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Thread: Rim tolerances

  1. #1
    Join Date
    Jun 2002
    Paonia, Colorado

    Rim tolerances


    We were discussing this at the race at Lou Pirro's last weekend:

    Rear wheel rims are made to variable tolerances. By my measurement, Mosetti and Speedshop rims are pretty uniform at around 0.0010 to 0.0020" when measured on the axle and about 1/16" in from the outside edge of the wheel. Others are more variable, and the little plastic 1/32 rims are way off. And occasional used rims measure way off regardless of manufacturer, presumably due to crashes. Interestingly, some of the rims that are way off look just fine on simple inspection.

    On one hand this would seem to be important in terms of balance. On the other, the tire truer assures that the wheel is round in the end. How important is rim trueness in actual racing?

    Thanks as always!


  2. #2
    Join Date
    Jun 2007
    The Czecks and Brian Saunders are now using carbon fibre hubs at great expense


  3. #3
    Join Date
    Jun 2002
    I would say the Speedshop rims and tire combo are the truest out of the box.

    I have a pair of the Cahosa Czech carbon fiber rims but I don't see any advantage to them. Other than expensive 4.50 euros (6.50 US) plus shipping. Theroetically they won't collapse or bend (as easy). I would think the tolerences on the CF would be harder to balance out since they are molded not trued (and most have inserts). The molded inserts is thier weak point in a crash as well as throwing off tolerences.The pair I got when put on the dial, was slightly out of true/tolerence but that could have been a fluke. It was the outer rim portion opposite the setscrew. I don't think they can be trued to establsh trueness as CF shouldn't be cut sanded or phazed. Thier truness depends on how carfully the insert is molded into the rim CF. I also understand Mossetii has some CF rims that I haven't tried.

    I will go on a bit of a tangent here. On the other hand I do weight balance my gear, axle and tires on free float spinner then in the chassis itself. A bit overkill but it adds overall smoothness which I prefer. Vibration is your enemy. I got in the habit of replacing the rear axle bearing after a race or two. The tires are dependent more on the trueness of the whole cut of the tire/rim in general as well as in combination of the axle and gear.

    Last edited by Slapshot; 09-28-2008 at 11:26 PM.

  4. #4
    Join Date
    Jun 2002

    How Round is My Rim?

    Hi Greg,

    What you are looking for in a rim is lightness. We want to reduce rolling weight as much as possible. That is why I use Mag instead of aluminum. Mag costs more but that isn't a concern when you want to go fast. I think you will find that the carbon fiber rims are no lighter than Mag so I can see no advantage except that they would not bend in an accident. As long as the rims is designed correctly bending is not a problem. If you rim is light then a little out of round is not a problem. Of course, the rounder the better but we don't live in a perfect world. The other problem is the set screw weight. Speedshop uses a hollow set screw for to reasons. They hold better and are lighter weight. At the speeds our slot car tires rotate a little out of balance is no big problem.

    Another important factor stability. Your rim needs to be stable with no flex to give you consistent traction which leads to good handling. Today you will see plastic rims on the market. Plastic is not stable enough and will tend to give or flex in turns. My testing has shown that plastic rims are less forgiving than metal.

    More important is rim design. The rim diameter makes a big difference in how the tire performs. Speedshop makes both a .400 dia rim and a .490 rim. The bigger rim has more bite. Some cars like flexi cars need all the bite you can get so we always use the big rim. For other cars, Indy, GT 12, Open 12, C 12 and Euros we use the 400 rim. This rim is more forgiving with a little less bite. For 2mm axles Speedshop builds the spacer into the rim to protect the axle.

    As you can see there is more that a few factors when it comes to rims. The only way to find out what works best for you is testing.


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