Mini-Z Motor Mount Positions: RM vs MM and How Wheelbase Actually Changes

You bought a body you love, it says 98mm on the listing, and your car is 94mm. Or you moved from a low-grip RCP club to a high-grip carpet track and your car suddenly wants to swap ends everywhere. Both of those problems land in the same place: the motor mount. On a RWD Mini-Z, that one part decides where the motor’s weight sits and how long the car is, and most people buy a chassis without ever being told that.

This guide covers what RM and MM actually change, how wheelbase works on the MR-03 and MR-04, and what else has to come apart when you change either one. If you are still choosing a chassis, the MR-03 vs MR-04 buyer guide is the better starting point.

RM and MM: Where the Motor Sits

The letters describe motor position relative to the rear axle.

RM (rear motor) hangs the motor behind the rear axle. It is the classic Mini-Z layout and it is what most ReadySets ship. Having that mass behind the axle loads the rear tires, which generally means more rear bite and a car that rotates willingly when you get on the throttle. On RCP and other lower-grip surfaces, that is usually what you want.

MM (mid motor) moves the motor ahead of the rear axle, closer to the center of the chassis. Weight shifts forward, the front end has more to work with, and the car feels calmer at the rear. On high-grip carpet, where a rear-heavy car can push the front and then snap, MM is the configuration most of the fast local cars end up in. The MR-03 EVO family ships MM for exactly this reason.

You will also see LM and HM in older listings and on third-party chassis from Atomic and GL Racing. Those describe further variations in motor height and placement, and they are worth understanding if you end up on one of those chassis, but they are not a decision you have to make to run a Kyosho car well. RM and MM are the two that matter for most racers.

Neither is faster in the abstract. They are answers to different grip levels, and the honest version is that your track surface and your tires decide which one suits you more than any spec comparison will.

Wheelbase Is a Part, Not a Setting

Here is the thing nobody tells new buyers: on the RWD platforms, the motor mount is what locates the rear axle relative to the chassis, so the mount length is your wheelbase. There is no adjustment screw. Going from 94mm to 98mm means buying the mount in the length you want.

The common lengths you will see listed are 94mm, 98mm, and 102mm, with wide-track variants alongside them. The body compatibility guide has the chart of which shells run which lengths, and that is the chart to read before you order anything, because the body is the constraint. A shell is cut for one wheelbase. The chassis is the flexible part.

Wheelbase also changes how the car behaves, though less dramatically than people expect at this scale. A longer wheelbase is generally steadier in fast sweepers and lazier in tight switchbacks. A shorter one turns in harder and punishes sloppy throttle. If you are choosing purely on handling and not on a body you already own, run what the layout at your club rewards.

One thing wheelbase does not change: track width. That is wheel offset, and it is a separate purchase covered in the wheel offset and width guide.

What Else Has To Change When You Swap

You have the mount in the cart and you are about to check out. This is the part where people get a chassis stuck on the bench for a week waiting on a $12 part.

The T-plate has to match. The plate bolts to the motor mount, and RM and MM mounts locate it differently. An RM plate on an MM mount is not a tight fit, it is a wrong part. Order the plate at the same time, in the same configuration. The T-plate setup guide covers which flex rating to pick once you know which flavor you need.

Gear mesh resets to zero. Moving the mount moves the pinion relative to the spur, so whatever mesh you had dialed in is gone. Set it fresh, and if the term means nothing to you yet, the gear mesh guide walks through it. Skipping this is how a mount swap turns into a stripped spur.

Your setup sheet numbers are no longer comparable. Static weight distribution just changed, which means ride height, droop, and spring behavior all shifted with it. Reset to baseline and re-measure rather than carrying old numbers across. How to read a setup sheet covers the fields worth re-recording.

Balance may want brass. An RM to MM conversion in particular can leave the rear light enough that exit traction suffers on low-grip surfaces. A couple of grams in the right place is cheaper than a tire experiment; the chassis weight tuning guide covers where it goes.

Parts sharing is not guaranteed across platforms. Kyosho lists some RWD motor sets as fitting both MR-03 and MR-04, but a shared motor mount does not establish shared gear compatibility, and Kyosho’s spur (MZW705-1) is listed for both the MR-03 and MR-04, with the 6T-9T MZ6BK pinions in the MR-04 parts lists too. That’s Kyosho’s own parts, though. Check the vendor’s stated fitment for the specific part, not the pattern in the part number.

When a Conversion Is Actually the Answer

Most of the time it is not. If the car is loose on exit and you have not touched tires, springs, or T-plate flex, those are cheaper and faster levers, and the diminishing returns trap argues that case in detail. A layout change is the right call in a narrower set of cases:

• You moved to a meaningfully higher-grip surface and the car pushes into the corner then snaps at the rear, with correct tires already on it.
• You bought a body at a different wheelbase and you want to run it.
• Your class rules specify a configuration you are not currently in.
• You have run the car long enough to describe what it does wrong in one sentence, and the answer to that sentence is weight distribution.

If none of those apply, spend the money on tires.

What to Buy

The mount itself is the cheap part. The plate and the tools around it are where the real cost sits.

• Motor mounts are sold by wheelbase and configuration, and specific SKUs rotate in and out of stock constantly. Match the length and the RM/MM flavor to your chassis: Mini-Z RWD motor mounts on Amazon. Verify the listing names your platform before ordering.
• Kyosho MZW404S carbon rear suspension plate for MR-03: → on Amazon. Consistent flex and it fits stock geometry without drama.
• Yeah Racing graphite T-plate for MR-03 and MR-04 MM chassis: → on Amazon. Lighter than carbon, same predictable behavior.
• A digital caliper, because “it says 94mm on the box” is not a measurement: → iGaging 6” digital caliper on Amazon.
• A bearing kit while the rear end is apart. MR-03: → FastEddy sealed bearing kit on Amazon. MR-04: → FastEddy kit for MR-04EVO2 on Amazon. You will never have easier access than right now.
• Brass weight if the conversion leaves the rear light: → brass chassis weights on Amazon.
• A proper driver set, if you are still using the wrench from the box: → Yeah Racing Mini-Z tool set on Amazon.

Budget somewhere in the $40 to $70 range for a mount, a matching plate, and fresh bearings. That is a real purchase, which is the argument for being sure the layout is your actual problem before you make it.

The Short Version

RM is rear-biased and forgiving on low grip. MM is balanced and calmer on high grip. The mount length is your wheelbase and the body decides which length you can run. And whichever way you go, the T-plate, the gear mesh, and your setup sheet all come along for the ride. Plan the whole swap before you order the first part, not after the car is in pieces.