7

My problems with this bar started with it slipping every time I jumped or yanked the bars back. So I had to tighten the screws frequently. Tightening probably caused it to deform and become oval shape before it cracked completely.

Cracked bar

It seems obvious that this handlebar needs replacing but how do I prevent this deforming in the future?

5

The other answer is too general for BMX and some things don’t apply, or are even bad advice for BMX handlebars.

Preliminaries

  1. Check that both the stem and the bar are not "crushed" (if they’re brand new, they’re obviously fine)
  2. Remove paint from clamping areas. Usually stems do not have paint on the inside clamping area. On the other hand, some handlebar manufacturers do not bother protecting the clamping area during painting. If you have paint on the clamping area, this reduces the overall clamping friction. You should sand that paint, but it’s not 100% mandatory.
  3. Remove dust, debris, etc. There should be nothing in between the stem and the bar that could cause slippage. DO NOT GREASE THE CLAMPING AREA.

Tightening the bolts

The main reason why handlebars get crushed like this is because of uneven tightening of bolts. BMX stems almost always have 4 bolts (either top-load, or front-load).

Here’s an example of a typical top-load stem:

enter image description here

When tightening the top bolts of the stem to your handlebar, you need to follow an even cross-like tightening pattern, such as this one:

enter image description here

  1. Screw all 4 bolts into the stem by hand until you feel a bit of resistance.
  2. Make sure that the top cap of the stem has even spacing all the way around. If not, screw/unscrew slightly the bolts until it does (see image below)
  3. Tighten the bolts 1/2 turn at a time, one after the other, following the above pattern. This will ensure that all bolts have roughly equal tightness and that the stem cap is evenly spaced.
  4. Tighten until it becomes hard to tighten by hand. Don’t overtighten the bolts. You should grease the bolts to allow better tightening (the bolts, not the clamping area!)
  5. Check that everything’s okay by pulling and pushing hard on the bars. They shouldn’t slip at all.

Regarding 2, here’s a quick sketch up of what I’m talking about. This is a side view of the stem:

enter image description here

The spacing in 1 is even, this is good. The spacing in 2 (exaggerated) is uneven, this is bad. This spacing must be even on all 4 sides.

  • Note that the part of the previous answer about using a torque wrench to get the correct amount of torque on the bolts is a very good idea. 5Nm is not a lot of force compared to what a simple ratchet will apply. – DavidW Oct 7 at 16:26
  • This is a great answer that very much applies to front-loading road and MTB stems (with the caveat that bolts on road and even MTB stems might not need as much torque as a proper BMX bike). – Paul H Oct 7 at 17:44
  • 1
    The old cheap bar was high tensile steel, swapped it to a CrMo bar as suggested by LBS. – akort Oct 8 at 22:44
3

To prevent overtightening there are 7 steps

  1. Make sure that the bar and the clamp match (same) diameter
  2. Check the clamp for deformation or a compromised surface
  3. Grease the contact surfaces slightly to prevent surface rust and electrolytic fusion
  4. Tighten the bolts with the correct amount of torque using a torque wrench. The correct figure may be stamped on the clamp and/or bar
  5. Re-check the tightening after a short ride (~20-30km)
  6. Check 3-months intervals or after a rough ride (I guess the bike in the question is a BMX)
  7. (as suggested by @Criggie) Check the bolts, or maybe even better, replace them straight away.

One last thing: Stems like handlebars are not for life. They should be replaced from time to time as they are submitted to quite a high stress.

  • 2
    Another idea - check the bolts haven't lengthened/distorted. That would lower the clamping force over time as they slowly stretch. – Criggie Oct 6 at 19:13
  • 1
    @Criggie It should require a huge force to stretch a steel bolt beyond its elastic limit. Is that really feasible? – David Richerby Oct 7 at 7:34
  • 1
    @DavidRicherby yes bolts can stretch. Its surprisingly easy given the leverage a BMX bar can exert, and bolts may not be as hard as one would expect. Chinese Cheese bolts stretch, deform, strip and bend and break at far lower pressures than a proper bolt. Note OP has tightened the stem bolts enough to squash the round handlebar. – Criggie Oct 7 at 10:05
  • 1
    You should never grease the contact surface. This will cause slippage. This might not be a problem for road bikes but it definitely is one for BMX bikes. – Fatalize Oct 7 at 13:43
  • @Fatalize Carbon mounting paste is an alternative to grease even with metal to metal contact. There should always be a non-metallic medium between to metal surfaces in contact to avoid electrolytic activity. – Carel Oct 8 at 8:57

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