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Truing BRM rubber tires (any shore)

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  • Truing BRM rubber tires (any shore)

    Gather a few things- your car, a clean single-cut flat file, something to electrically isolate it from the track (it gets laid across the power rails), and a wire brush (to clean the file if needed).

    Single-cut Flat File Close Up (yes, it's seen better days but it still works):

    What to do:

    1 - Lay the insulation (in my case, a sheet of wet/dry 400 grit) across the rails.

    2 - Place the file on the paper, perpendicular to the slot, within reach of the rear tires when the guide is in the slot. Note in the above photo that the file's teeth only cut in one direction- when pushed away from the user. You will need to orient the file on the track so the teeth cut into the tire as it spins, otherwise you're in for a long night. On a "normal" file, if you place the file on the track as you'd hold it if using it, the car will need to be pointed to the left. If that isn't convenient, don't bother switching polarity, just place the handle end of the file away from you.

    3 - Place the chassis on the track, with the guide in the slot and the rear tires approximately centered on the file (as shown in the photo). I personally recommend using the chassis only as body rock will allow vibration and make truing the tires a fruitless and frustrating endeavor. It's also reasonably imperative to ensure your bearings are slop-free as well.

    Now comes the "fun" part...

    4 - Holding the file down, the chassis up, and the controller in your hand, squeeze the trigger to get the wheels spinning and gently lower the tires onto the file. Watch that the file doesn't get shot out from under the tires. It's awkward at first. In the above photo, my left hand is doing what it normally does but I am taking the pic with my right. Normally it'd be holding the controller, squeezing the trigger and securing the opposite end of the file. If I applied throttle as is, the file would jet...

    5 -Maintain a light but consistent pressure across both tires to keep their diameters even. Let the file do the work, not the pressure.

    As you work the tires, you will see the surface change as it's cut. The high areas get distressed and the low areas retain their moulded surface finish. Check them as you work them but you will feel, hear and see the changes as they get rounder and rounder. They will go from vibrating to smooth. The cutting action at the high spot on this tire is visible and you can see the shoulder is yet to be cut:

    The Shore 30 tires throw off a fine dust of rubber. The Shore 15's and 22's will throw off chunks if you push them. Take your time, don't over heat them, let the file do the work. It is well worth the results!

    I have done this with all BRM's compounds and all their cars which I own- 88C, 962 and 917. I have used the file with NSR and Slot.It tires as well- the Super/Ultra Grips and the F15/22's and the N22's as well. The file doesn't care whose rubber it's cutting, it works... Give it a shot. I forget exactly how much but I think the files were <$10 at Home Depot.

    I think those were all my tricks. Round tires make a huge difference!


  • #2
    There you go

    See 90 degrees I told you!!

    Chris you are spot on, cut the tires slowly, get them round, then buff them at high rpm with almost zero pressure.

    You will end up with an extremely tacky tire with very good grip.

    Take care to cut the tire slowly, and once you have it round and clean of marbles, then buff with 2000 grit at higher rpm, this creates the finish surface which will be tacky and very grippy. Take care to rub off the marbles or they will stick back to the tire creating a lump, not good.
    Do not spin up so high the tire distorts or you will not create a round tire!!

    We usually clean the tires with a wet thumb.
    Alan Smith
    SCI Owner.



    • #3
      Originally posted by Scaleracing View Post
      then buff them at high rpm with almost zero pressure.
      I add a bit of oil to the paper for polishing, no pressure needed just the weight of the car. Water could be used I guess but..

      I will have to try the file method. The only time one can use the word ******* without any pangs of impropriety.

      TIP: If one cannot find anything greater than 1000 grit simply rub two pieces together to get a higher grit.


      • #4
        OMG.! now, this is something NEW i learned Today!! Thank you!


        • #5
          Nice article Chris. You needed four hands!

          Mike, If you are racing to Alan's BRM rules you might not want to use oil in the polishing prossess as that could be considered as chemicaly treating the tires. Water works well.