FM-A Chassis Tuning Guide
FM-A is one of Tamiya's front-motor Mini 4WD chassis platforms, and it tunes differently from the mid-ship MA/MS/ME family and rear-motor chassis like VZ or AR: the motor sits ahead of the wheelbase centre, so weight bias, jump attitude and damper placement all follow from that layout. This guide covers FM-A's published dimensions, why front-motor layout changes what you tune first, which motor and mass-damper rules apply, and how to set weight distribution against the class minimum-weight rule. Treat every rule below as a snapshot — organiser rules and on-day technical inspection are final, so confirm against current official regulations before an event.
FM-A chassis specifications
Tamiya's Chassis & Compatible Parts Catalogue lists FM-A as a front-motor layout, 150mm long and 97mm wide, with an 83mm wheelbase and 60mm tread. Weight is published as 71g, with 2.5mm ground clearance. Tamiya does not state what that 71g includes, and it cannot be a complete machine — the race regulations set a 90g minimum with motor and batteries fitted — so read it as the chassis unit, not a race weight. It also carries a low-friction front skid bar, four rollers including 8mm-thick rear rollers as standard, and "aircraft-inspired" aero side stays sized for Grade-Up option parts without extra machining.
| Spec | FM-A |
|---|---|
| Motor position | Front (ahead of wheelbase centre) |
| Length | 150mm |
| Width | 97mm |
| Wheelbase | 83mm |
| Tread | 60mm |
| Weight (as published) | 71g — chassis unit, not a race weight |
| Ground clearance | 2.5mm |
The catalogue's other current chassis are published on a different basis, so their numbers are not comparable with FM-A's: MA 125.9g, MS 124.7g, AR 123.5g and VZ 108.3g are complete-machine figures — the ME entry spells this out, quoting 117g “when using low-profile tires and alkaline batteries” — while FM-A's 71g is not. Set two assembled cars on the same scale rather than subtracting one published figure from another. Its 83mm wheelbase matches Super FM, its front-motor sibling — useful context when moving a setup between the two rather than starting from a mid-ship platform.
Why a front-motor layout changes your setup priorities
On a mid-ship chassis the motor sits close to the car's centre of mass, so pitch under acceleration, braking and jump landings stays relatively balanced front-to-rear. On FM-A the motor mass leads the front axle, so the nose carries more of the car's weight before a single option part is added. That gives one clear upside on elevation-heavy courses: a front-weighted car tends to hold a flatter, more predictable attitude on the way up, because the heavy end is already leading. The downside shows on landing — the same front bias means the nose plants hard and digs rather than skipping cleanly into the next section, which is the single most common complaint from FM-A builders and the problem the rest of this guide works toward. It is a mechanical consequence of where the mass sits, not a defect, so the fix is in setup order below, not in fighting the layout.
Motor rules that actually apply to FM-A
Tamiya's regulations restrict ME, MA and MS chassis to the PRO motor family — those three "may only use the kit-included normal motor or motors designed for use with the Mini 4WD PRO series." FM-A is not in that list: it is a standard single-shaft chassis, taking the standard single-shaft motors — the kit motor through the Tuned and Dash tiers, within the permitted list — driven through a propeller shaft, as covered in the motor guide. Disassembling a motor to alter its coil count is prohibited regardless of chassis. One note on any chassis swap: moving a proven motor onto a fresh FM-A, re-run break-in rather than assuming old numbers transfer — gearbox mesh and propeller-shaft alignment are new, even if the motor is not.
Fitting dampers and mass dampers around the front motor housing
The front motor housing occupies the space a rear-motor chassis would leave clear for a conventional front lantern-style damper arm — the layout problem every FM-A tuner eventually runs into. The rules have gotten more permissive here: current regulations state "the installation positions of rotating mass dampers are no longer limited," so there is no rulebook reason to avoid an unconventional mounting point — the constraint is physical clearance against the motor and gearbox, not the rulebook. What you're allowed to do to the chassis to create that clearance still depends on class. General race regulations permit "cutting away plastic material or cutting holes out of the stock chassis," while Stock Class rules go further and prohibit "cutting and shaving down of the chassis" entirely, alongside a ban on homemade chassis in both. Practically: start with bolt-on brake and damper adjustments and stay-mounted geometry before reaching for a fabricated arm, both to stay inside Stock Class limits and because a bolt-on change is faster to test and reverse. For stay material, see the FRP and carbon stays guide; for roller height, see the roller guide.
Weight distribution and the minimum-weight rule
Official regulations set a minimum total car weight of 90g including motor and battery, maximum dimensions of under 105mm width, under 70mm height and under 165mm length, and a minimum ground clearance of 1mm. A finished FM-A clears that 90g floor comfortably, so ballast on an FM-A is a tuning choice rather than a compliance chore: where it goes is a second, free lever on the nose-heavy handling described above. Weight added toward the rear works against front-digging; weight added forward compounds it — treat it as a deliberate variable, not an afterthought on a free screw hole. The 2.5mm stock clearance gives margin above the 1mm minimum, but recheck it after fitting larger rollers or lowering ride height.
| Rule | Limit |
|---|---|
| Maximum width | Under 105mm |
| Maximum height | Under 70mm |
| Maximum length | Under 165mm |
| Minimum total weight | At least 90g, including motor and battery |
| Minimum ground clearance | At least 1mm |
Frequently asked questions
Does FM-A need a different tuning order from a rear-motor chassis?
The order is the same in principle — drivetrain efficiency first, then roller geometry and brakes, then damping — but weight it toward the landing behaviour discussed above. Because the front carries more mass by default, roller and brake settings that control front pitch on landing pay off before cosmetic changes elsewhere on the car.
Can I run a Dash-series motor in FM-A for an official race?
Yes, subject to the current permitted-motor list — FM-A is a standard single-shaft chassis, not one of the PRO-restricted ME/MA/MS chassis, so it is not limited to PRO motors. The motor guide covers which motors sit on the permitted list versus which are excluded from tournament play. Always check the current official rules and technical inspection checklist before entering, since permitted lists can change.
Why does my FM-A dig its nose after every jump?
Almost always because front-biased weight (see above) lands with too little damping resistance and too much unsprung roller travel. Work through stay rigidity, roller height symmetry and brake sponge contact before assuming the fix is removing weight — that can just as easily push the car under the class's minimum-weight rule.