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Spec Miata Rules Interpretation: Bump Stops

Read this rule and see if you understand it on the first pass:

Maybe again, slower:

"All cars may use the Fat Cat Motorsports bump stop kit (p/n FCM-MT-KIT-SM) or the unmodified Mazdaspeed bump stop (p/n 0000-04-5993AW) in conjunction with the 1999-up stock upper mount assembly consisting of the upper mount (p/n: NC10-28-340C), the upper mount bushing (p/n: NC10- 28-776) and the upper mount washer (p/n: NC10-28-774), and shock body spacer over the shock shaft (p/n 1234-56- 789-AW). All other OEM upper mounting hardware shall be discarded. Non-OEM equivalents may be used in place of the upper mount, upper mount bushing, and upper mount washer only. No other modifications are allowed."

Now see if you understand it better after I re-wrote it:

"All cars may use the Fat Cat Motorsports bump stop kit (p/n FCM-MT-KIT-SM). Alternatively, cars originally built to the earlier rulesets may also use an unmodified Mazdaspeed bump stop (p/n 0000-04-5993AW) in conjunction with the original stock upper mount assembly hardware from the 1999-2005 Miata, which consists of: the upper mount (p/n: NC10-28-340C), the upper mount bushing (p/n: NC10- 28-776), the upper mount washer (p/n: NC10-28-774), and shock body spacer over the shock shaft (p/n 1234-56- 789-AW). All other OEM upper mounting hardware shall be discarded. Non-OEM equivalents of similar construction and material may be used in place of the upper mount, upper mount bushing, and upper mount washer only. No other modifications are allowed."

Why the rules still specify the use of the the Mazdaspeed bump stop is senseless other than the fact that there might be cars built to the original rules still racing and the organizations don't want to force people to have to upgrade to far superior Fat Cat Motorsports kit. These bump stops were the original specified bump stops until the Fat Cat kit was allowed. They were designed to work with the OEM 90-97 Miata shock hats and do not interface well with the 99-05 Miata shock hats, which causes them to pivot within the hat and separate, eventually destroying themselves. The Fat Cat Kit was specified to eliminate this issue, and as a way to allow the 90-97 cars to be on an equal suspension/shock assembly to the 99-05 cars. The Fat Cat Kit is also a marked improvement over the Mazdaspeed bump stop in both gain in shock travel (36mm bump stop is shorter) and handling (ability to lower the car more without being on bump stops, and superior micro-cellular foam (MCU from hereforth) bump stop vs. rubber and aluminum bump stop that will separate). Why anyone would still be on the original Mazdaspeed bump stop is beyond me, and I question on whether or not they're even still available through Mazdaspeed Motorsports Development.

Neverminding the issue above, the rule as written is confusing regarding the Mazdaspeed bump stop. Basically it is saying you can use the Mazdaspeed bump stop on ANY car, but if you do, you also have to use the 99-05 Miata shock hats (which don't interface correctly with the Mazdaspeed bump stops) along with the 99-05 Miata OEM top washer, and 99-05 Miata OEM rubber mount bushing between the top washer and shock hat base. What about the cars that were built to this spec originally using the OEM 90-97 shock hats?! Do they have to upgrade to at least the 99-05 mount and oem washer and bushing? Can they not use the original 90-97 shock hat anymore? If that's the case, the rule is senseless because they might as well just use the Fat Cat Kit!

With that cleared up (maybe), we now get into the knock-off or home-made Fat Cat kits that exist and find their way onto Spec Miatas across the country.

The Fat Cat kit is clever, as it was an evolution and improvement of the 99-05 Miata shock mounting solution for cars using lowered suspensions, like Spec Miatas. It replaces ALL of the 99-05 Miatas OEM shock mounting bits save for the shock hat. More speficially: the rubber upper and lower shock mount bushings are replaced by precision cut, firmer spring rate MCU bushings. The bump stops are 36mm and are of the same material as the upper and lower bushings. They replace the original 56mm OEM soft bump stops. Not only does this provide a more race-ready bump stop designed to handle the abuse of a racecar, it also is shorter than the 56mm OEM piece, which gives you approximately 20mm more shock travel. This might not seem to be too much to the daily driver, but to a racecar driver that can now lower their car another 20mm and still have the same amount of shock travel, it's HUGE! Along with the superior bump stops, the Fat Cat kit also uses proprietary shoulder nuts that ensure proper engagement to the shorter shock shaft of the 90-97 Miata Bilstein shocks. Add in the appropriately sized steel upper washer and lightweight aluminum lower washer and you have a super nice system using components designed specifically to work with each other, not a frankenstein'd mess of off-the-shelf parts cobbled together to "work" with each other.

When looking at the Fat Cat kit, it's enticing to say "I can make one of those and save a ton of money.". I've been there before, and tried. Wasn't as easy as it looked and I abandoned the effort. Why?

  1. Cutting bump stops: It's common knowledge that a Fat Cat Kit uses cut sections of Speedthane bump stops, right? Not really. The Speedthane MCU parts in the Fat Cat Kits are indeed Speedthane bump stops originally, but not in their final form. They are actually precision cut by Speedthane to Fat Cat specs, and not publically available. We know this because we (5X Racing) are the only ones in the world besides Fat Cat that is an authorized purchaser of them. When we tried to cut the 76mm Speedthane bump stops to the same specs as the Fat Cat kit, it was a disaster and a mess. We were left with a bunch of uneven pieces of bump stops destined for the trash can and out around $100. 
    1. Knock-off kits try to imitate the precision cut bump stop pieces, but end up with a sloppy product and are most likely not even using the right density of MCU bump stop
  2. Sourcing shoulder nuts: Not an off-the-shelf product, so good luck trying to find the right size shoulder nuts. Toughnutz? Not long enough and will leave you with too little thread engagement. The correct shoulder nuts are custom designed and manufactured for us to very specific tolerances and material.
    1. Knock-off kits? Who knows what they use, I wouldn't trust them.
  3. Sourcing the right size washers (silver and red): Another tale of custom sourced and sized parts. Not an off-the-shelf product
    1. Knock-off kits use generic off-the-shelf parts that are not of the same size

Also, it's not even legal! The rules specify the use of the Fat Cat Motorsports kit and nothing else besides the Mazdaspeed bump stops and OEM 99-05 upper mounting hardware! Where does it say you can make your own kit or use a knock-off? It doesn't, and it's easy to spot to anyone who's ever seen a Fat Cat kit. Compare a knock-off kit to an authentic Fat Cat kit next to each other and you'll see plain as day that it's a cheap representation, and not authentic. Note the rule "Compliance of such parts will be determined by comparison to new parts from the supplier." We can't use sway bars that are not Eibach, so why should we be able to use a knock-off Fat Cat kit? We shouldn't, and you shouldn't either. It's an easy DQ in the tech shed to anyone that's looking to protest you. Be smart people!

  • Posted on   02/14/17 at 12:00:00 AM   by John  | 
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Tagged with Spec Miata rules, Spec Miata bump stops, Fat Cat Motorsports Spec Miata Kit