3

I just bought a dual suspension MTB. I have pumped up the rear shocks to the correct pressure, but whenever I’m doing jumps the rear tire just rubs against the thing that holds the cable for the front derailleur. You can see the paint is chipping a bit from the tire rubbing.

Image of the chipped-paint part

I have no idea how to fix this, the only way for me to stop the rubbing is just to harden up the rear shocks so it doesn't compress as much, or just to remove the front derraileur and cut the thing that holds the cable. Is there another way to fix this?

14
  • 3
    Could be lots of things - wrong shock letting suspension travel too far comes to mind as most likely. Shock should bottom out before moving parts start connecting to non-moving parts. What wheel and tire is on the bike - is it original spec size of has someone 'upgraded' from say 26" to 27.5"? Can we have more detail and photos of the rear triangle, including with air out of shock and suspension fully compressed. – mattnz Nov 9 '20 at 7:15
  • 1
    Bike is fully stock, 27.5 both front and rear. The Tire will only rub whenever The shocks are almost fully compressed. The shocks can still compress a bit but not fully – B- boy Nov 9 '20 at 7:23
  • 2
    This seems like a poorly-designed frame and poorly-spec'd bike. You could add volume spacers to the shock to make it's spring rate more progressive, you could use smaller tires, or you could commit to 1x drivetrains and file the cable stop down – Paul H Nov 9 '20 at 8:07
  • 3
    @B-boy the cable hanger looks much longer than necessary. You could get a slightly narrower rear tyre, or have that hanger cut down and welded closer to the frame by an aluminium welding specialist – JoeK Nov 9 '20 at 8:07
  • 2
    I'd return this to store as unfit for purpose – Andy P Nov 9 '20 at 10:59
11

I am far from convinced the bike is stock, I would be surprised if any bike manufacturer or seller would ship a bike in this condition, even a cheap BSO. The rear tire is definitely not new, so its probably not a new bike. What little I can see of the shock it looks out of place (Aircan shock on a cheap looking frame). I therefore suspect the shock has incorrect stroke - e.g. 210 shock comes in both 50 and 55mm stroke. A 55mm stoke where a 50mm is require would do exactly this, have the wheel foul on frame components when at full compression. The difference between 50 and 55mm stoke is often an internal spacer so it can hard to tell if the shock is correct just by looking at it.

Ideally find the bike make/model/year, look up the specifications and compare to the shock you have on the bike. If the shock has no measurements, let the air out and measure its length uncompressed and fully compressed to determine its stroke.

If you cannot confirm the requirements for the frame from make/model etc, and the shock is length is a common size with the longer of two common strokes, a shorter stoke will stop the tire rubbing when you bottom out.

If the shock is the correct size, then look at the wheel - its possible the frame is designed for 26" wheels and has had 27.5" fitted (maybe from new). This increases the diameter and would cause the interference you are seeing. You could install a 26" on the rear (Presuming disc brakes) which will most likely solve the problem.

Hacking the frame should be a last resort - however will almost certainly be cheaper than a new shock or new wheel(s).

7

It's a clamp-on FD, so the seat tube is standard diameter. It doesn't appear there would be anything holding you back from using a clamp-on housing stop, probably positioned a little lower and offsetting it as needed to clear the tire. You'll need new cable and housing. Get it working and cut the old stop off.

It's a curious situation, so before cutting, it would be wise to let all the air out of the shock and swing up the rear end to make sure that there won't be a new thing the tire bumps in to after the stop is gone.

4

Given you just bought the bike, my first response would be to contact the seller and ask for advise and suggestions


There are two possible modifications I can see. You will void any frame warranty by doing either of these.

  1. Mod the cable stop - shorten the total length of the boss and drill a two-step hole in the stub. I would not add a slot as per the original, for strength and to reduce water/mud ingress.
    The risk is whether the width of this stub has enough "meat" to take a hole wide enough for the cable's ferrule.

Picture of the back of the seat tube showing the cable stop for the front derailleur with the suggested modification to shorten it sketched on top

  1. The other option is to buy a clamp-on cable stop. There are several manufacturers, you just need to match the clamp to your tube's outer diameter. Note the angle's not quite right. A mockup:

Picture of the back of the seat tube showing the cable stop removed and a clamp-on stop sketched in place

1
  • 1
    The second solution is by far the cleanest since the cable stop may be rotated to get the cable in line with the original routing. – Carel Nov 10 '20 at 11:00
3

This is a stupid manufacturer's mistake, but if it really happens only on the hardest impacts then it's probably not that bad of an issue. I'd be most worried about the hanger puncturing the tyre, so what I would do is just to file the upper edge of the hanger (where the paint is anyway chipped already) completely round and then covering the spot with some electrical tape or maybe a drop of hot glue, as a harmless consumable to be chipped away in future impacts.

Many people consider converting their bikes to 1× these days. I generally don't think much of that (IMO, for hardtails and road bikes the advantages of 2× outweigh those of 1×), but if you're otherwise happy with this bike then this could be an instance where it makes sense. Think however about whether that's really economical – a good 1× drivetrain can cost more than this entire bike is worth...

4
  • Even thought the frame has been noted to be AL, not steel, it will still corrode. If you file down the hangar, you'd want to have a good coat of paint on there to protect the metal, then possibly add a frangible wear surface on top of that. – FreeMan Nov 10 '20 at 14:02
  • Yes, but does it matter? That part is anyway basically scrap, so no point in wasting any effort in preventing corrosion there. – leftaroundabout Nov 10 '20 at 14:30
  • Corrosion could start there and work its way into the seat tube. Probably not as fast in aluminum as steel, but it's still a possibility. – FreeMan Nov 10 '20 at 14:35
  • 1
    @FreeMan That piece looks to be a solid aluminum block, so it would be really hard for corrosion to get to the seat tube. Corrosion isn’t a contagious virus. – MaplePanda Nov 10 '20 at 16:24
2

What bike is that? I’m curious as to which brand is specifying air suspension with a Tourney drivetrain.

You could switch the front derailleur to a side swing design like this one, eliminating the need for that cable stop: https://bike.shimano.com/en-US/product/component/alivio-m3100/FD-M3120-M-B.html

Finally, your rear tire looks kind of worn out. Maybe just keep an eye on it.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.