Mini 4WD Stock Class and B-MAX Setup
Stock Class and BMAX both cap what you can bolt to a Mini 4WD, but they cap different things — and a setup that is legal in one can be a disqualification in the other. This is not a rules explainer; that ground is covered in Stock Class vs BMAX: what you must check. This post is a setup guide: given each ruleset's actual restrictions, which specific parts and adjustments are worth making, and which ones will get a car pulled at inspection.
The baseline every legal build shares
Both classes sit inside Tamiya's official Mini 4WD race regulations: under 105mm wide, under 70mm tall, under 165mm long, at least 1mm of ground clearance, and a minimum all-up weight of 90g including motor and batteries. Tyres must be 22–35mm in diameter and 8–26mm wide, and only Tamiya-branded AA batteries are permitted at an official event. Neither Stock Class nor BMAX loosens any of these — they only add restrictions on top. So the first setup pass, regardless of class, is the same: weigh the finished car on a real scale (not an estimate), measure ride height at all four corners, and check tyre diameter with calipers rather than assuming a fresh Tamiya tyre is exactly on spec.
What Stock Class specifically restricts
Tamiya's own Stock Class regulations (updated 11 May 2026) go well past the general rulebook. Chassis cutting and shaving are not allowed at all, and homemade chassis are banned outright — this is the single biggest difference from open racing, where controlled chassis lightening is permitted. Non-body parts can't be modified except for one narrow exception: existing screw holes in the chassis may be countersunk, to 2mm. Roller combinations are limited to Tamiya's permitted list — you can't mix arbitrary diameters front and rear. HG aluminium setting boards, setting gauges and pinion pullers are explicitly barred from being attached to the car. Battery terminal parts must stay kit-standard or use Grade-Up parts exactly as instructed — soldered terminals and doubled-up terminal contacts are both prohibited. Any additional part you add has to come from the Tamiya Mini 4WD, R/C Mini 4WD, or Dangun Racer catalogues; nothing outside those lines is legal, no matter how well it fits.
What Stock Class does allow: ball bearings, carbon plates, clear or polycarbonate bodies, body shaving and cutting away obstructions on the body itself, brake sponge cutting, and pushing shafts through wheel holes for a tighter fit. Dash-series motors are confirmed legal in the class FAQ. A "forbidden zone" between the front and rear axles restricts what can be mounted there to essentially the body and driver figure — this is where a lot of first-time Stock Class builders get caught out, mounting a mass damper or option part in a space that reads as fine on an open-class car.
Setup choices that hold up under Stock Class inspection
| Setup goal | Stock-legal approach | What to avoid |
|---|---|---|
| Reduce weight | Ball bearings in place of plastic bushings; lighter body trimming within the body itself | Shaving or drilling the chassis — flatly banned, not just discouraged |
| Stiffen roller mounting | Carbon reinforcing plates (e.g. HG Carbon Reinforcing Plate Set, 1.5mm) bolted on as designed | Countersinking anywhere but existing holes; adding a setting board to the car |
| Tune braking | Cutting or shaping the brake sponge itself; adjusting sponge height with kit-legal spacers | Non-Tamiya brake material or a home-fabricated brake stay |
| Improve motor output | Any Dash or Tuned motor from the permitted list | Opening the motor case to change coil winding — explicitly banned in the general regulations |
| Adjust roller stance | Roller sizes and combinations from Tamiya's permitted list only | Mixed diameters outside that list, even if the car handles better |
What BMAX changes, and why it isn't Tamiya's rulebook
BMAX is not a Tamiya-published class — it's an organiser format, most commonly run under community rulesets such as Basic-MAX GP, and its exact restrictions vary by event and by version of that ruleset. That matters for setup because a part or modification legal under Tamiya's own Stock Class is not automatically legal under a given BMAX ruleset, and the reverse is also true. Community BMAX rules have historically leaned toward the same core restriction as Stock Class — no chassis cutting, no homemade chassis, loose or unsecured plate mounting disallowed — but the permitted parts list, roller rules and forbidden-zone details are set by whoever is running that specific event or ruleset revision, not by Tamiya. Before you finalise a BMAX setup, get the exact rules document the organiser is running and check the version date; do not assume last year's sheet still applies, and do not carry a Stock Class part list over to BMAX without checking it against that document.
Practical setup order for either class
Build in this order and you avoid most inspection failures: assemble to kit spec first and confirm weight and dimensions before touching anything else. Fit and align rollers using only combinations your class permits, checking left/right height matches within a millimetre. Set brake height and sponge angle for your track, using only sponge material that's legal under your ruleset. Fit ball bearings and any permitted reinforcing plate last, since those are the parts most likely to have a narrow, specific legality window. Re-weigh and re-measure the finished car — a setup that passed inspection last month can drift out of spec as bearings bed in or a sponge compresses. For general drivetrain and motor selection that applies to both classes, see motor and gear ratio matching and the motor guide.
Frequently asked questions
Can I use the same car for Stock Class and BMAX without changes?
Sometimes, but don't assume it. Both classes ban chassis cutting and homemade chassis, so a clean, unmodified chassis is a safe starting point for either. Where they diverge is the specific permitted-parts list, roller combinations and forbidden-zone rules — those are set separately by Tamiya for Stock Class and by the organiser for BMAX, so the same reinforcing plate or roller pair can be legal in one and not the other.
Does adding ball bearings break Stock Class legality?
No — ball bearings are explicitly permitted under Tamiya's Stock Class regulations. They're one of the few weight- and friction-reducing changes allowed given the ban on chassis cutting and shaving.
Who has the final say if my setup gets flagged?
The event's technical inspector, on the day, using that event's current rules document. This article summarises the published Tamiya Stock Class regulations and general BMAX practice as sourced above, but organiser rules and on-day inspection always prevail — confirm directly with the event before you race.