Miata Suspension Bumpstops

Written and conceived by Charles Cox

These are some of the development issues I've dealt with while trying to improve the Miata's handling (a difficult task). My current setup is available by clicking here.

I had some issues with the car handling.  Seemed too stiff (rides too rough) and adjustments didn't help.  I cut the dust covers (boots) off to see what was going on.

Photo is of the Right Front at static ride height (12 1/16 Fender Lip to Hub Center). Bump-stop is compressed through 1 3/16" of wheel travel, from first contact with bump-stop to static ride height.  Bump-stops are 2 and 1/8” thick and compressed 7/8 of an inch at static ride height.  Essentially very little to no front travel available.  All travel is limited by remaining bump-stop material, which isn't much.   


     Front unloaded to show bump-stop style and relative size:

     Right Rear under load at existing static ride height (12 and 9/16 from fender lip to hub center-line).  Appears to be approximately ¼” away from contact with the cut off bump-stops and is touching the bump-stop with my weight in the car.  Bump stops were cut according to install instructions by alignment shop that did the installation.  Springs were installed according to written instructions as well, but I was told afterward that the instructions were wrong and springs were installed upside down both front and rear.   
     If the rear bump-stops weren't cut-off  the car would be riding on compressed bump-stops the thickness of what was cut off (I would estimate he cut off around 3/8").     


     Car rode roughly on anything but smooth tarmac, didn’t go around corners that badly, but over-steered even with rear bar full-soft and front bar full-stiff.  I feel it had to do with the rear contacting the bump-stops when cornering and associated exponential increase in relative spring rate (inclusive of the bump-stop and spring) in the rear and the lack of any suspension movement in the front.  

     Measured ride from hub center to fender lip with first modified setup:
(static, without driver, ¾ fuel):

LF: 11  11/16  RF: 12  1/16
LR: 12  1/16  RR: 12  9/16

   
OA ride height from Fender Lip to ground:

LF: 22 ½  RF: 23  ¼
LR: 24  1/8 RR: 23  7/8

     Just finished installing Ground Control Coil-Over springs with a front set at 13" rear 13 1/8".  This raises the front about 1" and the rear about 1/2" from the previously modified setup, which is about 1/2-3/4" lower than stock in the front and about 1 1/8-1 5/8" lower in the rear than stock ride height. 

Measured ride from hub center to fender lip with current setup 
(static, without driver, 1/4 fuel):

LF: 13  RF: 13
LR: 13 1/8  RR: 13 1/8

   
OA ride height from Fender Lip to ground:

LF: 24 1/16  RF: 24
LR: 24  5/8 RR: 24 1/2

     I would like it to be lower, but there isn't enough suspension travel with the shocks as long as they are.  I will try to find shorter shocks for a later modification, but is good now as it doesn't scrape getting into the driveway and makes the car more useable for the street and is still lower and stiffer than stock and makes the wheels and tires seem more like they fit the fender openings.


    updated 2/16/2002-After 10 events last year and trying whatever off the shelf suspension components I could find, it appears there just isn't anything totally suitable for my application (2001) so having to do the R&D myself to sort it out.

FRONT END: Currently, I have a 1.25" front tubular sway-bar coming from Racing Beat (this bar is equivalent to a 29.64 solid bar), after numerous delays...has been on order for quite a long time.  I need to increase the front roll stiffness with the sway-bar and reduce the spring rates so the brake bias is more suitable.  Currently, with 600# front springs in the front, the front brakes like to lock up without much help from the rear.

     Based on my calculations and many considerations, I will be installing 350#  2.5 X 7" front springs to match the 1.25" sway-bar. This will give the equivalent roll stiffness of 650# springs with a 1" bar.  I had bottomed out with the 600# springs and 1" bar in the front in a bumpy turn on the street, so I wanted just a bit more roll stiffness (although when I buy the shortened shocks, this issue will be moot). It is also interesting to note that all the vendor recommended springs utilizing the coil-over setup, are too short to have adequate wheel travel without coil-bind.  I had to go through the charts to come up with my own spring sizes at different spring-rates that would still allow adequate wheel travel.

     The other problem I had with the front, the last event I ran at Marina, the car over-steered (again) very badly.  I thought it was due to the worn-out Hoosiers (a lot of it was), but when I got home and checked the sway bar settings, I discovered the front sway-bar link had broken.  It seems that for the front, the Jackson Racing heim-joint type links are just not strong enough for this application.  What broke was the joining stud between the two joints.  Jackson sent a new stud, but still looks too flimsy to me, so I ordered new male, teflon heim joints from Baker that are the heavy-duty type (with 7/16" shafts, instead of 3/8"). I found some couplers at Orchard Supply and will be using this set-up with the new 1.25" front bar. They will be long enough and should be quite a bit stronger.  Only problem is cost...at around $22 each joint, will have around $100 just in the front sway bar links...if they don't break, it will be worth it, though.

     I also will be taking off the lower control arms during the spring and sway bar install as I have reinforcing tabs to weld on the lower control arms for the sway-bar links (so it is as strong as the earlier cars...the new ones like mine are not nearly as strong) and I have some lower control arm inner polyurethane bushings to put in (will be gradually replacing the rubber stock ones with poly as I go along).

     I also have acquired the MazdaSpeed front reinforced sway bar mounting brackets.  These are expensive for what they appear to be, but look very, very strong and should eliminate any potential bracket strength issues.  I have heard of a number of others breaking the factory brackets, so thought this was a good time to replace them (although I think the Racing Beat solution to this problem is probably adequate and has been so far with my current installation). (Note: they don't work with the RB 1.25 bar brackets)

     There are a number of vendors building front bars that fit this application, but none of them are stiff enough.  Only the Racing Beat 1.25" bar is suitable for the 2001, unless either making a custom bar or installing a NASCAR type tubular/torsion bar with splined ends for removable arms (which I would have done had I known how long it would take Racing Beat to deliver their bar).  I feel this may be the way to go for eventual adjustability of the front end.  They are easy to replace and cheap once you get the arms and pillow blocks (just the torsion bars can be as little as $65). Note, I haven't tried the fit, but it appears the MazdaSpeed brackets (which would be required for this installation) have an extra set of wider spaced mounting holes that I feel certain will accommodate this installation...worthy of further r&d later.

REAR: In the rear, I decided that I must have an adjustable sway-bar to for the different track surfaces I run on...they are that different in how they affect the handling.  I reduced the rear spring rates in half to 150# (was running 300# rears with no sway bar) and installed a 9/16" Suspension Techniques adjustable sway bar. This is where I discovered the spring bind issue and had to calculate spring rate v. length to make sure it wouldn't coil bind, like the initial 150# did.  I think this will still be too much roll stiffness to balance with the front, so I ordered a custom made 1/2" bar from Saner in Florida. It should be here in a couple of weeks.  Even if the 9/16" seems to have reasonable balance (still have to test it), I'd want the softer bar and stiffer springs than 150# to keep from bottoming out in the rear on the street.

Interesting numbers:
Stock Sway-Bar: 11mm
R model rear bar: 12mm
After-market bars:
     Many make the 5/8" bar, but it is 430% (4.3 times) stiffer than the stock bar and makes the car over-steer virtually uncontrollably and what most vendors try to sell you...don't fall for this one.
     Suspension Techniques has a 9/16" bar, but this is even 2.7 times stiffer than the stock bar.
     As you can see by these numbers, it is very difficult to get spring rates to overcome these massive increases in roll stiffness and why you either need to retain the stock rear bar (as Racing Beat recommends for the 2001 model) or have one custom built (as I have chosen to do and compensate with changes in spring rates.) Custom 1/2" bar should be around 61% stiffer than the stock bar, which is within the range I can adjust spring rates.
     Interestingly, even the difference between the 1/2" and the 9/16" is still 61%, so the 1/2" bar will be essentially half the stiffness increase the 9/16" bar provides.  Also interesting of note the difference between the 1/2" and 5/8" is 243%! No wonder it wouldn't work!

UPDATE:  I am now in a position to try most of these changes over the next two to three events at which time I should be able to come up with final relative spring rates so I can finally have a set of Koni shocks built for the car.  I will probably initially order a set of standard single adjustable shocks and have them custom shortened and valved for the spring rates I'll be using.  (shortening runs only about $120 per shock, so is the least expensive way to accommodate adequate wheel travel)

So now I have in stock having tried:

Eibach springs:
2-2.5 X 6" 700#
2-2.5 X 6" 600#
2-2.5 X 6" 550#
2-2.5 X 7" 350#
2-2.5 X 7" 300# (for rear...worked reasonably well with no rear sway bar and 550# fronts with 1" bar)
2-2.5 X 7" 250#
2-2.5 X 10" 175#
2-2.5 X 8" 150# (too short  coil bind)
2-2.5 X 10" 150#

Sway bars:
1" Flyin Miata Front
1  1/4" Racing Beat Front Tubular (3/16" wall) Bar (now on car)
5/8" Flyin Miata rear
9/16" Suspension Techniques (currently on car)
1/2" Custom made rear adjustable by Saner
11mm Stock rear

Setup (3/02):
Front Springs: 350# 7" Eibach springs on KYB AGX with Ground Control Coil Over conversion.
Rear Springs: 175# Eibach springs on KYB AGX with Ground Control Coil Over conversion.
Front Sway Bar: Racing Beat 1.25" 3/16" wall tubular bar with 2 adjustments.
Rear Sway Bar: Suspension Techniques 9/16" 3-position adjustable bar.
Front Lower Control Arm sway bar reinforcing plate from Racing Beat welded on: click here
Front Sway Bar bracket reinforcement kit from Racing Beat (note: their 1.25" bar comes with cast aluminum mounting brackets that won't bolt up to the MazdaSpeed reinforcing brackets available from Mazda, so had to use the Racing Beat Kit to keep the brackets from breaking. Think they do the job fine, though: click here Powerflex lower front control arm bushings (these are great...your suspension will actually work smoothly without binding when you use these bushings and no difference in ride quality. Can't recommend these more highly!) See: Spa Technique (will change the rest of the suspension when I can)
Alignment:
Front: as much -camber and +caster possible (currently 1.8°
and need at least 2°) toe is 0° right now, but going to 1/8" toe out.
Rear: -2° camber...seems about right (2-2.5°) 1/16"-1/8" toe in.

Using Kumho Ecsta 700 225-45/15" tires on Kosei K-1  16X7" rims. Tires are weird...can't get proper temperature readings.  Seem to want way more air than they should, but still trying different pressures. Hoosiers were much easier to read (temperature and pressure wise) but don't think the Hoosiers had enough more traction to justify the price difference.  New Kumhos are wearing much more rapidly than I expected, though. I am sure I won't get 1/2 a season out of them, when I was expecting a full season.  Seems I need from 2-3 sets of tires per year at the number of events I run.

Still don't notice any difference among the adjustments on the KYBs...think the shocks basically suck! Will be getting Koni's and having them shortened as soon as I can afford it.

5/2002

Just installed the new custom shortened and valved Koni, single-adjustable shocks.  The front springs remain the same (350#) but did install the new sway-bar end links I custom made, due to the Moss Motors ones breaking.  In the rear, I have installed 250# (8") springs and the new 1/2" custom made adjustable sway-bar from Saner.  Front shocks were cut 1.5" (body), rears about 3/4".  This gives much more travel and can now lower the front to get more negative camber without bottoming out on the bump stops.  Will setup with 2° - camber in front and 2.5° in the rear.  Front, 1/16" toe out and rear 1/16" toe in. Find the Kumhos like much lower tire pressure than originally anticipated.  Seem to like about 34# hot, so usually starting at about 30# front and rear and adjust according to handling.

I have also finally finished installing all the polyurethane suspension bushings...they really are great and allow the suspension to travel without binding due to the damn rubber bushings in stock.  It was a real pain to remove the existing rubber bushings. I built my own puller out of threaded rod, nuts, washers and some pipe fittings...worked ok but still a lot of work. Glad they are finally out.  The front upper ones require that the long bolt holding the upper a-arm be pulled out forward, so the Racing Beat reinforcing blocks for the sway-bar brackets, had to be removed...glad I hadn't put in the mazdaspeed brackets yet, you can't take the bolt out without removing them, if you've installed them.

Will be interesting to see how the car handles now. I will be setting up the ride-height tomorrow, but want to get the front low enough to get the two degrees negative camber and no more...at the current ride height (could only get 1.8° with the previous set up and ride height...this wasn't quite enough and had some bottoming problems with the bump stops as it was). The front left tire rubs the fender, so need to raise the front end...will be better geometry as well, so doing that tomorrow.

6/2002 Update: Just installed 450# front springs to compensate for quite a bit of oversteer on first trial of new shocks and suspension setup.

Did some measurements and calculations and put this chart for negative camber together:

Will make an update after seeing how the existing setup works.

8/2002 Update: After a couple of spring changes (front) I'm back at the 600# front springs trying to keep the front tires from rubbing the upper spring perch and inner fender.  Raised the car a little bit to help too.  Last event at Marina, the car handled very satisfactorily, however at Golden Gate Fields, the car oversteered very badly.  Found the front tires were rubbing and shredding the inside corner on the spring seat (upper)...think I can still use the tires for a bit, but very disconcerting.  This was with 550# front springs.  I know wheel spacers would do the trick, but I already have the calipers, backing plates, rotors and stainless brake lines to do the back-dating to smaller diameter brakes so I can fit on the 13" diameter wheels, which should be here today...with those, inner clearance won't be a problem but fender rub may...won't know until I try them. So as of now, I'm running 600# front, 250# rear springs, with the shortened Koni single adjustable shocks, 1.25" Racing Beat tubular (3/16" wall) front and custom made 1/2" rear (solid) sway bars.

     Current ride height in front only allows for 1.76° of negative camber (I perfer 2°) but that's as good as I can do at this ride height (approximately 12.5" fender lip to hub center) after raising it about .5" for clearance purposes.  I also lowered the rear to help balance so the front and rear are about the same.

     I also had one of my custom made sway-bar heim-joint links start coming apart, so replaced it with the Moss Motors/Jackson racing ones that I ran before (and broke).  I now have the Racing Beat polyurethane type links but won't install them until I have to...there just isn't any articulation in the joint, which I feel is necessary for smooth and proper movement, but will put them on and try before condemning them out-of-hand...was worth the purchase to find out, but they appear to be a much stronger solution to the end links than the heim joints due to grade 8 connector and bolting hardware.

     Here is a link to Racing Beat's suggestion for cutting bump-stops for their replacement springs...probably the best way to cut the stock stops if you are going to do so: Racing Beat Bump Stops

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