The Carpet season is almost upon us, and with the hope for more mini's on the rug this year, comes the need for proper setup info.

Enter: Mark Brown and his M-chassis expertise. This write-up is from 2010, but still relevant now.
Have a look, and take notes. You just might have a chance at staying on the same lap as Mark this winter.

-Jesse

M_

Tamiya Mini Carpet Techniques

by Mark Brown[1]

1 December 2010

Introduction

This short document summarizes techniques for making an M03 or M05 Tamiya Mini perform well on carpet.

Setup Items

Here are the aspects of setup that differ significantly between an outdoor and an indoor setup, in approximate priority order.

Tires and Inserts

Tires and inserts are the most important part of setting up your Mini. It can be a lot of work to achieve the ‘perfect’ setup, but getting close is usually easy.

Fronts

Carpet generates a lot of side grip. Running a really grippy front tire, like a Type A slick, on high-grip carpet will give you too much side traction. Your car will traction roll.

You can prevent traction rolling by applying super glue to the outer edge of the front tire, as explained in more detail below. But if you use this technique with slicks, the most likely result is a car that spends a lot of time on two wheels in the turns! That’s not the quickest way around the track.

The usual front tire on high-grip carpet is the M-Grip radial tire (TAM 50684). It gives more grip than the kit tire, but less than the S-Grip radial (TAM 53254) and a lot less than the Type A slick (TAM 53340). On a carpet track with less grip you can go faster running a grippier front tire.

The insert for an M-Grip or S-Grip on the front should be something that does not alter the flat contact patch of the tire. A harder tire like the M-Grip does not need a highly supportive insert to work; the kit foam insert (TAM 53204) can work fine. S-Grips seem to like a small air gap between the tire and insert; this results in a tire that is slightly concave when unloaded, but gives a larger contact patch when loaded. If you have one of the old 55D shaped inserts (TAM 53223), use it; otherwise cut and glue a Tamiya TC insert to make the correct diameter, then fill the gap between the wheel and this insert with a Tamiya foam insert, cut as needed.

Rears

Rears are all about balancing the car, given your car’s setup and especially the fronts.

Most people seem to like Type A slicks in the rear. Most of the time I don’t; they usually nail down the rear of my car too much, and when I get the rear free enough it becomes inconsistent and does not carry good speed.

I generally end up with S-Grips on the rear. No glue on them, unlike the fronts. And a fuller insert than in front; I want something that makes the contact patch a little bit convex, so the grooves in the tire will generate side-bite. Double-stuffing with a cut/glued TC insert on the inside and soft Tamiya foam (TAM 50686) on the outside is one combination that works for me.

Glue

To glue the outside edges of your front tires, first clean the area you’ll be gluing with isopropyl alcohol or motor spray. Use a good quality glue like Losi or ProLine that accepts a micro tube. Cut about 15mm of tube and insert it partway into the opening of the glue bottle. The micro tube will let you put the glue exactly where you want it, which is: from the groove on the sidewall of the tire, up over the edge, and covering the outer ½ of the outermost tread block. I’ll apply glue to about 1/6^th of the tire, then take a toothpick and run it over the glue to be sure it covers the entire area and gets down into the cracks. If I don’t have the micro-tube, I’ll drip glue onto the end of the toothpick and use the toothpick to apply the glue to the tire. This is slow but it ensures that I don’t put glue where it doesn’t belong.

Be sure to let the glue dry completely before running the tires!

Keep an eye on the glue because it does wear down. You’ll probably need to renew it every 3-4 race days.

Sauce

I rarely sauce the full tire on high-traction carpet. On the front I might sauce the inner 2/5 of the tire (two ribs out of five), while on the rear I’m saucing the inner 3/5. Small adjustments in sauce are a great way to achieve the exact handling balance you are looking for.

I like to clean rubber tires with WD-40 between runs. However I only clean the area that I’m saucing that day. If I clean the whole tire then the whole tire will pick up traction with each run, and that can lead to disaster, especially if the front picks up more than the rear. If I decide I want to reduce the amount of tire I’m saucing during the day, I’ll clean the “abandoned” area of the tire with motor spray to reduce its grip. When the race day is done I clean the entire tire with WD-40.

Diff

On carpet you want to run a diff with a lot of internal resistance; this is called a “closed” diff. With an “open” diff, when your Mini is leaned into a turn and unweights the inside wheel, you’ll lose drive from the outside wheel and your car will push. I see a lot of cars on the track that suffer from this.

There is a trade-off: The more closed the diff, the less off-power steering you will have. But on a typical track the Mini spends a lot more time turning on-power than off-power.

I never run a fully locked diff. With a locked diff, if you crash the car and one of the drive wheels stops suddenly, you are likely to break a tooth off of one of the internal gears: game over. It only takes a little diff action to eliminate this problem.

Here’s a way to build the Mini’s gear diff that works well for me on carpet:

Get two containers of Tamiya Anti-Wear grease (TAM 53439). Also get two extra Tamiya 9mm washers (TAM 2300010), in addition to the two that come with the gear diff in your Mini kit.
Glue the two 9mm washers that come with the diff to inside of the plastic case of the diff, where the washers go when the diff is assembled. One washer is glued inside the drive gear portion of the diff case, the other inside the screw-on side portion of the diff case. Let the glue dry to full strength before assembling the diff.
Now assemble the diff as usual, using the two extra Tamiya 9mm washers to replace the two washers your glued to the diff case, resulting in two washers on each side (one glued and one free). As you assemble the diff, cover all internal parts with Tamiya Anti-Wear grease and fill all cavities with Tamiya Anti-Wear grease. The diff will hold more than two full containers of grease, but two gets close enough.
With the diff fully assembled and the case screwed together, seal the seam in the case using a small amount of Shoe-Goo. Let the goo dry fully before running the diff.

Here’s what assembling the diff this way accomplishes:

· Gluing the washers to the inside of the diff case prevents the washers from rubbing on the diff case and wearing through it. This makes the diff last longer and work more consistently.

· Running two washers on each side of the diff, instead of one, helps reduce grease leakage through the “thrust bearings” of the diff, and also adds friction to the diff action.

· Filling the diff case with grease prevents the grease from being thrown off the gears, and thereby maintains a high level of friction in the diff action.

· Sealing the diff case seam with Shoe-Goo eliminates grease leakage through this seam.

The result of building the diff this way will be a diff that is almost locked at first. The first couple of times you run the car the diff will open up a little as the grease migrates around inside the case, then it should stabilize and stay consistent without any maintenance.

I’ve tried filling Mini diffs with heavy silicone fluid (500K cSt) as used in the diffs of gas buggies. The silicone fluid gives a nice heavy diff action, but it leaks more than grease, so I prefer grease.

Ride Height (Camber)

Ride height is a very important adjustment on the M03, because you adjust camber with ride height. Lower ride height gives more camber and vice-versa. Adjustable camber links are now available for the rear of the M03, and for both ends of the M05, but still I adjust camber at both ends with ride height.

I like my M03 to have lots of steering so I run the front end fairly low and the rear end fairly high. You should experiment with ride height and see how the handling of your car changes.

Beware running the car too low in the rear. On the M03 with the plastic rear uprights, running low in the rear can take away too much up-travel and make the rear end break away suddenly in the turns. On the M05 with the receiver and ESC mounted on shelves outside the main chassis, running low in the rear can cause the shelves to drag on the track, taking away grip and possibly causing the rear of the car to hop. You should be able to make the rear end of your car stable without running below 5mm in the rear. Perhaps you are running the wrong rear tires?

Shock Length

Shock length controls droop on the Mini. Running less droop makes the car more responsive and less prone to traction-roll. Running more droop makes the car drive better over bumps and generally more predictable.

On high-grip carpet you want the shocks fairly short. On a lower-grip surface you lengthen the shocks.

TRF TC shocks are long for a Mini and will prevent you from running really small amounts of droop. Even so it is possible to have a good setup with these longer shocks. The shorter TRF M-Chassis shocks (TAM 54000) were not available back in 2003 when I built my M03, and so I still run TRF TC shocks. My car’s shock length on high-grip carpet is about 54.25 mm end-to-end.

Springs

I run stiffer springs on carpet than I run outdoors. Springs are not a major tuning point on the Mini but going stiffer in high-traction conditions generally seems to work better, even if it does accentuate the car’s tendency to traction-roll.

Sample Setups

M03

This is my base carpet setup.

Front tire: 60D Super Grip Radial (53254). Half of outer tread block covered with CA.
Front insert: 55D shaped insert (53223)
Rear tire: 60D Super Grip Radial (53254)
Rear insert: Inner: hard TC insert (53435), cut/glued to fit Mini wheel. Outer: soft sponge (50686)

Front ride height: 6mm (measured at front of chassis)
Rear ride height: 6.5mm (measured at rear of chassis)

Front toe: 0.5 degrees out
Rear toe: 2 degrees in (53345)

Front shock: TRF TC damper (49198) with 3-hole piston, 60wt Associated oil. Length 54.25mm.
Front spring: blue (53632)
Rear shock: Same as front except 40wt Associated oil
Rear spring: yellow (63631)

Wheelbase: Long (BMW Mini Cooper)

Diff: gear diff filled with AW grease (53439)

M05

Troy Crabtree posted this setup on rctech. He ran it successfully at Hangar 30 in Seattle.

Front tire: 60D M-Grip Radial (50684)
Front insert: 55D shaped insert (53223)
Rear tire: 60D Super Grip Radial (53254)
Rear insert: Inner: hard sponge (53255)

Front ride height: 4.5mm (to achieve camber specified below)
Rear ride height: 5mm (to achieve camber specified below)

Front camber link: upper hole (2 degrees camber, fixed link)
Rear camber link: lower hole, (2 degrees camber, adjustable link)
Front toe: 2-3 degrees out. Outboard steering rod end in forward hole of knuckle, shimmed to make steering rods near-level at ride height.
Rear toe: not specified; probably 1.5 degrees

Front shock: TRF M-Chassis damper (54000) with 2-hole piston, 40wt oil, no foam above the bladder, one o-ring under the piston to shorten the shock length.
Front spring: white (53633)
Rear shock: Same as front.
Rear spring: yellow (63631)

Wheelbase: Mid (Suzuki Swift)

Diff: gear diff filled with AW grease (53439)

Conclusion

None of the techniques described here is especially difficult, so give them all a try and you’ll probably end up with a Mini that works really well on carpet. Have fun!

Revision history

1 December 2010. Initial version.

Saturday, August 3, 2013

Mini's on Carpet