Are any of you guys running your MM control arms in the upper holes on the MM kmember?

Ive got the car torn down for a little winter tune up and I'm wondering if I should do this while I'm in there. I'm going a little stiffer on the front coil overs so it's not much more of a step to raise the control arms to the upper bolt holes.

My car is not terribly lowered but it's on coil overs all around so it's easily adjustable.

*When is it advantageous to use the upper holes Vs. the lower holes?*


91 Road Racing Fox. N/A 383W. Full MM setup front and rear with Bilstein coil-overs. 14x1.25 Wilwood 6 piston fronts with Wilwood 13x1 single piston rears.
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post #2 of 24 Old 01-16-2017, 02:48 PM
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Once your A-arms go beyond level and start to angle upwards towards the wheels then it's time to use the upper holes. Of course that's in a static position. There is probably something to be said about how a-arm angle will change as the suspension is cycled. Someone with more trustworthy information than I should comment on that. I use the lower holes right now with my A-arms dead level.

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post #3 of 24 Old 01-16-2017, 02:53 PM Thread Starter
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Perhaps as I'm moving up from a 400lb spring to a 500lb spring and MM valves race bilstein there will be much less A-arm "roll" and it would be more advantageous to use the upper holes?

91 Road Racing Fox. N/A 383W. Full MM setup front and rear with Bilstein coil-overs. 14x1.25 Wilwood 6 piston fronts with Wilwood 13x1 single piston rears.
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post #4 of 24 Old 01-17-2017, 01:12 AM
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I use the upper holes for a better camber curve, as they're closer to level in that position, but Jack at MM once pointed out some downsides and I'm damned if I can find the communication. I've considered moving back to the lower holes but I don't want to add another inch to an already long bumpsteer stack.

- '92 5-sp LX coupe, TKO, 3.55s, pulleys, Al DS, 1-5/8 shorties, Magnaflow spun metallic cats, 2.5 Dynomax VT's, Pro5.0 w/ Steeda Comfort Pro
- GT40Y's, E cam, Cobra intake, tmoss ported lower, FRPP 65mm TB, BBK 75mm MAF, 155 lph pump, FRPP oil cooler, Al rad, 130amp alt, Energy mounts
- Autopower 4-point bar, 5-pt harnesses, MM weld-in subs and STB
- MM K-member/A-arms/Coilovers, 425# springs, MM cc plates, Koni Yellow DA's
- MM adj rear LCA's/torque arm/panhard bar, 500# rear springs, Eibach swaybar, Koni Yellow SA's
- SN95 Cobra brakes, FR500 17x9 wheels, Raybestos ST47 pads
- 3400 lbs with me and 1/2 tank of gas
- Regional autocross champ 99-02, 10,100 miles on 17 tracks
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post #5 of 24 Old 01-17-2017, 07:23 AM
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Quote:
Originally Posted by MFE View Post
I've considered moving back to the lower holes but I don't want to add another inch to an already long bumpsteer stack.
Wouldn't lowering the inboard side of the LCAs reduce the length of bumpsteer stacks here?


Norm

Mine: '08 GT, 5MT, black/light graphite, un-Fstock (DD, occasional track day & auto-X)
Wife's: '10 Legacy 2.5GT (DD, six-speed manual)
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post #6 of 24 Old 01-17-2017, 08:38 AM Thread Starter
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Yes, if you lower the control arm to the bottom bolt, you would have to REMOVE approximately 1" of your bumpsteer stack.

So you would be better off as far as your bumpsteer stack of spacers go MFE. If this is something you decide to do PLEASE report back and let me/us know what kind of difference it made 😊 If you happen to find the communication between you and Jack on this subject I would be interested in that too!

91 Road Racing Fox. N/A 383W. Full MM setup front and rear with Bilstein coil-overs. 14x1.25 Wilwood 6 piston fronts with Wilwood 13x1 single piston rears.
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post #7 of 24 Old 01-17-2017, 10:50 AM
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Ive been using the upper holes since day one. I figured this would allow me to keep the car lower and still keep the arms level.


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post #8 of 24 Old 01-17-2017, 11:48 AM
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I have mine in the lower holes with the A-arms level. I've considered going to the upper holes but due to tire to fender clearances I wouldn't be able to have the arms level. They would angle down from the kmember toward the wheel/tire. Are there any downsides to the arms angling in this fashion?
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post #9 of 24 Old 01-17-2017, 12:17 PM
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back later
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post #10 of 24 Old 03-05-2017, 07:52 PM
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Quote:
Originally Posted by MFE View Post
back later
How much later? I have my MM K-member install going as I type. I'm going with the lower holes. My car is low, but I see lower ones regularly.
I still have a couple fingers of room in the non-modified fenders.


'88 LX hatch. Full MM suspension minus the Torque Arm, Cobra Brakes. Not as slow as it used to be.
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post #11 of 24 Old 03-05-2017, 09:05 PM
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I made the switch a couple weeks ago but I won't be back on track for a couple weeks more.
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post #12 of 24 Old 03-05-2017, 09:18 PM Thread Starter
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So what made you decide to switch? What are the advantages and when is it advantageous to do it?

Or, is this more of a test to just see the difference? Very very curious about your "testing" results one you get it on track. Possibly notice anything on the street with it? Did you have to remove or add an inch of bumpsteer shims to correct bumpsteer?

PLEASE PLEASE PLEASE report back after you have a chance to put it on track.

91 Road Racing Fox. N/A 383W. Full MM setup front and rear with Bilstein coil-overs. 14x1.25 Wilwood 6 piston fronts with Wilwood 13x1 single piston rears.
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post #13 of 24 Old 03-06-2017, 02:18 PM
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As far as bumpsteer stack goes, I swapped to S198 from SN95 spindles to reduce it by about 1".
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post #14 of 24 Old 03-07-2017, 09:32 AM
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Quote:
Originally Posted by LT1HAHA View Post
So what made you decide to switch? What are the advantages and when is it advantageous to do it?

Or, is this more of a test to just see the difference? Very very curious about your "testing" results one you get it on track. Possibly notice anything on the street with it? Did you have to remove or add an inch of bumpsteer shims to correct bumpsteer?

PLEASE PLEASE PLEASE report back after you have a chance to put it on track.
I removed about an inch of spacer stack. I say "about" an inch because I forgot to record exactly how high my rack was mounted before I took it down, so I re-measured and re-adjusted the bumpsteer for the location its in now. Assuming nothing changes, you can remove exactly an inch of spacer stack. Interestingly, the resulting stack is shorter than the shortest tapered spacer MM provides.

As for why I changed it, I've been chasing a rather persistent understeer condition for a long time. I have the rear where I want it with 500-lb springs and an Eibach bar. Meanwhile I've also been chasing uneven tire wear in the front. No matter what, I wear out the edges of the tires long before the rest of the tire is gone.

Jack Hidley once had this to say about the location of the arms:
Quote:
Lowering the FCA in the k-member increases the distance from the roll center to the CG. This results in less roll stiffness at the front. Less roll stiffness means less load transfer, AT THAT END OF THE CAR (FRONT). This means each of the front tires is more equally loaded and therefore can provide more total grip.
This is pretty basic, and it's always made me wonder if I was sacrificing grip for a better camber curve. Meanwhile:

Quote:
Another advantage is this. The higher the roll center, the greater the jacking forces created from cornering forces. The jacking force actually causes the sprung mass to lift. Tire scrub tends to hold it up. Higher sprung mass cg, increases the total weight transfer of the car, which reduces total cornering grip. This is why fairly low roll centers are important.
Hmmm...jacking forces, tire scrub, reduced grip...it's been getting me thinking. Furthermore:

Quote:
One of the local guys that runs AI with our parts dropped his FCAs in the k-member, added 30% more spring rate to increase the roll stiffness and dropped his lap times about 1.5 seconds due to the decreased understeer.
Sold. Except I'm not increasing the front spring rate beyond the 425's I'm already running, so this should be educational. Unfortunately I'm not a consistent enough driver to nail it down to lap times (I took 3 seconds off my best lap time on Arizona Motorports Park last time I was there, with NO changes to the car), and I'll be running different tires next time anyway. So my report back will likely come down to how it feels and how tire wear nets out.

- '92 5-sp LX coupe, TKO, 3.55s, pulleys, Al DS, 1-5/8 shorties, Magnaflow spun metallic cats, 2.5 Dynomax VT's, Pro5.0 w/ Steeda Comfort Pro
- GT40Y's, E cam, Cobra intake, tmoss ported lower, FRPP 65mm TB, BBK 75mm MAF, 155 lph pump, FRPP oil cooler, Al rad, 130amp alt, Energy mounts
- Autopower 4-point bar, 5-pt harnesses, MM weld-in subs and STB
- MM K-member/A-arms/Coilovers, 425# springs, MM cc plates, Koni Yellow DA's
- MM adj rear LCA's/torque arm/panhard bar, 500# rear springs, Eibach swaybar, Koni Yellow SA's
- SN95 Cobra brakes, FR500 17x9 wheels, Raybestos ST47 pads
- 3400 lbs with me and 1/2 tank of gas
- Regional autocross champ 99-02, 10,100 miles on 17 tracks
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post #15 of 24 Old 03-07-2017, 07:26 PM
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Been running in the lower mount for three years. Rack is as high as it will go. Bump-steer stack is very short. At my ride height, arms are level. But I'm in the process of measuring the mounts and running an analysis to see what this thing is doing in dive and roll; caster and camber gain, roll-center migration, etc. I know the MM front strategy is low travel/high roll. I'm interested how this compares to other strategies and if advantageous, whether the others can be accomplished on the Fox chassis.
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post #16 of 24 Old 03-07-2017, 11:26 PM
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To add to Fraser's comments in post #14.

The closer the roll center is to the cg, the faster the weight transfer occurs. The higher roll center does increase the roll stiffness some, but this is a smaller affect than the transient change.

With the higher front roll center, the car will turn in faster, but understeer more due to the slightly greater weight transfer.

Virtually all of the differences between a high and low roll center can be tuned in or out with different swaybars and infinitely adjustable dampers.

This works essentially the same as IC does in side view for the rear tires. More AS% allows the weight to transfer faster to the rear tires, but it does not affect the total weight transfer. This is still super important to have high AS% for maximum forward acceleration because if the engine and clutch deliver power to the rear tires faster than the weight transfers there from acceleration, then the tires will start spinning and all is lost, without lifting and waiting.

RC height in the front or rear affects how quickly weight transfers at that end of the car. This affects the transient handling balance. With a high rear RC and/or a low front RC, the lateral weight transfer is going to occur faster across the rear tires, so the car is going yaw faster. This will not affect its steady state cornering power.

In Fraser's case, I'm pretty sure that he still needs a larger rear swaybar.

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post #17 of 24 Old 03-08-2017, 08:38 AM
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Amazing the difference that a couple hundred or so milliseconds can make.


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Mine: '08 GT, 5MT, black/light graphite, un-Fstock (DD, occasional track day & auto-X)
Wife's: '10 Legacy 2.5GT (DD, six-speed manual)
Spares:'01 20AE Maxima, 5MT (also my bad weather alternate); '95 Mazda 626, 5MT
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post #18 of 24 Old 03-09-2017, 08:20 PM Thread Starter
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This is very interesting. Looking forward to your experiences with it...

91 Road Racing Fox. N/A 383W. Full MM setup front and rear with Bilstein coil-overs. 14x1.25 Wilwood 6 piston fronts with Wilwood 13x1 single piston rears.
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post #19 of 24 Old 03-20-2017, 12:44 AM
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As I suspected, my only data is mostly anecdotal. The one session I timed put me in similar times to what I've run before, but I did catch myself saying "feck me" like an Aussie several times, as the car wanted to turn in RIGHT NOW, it seemed significantly looser, and I had to dial way back on my usual trail-braking. I anticipate further disappointment in the tire-wear department ad the left front seemed to be wearing its outside edge pretty hard at 40 psi hot. I haven't taken the wheels off to look at the inside edges yet. Seems looser in turning left and tighter turning right which is obviously a tuning issue but I don't know what yet. Bumpsteer is perfect, and while it's not corner-weighted, both perches on each end of the car are within 1/4 inch of each other. Bottom line is the car seems like it wants to rotate a lot easier than before and is more lively to drive, but not sure of ultimate lap time change.


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post #20 of 24 Old 03-20-2017, 11:55 AM Thread Starter
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So you like it better overall?

You went from the top holes to the lower holes, correct?

91 Road Racing Fox. N/A 383W. Full MM setup front and rear with Bilstein coil-overs. 14x1.25 Wilwood 6 piston fronts with Wilwood 13x1 single piston rears.
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post #21 of 24 Old 03-20-2017, 01:47 PM
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Yes, I think I like it overall but this was one track day compared to dozens with the other setup. And yes, I moved it to the lower holes.

Last edited by MFE; 03-20-2017 at 01:48 PM.
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post #22 of 24 Old 03-20-2017, 05:32 PM Thread Starter
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Keep us posted as you further sort things out.

Ive decided to go ahead and keep mine in the lower holes for now then.

91 Road Racing Fox. N/A 383W. Full MM setup front and rear with Bilstein coil-overs. 14x1.25 Wilwood 6 piston fronts with Wilwood 13x1 single piston rears.
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post #23 of 24 Old 03-20-2017, 09:43 PM
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Lower holes it 'tis.

'88 LX hatch. Full MM suspension minus the Torque Arm, Cobra Brakes. Not as slow as it used to be.
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post #24 of 24 Old 03-22-2017, 12:49 AM
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Last season I was using a modified stock K member where the A-arm holes were relocated upward about 2" (equivalent to the upper holes on the MM k member); main reason for doing this was for a better camber curve. (Class restrictions prohibit use of the MM K member without a substantial weight penalty.) In conjunction with 2" drop spindles, this raised the front roll center to over 6" about ground. . I now fully realize this was highly counterproductive. LOL.

Anyhow, here's a picture of the car at full dive/roll at the limit of traction, where you can clearly see the "jacking" effect on the inside tire, which is actually lifting that tire off the ground. In this situation it understeered pretty badly.



I've since changed things around , including relocating the A-arms to stock height so now the front roll center is much, much lower (but still above ground), and hopefully more optimal.

Can't wait to see how it affects performance this year.

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