Mini 4WD AT Bumpers and Slide Dampers
A stock Mini 4WD chassis has no suspension in the RC sense — the plate is bolted directly to the roller stays, so every fence gap and wall clip transmits straight into the roller mounts and, from there, into the car's line. Tamiya answers that with two official Grade-Up part families built to move on impact: Wide Sliding Dampers and Mass Damper Sets. Separately, racers have long built a home-made pivoting bumper — usually called an "AT bumper," short for Auto Track Bumper — from ordinary stays, a single pivot screw and a spring, rather than buying it as a kit. This article covers how each mechanism absorbs a hit, the official parts that do the job, how a scratch-built pivot bumper differs, and what the Stock Class regulations say about which moving parts are actually permitted.
How a sliding damper absorbs a hit
Tamiya's Wide Sliding Dampers mount the roller assembly on a 2mm aluminium plate that slides against spring pressure instead of sitting rigid against the chassis. Low-friction POM resin bushings carry that plate so it returns to centre quickly, and the roller itself is a 17mm tapered type shaped to climb back onto the track surface if the car rides up a fence lip instead of catching on it. Each set ships with two spring rates — hard and soft — so resistance can match how rough or fast the section is: soft where you want the plate moving easily on frequent, low-speed contact; hard where you want it absorbing a fast hit without going slack.
Tamiya's official sliding and mass damper parts
These are the two families the regulations actually name as permitted moving assemblies — everything else on the car is expected to be fixed.
| Part | Item # | Position | Key spec | Listed chassis fit |
|---|---|---|---|---|
| Wide Front Sliding Damper | 15469 | Front | 2mm aluminium plate, POM bushings, 17mm tapered rollers, hard/soft springs | MA, AR, Super XX |
| Wide Rear Sliding Damper | 15467 | Rear | Same plate/bushing/roller/spring spec as the front version | MA, AR, MS, Super-II |
| Mass Damper Set (w/ ball connectors) | 15478 | Anywhere, via a stay | 6×6×32mm block weight on a ball-connector mount, fibreglass stay | AR, MA direct; MS, VS, Super-II, Super XX, Super X, Super TZ-X with a separate stay |
Sliding dampers absorb lateral wall contact by letting the roller plate travel. A mass damper works on a different axis: the block weight hangs on a ball joint and lags behind the chassis on a jump landing, so its inertia soaks up vertical shock instead of horizontal impact. Fit both when a course mixes wall-heavy corners with elevation changes — one does not substitute for the other.
Building a pivot ("AT") bumper
An AT bumper is not a boxed Tamiya part — it is a builder's technique for making a front or rear bumper stay rotate instead of sit rigid. The usual method replaces the normal two-screw rigid mount with a single pivot screw, so the plate can swing on that one point, and adds a spring or slim foam pad between the plate and the chassis to return it to centre and limit its travel. Some builds add a second, non-pivoting screw set slightly proud as a stopper, so the plate rocks but does not flop. The goal matches a sliding damper's — keep the roller in contact with the wall through a glancing hit instead of letting the impact deflect the whole chassis — but the spring rate, pivot friction and travel limit are whatever the builder sets, not a published spec. That makes it powerful for a specific, already-diagnosed handling problem and unpredictable as a blind copy: the same screw-and-spring recipe behaves differently depending on spring wire gauge, pivot-screw torque, and where the pivot sits relative to the roller.
What the rules actually allow before you build one
This is worth checking before committing a pivot bumper to a race car. The official Stock Class regulations state plainly that the only parts allowed to move are wheels and tyres, rollers, and the components inside mass dampers and sliding dampers — everything else that can contact the course wall is expected to be securely fixed to the chassis or a plate. "AT bumper" and "pivot bumper" are not categories Tamiya's regulation pages name at all; they are community and BMAX-scene terminology for a mounting technique, not a listed official part class. That leaves a genuine grey area: whether a given build reads to an inspector as an acceptable sliding-damper implementation or as an unfixed part depends on how it is built and on the event's own inspection, not on a general rule this article can settle. BMAX groups run rulesets defined by their own organiser, separate from Tamiya's Stock Class page, so a bumper accepted at one meet is not automatically accepted at another. Confirm against the official Mini 4WD race regulations and the Stock Class regulations, and read the event's own rules before entering — see the rules and technical inspection checklist for how that process runs in practice.
Frequently asked questions
Is an AT bumper the same thing as a Tamiya sliding damper?
No. A sliding damper is an official Grade-Up part — 15469 or 15467, for example — sold with a fixed spec sheet and a published chassis compatibility list. An AT bumper is a scratch-built pivot mount using ordinary stays, a screw and a spring, with no fixed spec at all. Both aim to keep the roller in contact with the wall through an impact, but only the sliding-damper family is a named part in Tamiya's own regulations.
Should I fit a sliding damper or a mass damper first?
Match the part to the impact. Sliding dampers address lateral wall contact — fence gaps, corner clips, chicanes. Mass dampers address vertical landing shock after a jump. If your car only struggles on jump sections, a mass damper set does more for you than a sliding damper, and vice versa; fit whichever matches the specific section you can point to on the track.
My roller assembly is loose or uneven — will an AT bumper fix that?
No. A moving bumper adds controlled compliance on top of a sound baseline; it does not correct one that is already wrong. Confirm roller symmetry, thrust angle and mounting stiffness first — the roller guide and stays guide cover that baseline — then add one damper system at a time and retest the same section before judging whether it helped.
For the vertical-shock half of this pairing in more depth, see the mass damper guide.