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I recently picked up a set of GT mags on Craigslist. I brought them home, pulled them apart, cleaned and re-greased them. Once I reassembled them and put them on my sons bike, I took it for a ride. The rear mag has a horrible hop in it! Both mags are as straight as can be but the rear hop is unreal. They were advertised for $40.00 with a new freewheel included and I ended up getting them for $25.00. I don't feel like I'm out a whole lot because the freewheel would have cost me $15.00 at the LBS but I'm bummed about the hop. When I inspected the mag, I can see that the center portion of the hub area looks out of round. I've included a picture but am not sure if you can detect the odd shape. The upper portion is the out of round part. Is there any way to fix this in a way that would still be safe to ride for my 9 year old? Otherwise, I guess I have some shelf filler in the garage........
From the photo it looks like the wheels use a sealed bearing cartridge pressed into the hub? And your photo shows this hub area to be slightly out of round? Does the bearing fit tightly in the hub, or is there some radial play? That would cause the whole wheel to wobble. From what I can see, that doesn't appear to be the case. So then the thing to check would be if the tire is seated on the rim evenly all the way around. I would also remove the tire from the wheel and check the rim on a truing stand or even just a quick spin while its still mounted in the frame. Brace a pencil as a reference point against the chainstay and hold it steady with the tip near the rim. Give the wheel a quick spin. Any runout/hop will be quickly evident. I'm guessing that if these wheels have been on a bike used for jumping, they could easily be out of round. There's a 100° gap between the three "spokes" with no structural support for the rim, other than the strength of the plastic used to mold it. A hard impact there could have permanently deformed the material. Your 9 year old might not notice or even mind a little wheel hop, But I sure wouldn't want him landing a jump on a weak wheel.
One day I'll think of something clever to put here....
Hopefully they just used an undersized hub...
I would pull that hub out and look at the concentricty of the hole for the hub vs the wheel. It would surely be a pain to mix up resin, fill part of the hole, and drill a concentric hole compared to getting the right sized hub.
The bearings on the wheel are typical loose ball bearings. The black rubber washer seals the bearing race from dirt or water but they are not a cartridge type bearing. When on my truing stand, you can definitely detect the up and down hop but no side to side wobble. I had thought that maybe these wheels took one too many hard landings as I don't believe they were made for any rough riding. My son does not do anything more than short ramps but I don't want to take any chances on these wheels anyway. I appreciate the replies!
no it dosnt matter if its a bmx bike or not.. wheel hop is wheel hop.. what are you talking about?..
my wheels is straight n true in my 2011 Fit eddie 3 'new school bmxer here'
i really dont know what your talking about, if my bmx bikes have a slight little problem i gotta tinker with it till its fixed right and true..
try throwing them mags in the freezer. some say a myth, but thats all your spose to do to straighten them mags out..
'i know it works on skyways'
While I can very much see it on the truing stand, I can feel it in the bike while riding it. I've experienced slight hop on tires not seated evenly in the wheel but this has been easily fixed with proper seating. This rear wheel has quite the hop beyond any other I've experienced with the tires seated well. I've set him up with a set of spoked rims in the meantime and he's loving the bike. I'll keep an eye out for a set of GT mags in the meantime.
6 posts • Page 1 of 1
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