Author Topic: RIM CHECKER FROM THE JUNK BOX  (Read 637 times)

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Offline Brooklands

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« on: November 28, 2011, 02:17:07 pm »
The jolly MoT man is likely fail your bike if the rims are out of true by more than 1/8th ", either side to side or out of circle. This simple tool lets you check both quickly and cheaply (or you could scare yourself with a dial gauge and watch the needle spin round and round). Make up the parts shown in the pics (you don't need to mark the lock nut) - if you haven't got any number/letter stamps use a punch or a piece of masking tape around the bolt head. Measure the threads per inch on the bolt.
Let's call it 24 Tpi. So for every 3 full turns of the bolt it moves 1/8" (.125") inwards, and if you turn the bolt just one face it moves only 7thou (accurate enough for most of us).
Clamp the bar to the mudguard stay and wind the bolt in until you find the lowest (inward buckle)  part of the rim ( the rubber nipple cover is to protect your chrome, if you're bothered or even have any). Chalk mark the tyre,  wind the bolt out & rotate rim to find the highest point, & chalk mark this point. Go back to both points and count the number of full & part turns of the bolt from one to the other and you've got the run-out. In this case say 1 full turn & 3 faces would be 41+21 = 61thou, well within limits. You can obviously use the tool to true up your rims to get them back within limits and I have to admit to tweaking my rims in situ with the tyres on and the tubes deflated, with good rim tapes, then taking them off to sort out the spokes & nipple heads, as you only have to re-fit the wheel once. Not best practice but as long as the tube isn't inflated or the bike wheeled on the flat tyre, no harm should be caused.
To check out of circle measurements cut an 1/8th vee in you bar and align with the rim/tyre line and rotate the wheel whilst watching the run out - anything outside of the V is outside of the MoT limit. 
Norfolk 'n good!