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Application guide for all supported vehicles
(updated 1/22/2014)


FCM bump stop selection matrix, 1990-2005 Miatas for Comfort, Sport and Race

FCM Bump Stop Selection Matrix for 1990-2005 Mazda Miata


FCM bump stop selection matrix, 2006+ Miata and 2004+ Mazda RX-8 for Comfort, Sport and Race

FCM Bump Stop Selection Matrix for 2006+ Mazda Miata and 2004+ Mazda RX-8

NOTE: Fine-adjustments of your car's suspension travel may be done via 'bump stop spacers' or 'packers'. Watch this video to understand how they're used.


  • FCM 36mm linear - for cars lowered 1/2" or more OR using front springs MORE THAN 250 lb/in. Recommended for low cars with stiff springs. Standard on FCM custom coil-over kits (300f/220r - 600f/375r), usually shortened depending upon spring rate used

  • FCM 36mm front/46mm rear - Recommended for moderately stiff lowering springs such as Tein S-tech, Eibach, Racing Beat, H&R or Flyin Miata

  • FCM 46mm linear - for cars at stock ride height/stock springs or slightly lowered. A good choice for street/mild sport applications.

  • FCM 54mm - superceded by either the FCM 46mm or FCM 58mm red bump stops. We've found these to be more durable than the 54mm over time.

  • FCM 58mm high-performance - available in red, white and blue (soft, medium, and hard) densities. For 1990-2008 Miatas used for aggressive driving (on stock springs) or Stock-class autocross to stiffen effective spring rate when stock springs must be retained. Mix densitities front/rear to achieve desired handling balance (usually one step stiffer rear vs. front when using a large front bar). May also be used on Penske/Koni 28-series shocks to prevent tire/fender contact and supplement primary spring rate ala NASCAR/Indycar

  • FCM 76mm high-performance - available in red, white and blue (soft, medium, and hard) densities. For 2006+ Miatas, 76 red may be used as a OE replacement bump stops. For custom applications stops may be cut to varying lengths

  • FCM 12mm bump rubber - very stiff engagement, >600 lb/in, only recommended as a suspension limiter on a trailered autocross car. For track or street use, a 36mm or trimmed 76mm Red/White/Blue are best.


Shock absorber application guide:
  • Bilstein R-pkg - original-equipment on the '93LE and '94-97 R-pkg, these are digressive, monotube shocks. Digressive means the forces decrease (or digress) at higher velocities. Digressive forces are important to prevent either rebound or compression forces from upsetting the chassis, either with too much compression (wheel jumps up over bumps) or too much rebound (wheel pulls down over bumps). The factory R-pkg shocks can be used with up to 450/350 lb/in springs as a budget race setup. A coarse revalve from Bilstein (usually only one or two internal shims changed) may be performed at a cost of ~$65 per shock. In the past (pre 2010) we offered revalve services on R-pkg/LE shocks but have discontinued this service due to the time/material needed to update the parts to current standards. See Bilstein HD for more information.
  • Background: very strong low-speed rebound damping and very weak low-speed compression
  • Usage: Best suited to smooth roads or stiffer springs. Users have paired with springs from 300/220 to 700/325 (Spec Miata rates). Our suggestion is in the range 400/300- 550/350. These truly come alive with an FCM revalve!
  • Revalve/rebuild? Can be revalved by Bilstein to achieve your goals. We no longer offer services on R-pkg/93LE shocks due to volume of orders for custom coilover packages using new Bilstein HDs.

  • Bilstein HD, 90-97 Miata - aftermarket, German-made, intended for 90-97 Miatas, may be adapted to 99-05 via FCM mounts
  • Background: soft low-speed rebound and compression, although rebound increases very quickly. very strong low-speed rebound damping and very weak low-speed compression
  • Usage: The valving was designed in Germany for German roads (which are smoother than most in the United States). Best suited to smooth roads or stiffer springs, up to ~450 lb/in front, 350 lb/in rear
  • Revalve/rebuild? Can accept a very low-mileage 90-97 Bilstein HD but prefer to start with brand-new. You achieve better performance and longevity if we revalve new HDs whether shock-only or as part of a complete coilover package. Revalve cost is $150 per shock and we shape the entire damping curve based on vehicle + passenger weights, driving style and your typical roads. Bilstein revalves are less refined than what we perform based on our modeling and detailed consulation process

  • FCM Elite shock or coilover using Bilstein body - based on brand-new aftermarket Bilstein HD shocks (for 90-05 Miatas). 2006+ Miatas we can use either new aftermarket Bilstein HD or an OE "crimp top" (non-serviceable) Bilstein that we can have converted to a serviceable unit. These shocks may be supplied individually or built into a coilover. For 2006+ Miatas and 2004+ RX8s, the factory shock mounts are re-used but new MCU bushings are provided. Valving is tailored to the customer's needs. A thorough phone interview is performed to determine optimal ride heights, spring rates, recommended front and/or rear sway bar, and alignment settings. No two customers are the same and one-size does not fit all
  • Background: Based on the proven Bilstein monotube design. Our complete coil-over kits (not including the single or double-adjustable damping versions) carry a MONEY-BACK GUARANTEE
  • Usage: Can be built to any specs for any application, from street to autocross to high-speed road racing. May include single-adjustable (double-acting rebound/compression), double-adjustable (independent rebound/compression), with external reservoir
  • Revalve/rebuild? Yes. Can easily accommodate +/- 100 lb/in change in spring rate before any revalving is needed.
  • Dyno graph: each shock is tuned specific to each customer's needs. Dyno curves are only provided with adjustable shocks to show full range.

  • Bilstein HD, 06+ MX-5 Miata, 04+ RX-8
  • Background: decent amounts of bump and rebound at mid and high-speed but overdamped at low-speed. A firmer ride than is needed but decent choice for tighter handling than stock or with mild lowering springs. Not recommended for coilovers without an FCM revalve.
  • Usage: The valving was designed in America so the profiles are less harsh than the 90-97 Bilstein HDs.
  • Revalve/rebuild? Can accept a very low-mileage 06+ MX5 or 04+ RX-8 Bilstein HDs but prefer to start with brand-new. You achieve better performance and longevity if we revalve new HDs whether shock-only or as part of a complete coilover package. Revalve cost is $150 per shock and we shape the entire damping curve based on vehicle + passenger weights, driving style and your typical roads. Bilstein revalves are less refined than what we perform based on our modeling and detailed consulation process

  • KYB GR2 - aftermarket, Japanese-made, twin-tube digressive shock. Non-adjustable.
  • Background: not having measured one, we can only comment on user reports and marketing. It seems to be geared as a stock replacement.
  • Usage: Stock springs only, not intended for lowering springs or performance applications.
  • Revalve/rebuild? Not serviceable.
  • Dyno graph: None

  • KYB AGX - aftermarket, Japanese-made, twin-tube digressive shock with a single double-acting (rebound/compression) low-speed bleed knob
  • Background: rebound is 3x stronger than compression throughout the shocks operating range. This causes jacking down and a rough ride over bumpy roads, though not as much as Bilstein R-pkg or HD.
  • Usage: The damping is not very strong and our calculations indicate a maximum front spring of ~230 lb/in. These paired with stock springs or Tein S-tech would be the best compromise. Note that ride quality will be less than Koni Sport (on softer settings), Tokico Illumina (also on softer settings), or an FCM-valved Bilstein.
  • Revalve/rebuild? Not serviceable.

  • Koni Sport (also Koni Yellow) - aftermarket, Dutch-made, twin-tube digressive shock with a single double-acting (rebound/compression) low-speed bleed knob
  • Background: designed as a stock-replacement, this shock works quite well with a stock spring except for having insufficient mid- and high-speed compression to prevent jacking. On stiffer rebound settings the shock is often used to 'control' stiffer springs. However, the control being exerted is only in the rebound direction so the result is jacking down, creating a loss of grip, unpredictable handling in the wet and reduced ride comfort. We used these shocks for 10 years and they served us well until we discovered monotube technology via Bilsteins and haven't looked back.
  • Usage: For touring and sporty driving, we find 300 lb/in front, 220 lb/in rear springs is a good complement to the Koni damping. Higher rates are typically used for STS2, CSP and more dedicated track cars.
  • Revalve/rebuild? At an authorized Koni center (no, we do not revalve Konis).

  • Tokico Illumina - aftermarket, Japanese-made, twin-tube digressive shock with a thru-rod double-acting bleed adjuster affecting rebound and compression simultaneously
  • Background: The Illumina has stock-like damping at the softer settings and since compression increases with rebound, it is able to prevent jacking down at higher settings. A downside is that since it is a twin-tube, there is a practical limit to how much compression can be generated via piston valve without causing hysteresis (non-ideal response).
  • Usage: Stock springs on softer settings (e.g. 2f/1r) up to 450/300 springs (Ground Control/Illumina coil-over kit).
  • Revalve/rebuild? No


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