Calculate clamp force on bike handlebar

[FLM]

Hi Folks

I am looking for some help in a calculation if possible.

I am looking at a bike design where the handlebars are clamped in place in the stem using only 1 bolt - see the picture attached or link below for an example.




There has been issues with the handle bars slipping forward when in use. I think that this is a manufacturing problem and not a design issue as this type of stem with one bolt is quite common.

However I would like to prove this mathematically. How do I calculate the force that the bolt and clamp have on the handlebars? I would like to compare a calculation of 1 bolt versus 2 and 4.

Any help greatly appreciated.

Thanks

[JAlberts]

FLM, there is too little information for anyone to calculate the clamping force for your assembly and the clamping force is only one element in determining if you have an effective design or manufacturing error. Unfortunately, unless there is an obvious mismatch of the bar diameter to the clamp bore and am inclined to believe your problem is most likely to be design related.

In that respect, before I address each of those, one critical issue I would like to initially discuss relates to the issue of friction between the clamp bore and clamped handlebar in your assembly. From the photo of your clamp the overall black finish of the clamp, including the bore, leads me to consider that your head assembly may be anodized aluminum. If this is true, then the anodized bore which results in a durable hard finish on the clamp bore also unfortunately also results in a surface with a realtively low coeficient of friction to resist rotation of the handle bar. A corresponding finish such as chrome if used on your handlebar likewise has a low coefficient of friction. As a result, it would be extremely difficult to develop a significant friction between the parts to resist rotation of the bar with any reasonable amount of clamping force. A number of the handlebars I have seen include a straight knurling on the outside of the bar in in the clamping region to address this issue.

If you will respond to this issue first, I will be glad to discuss other potential elements related to the clamp assembly and its indidvidual components.

[FLM]

Hi JAlbert thanks for your reply.

Apologies I only posted that picture as a reference. Please see attached for a picture of the design and relevant dimensions. Both stem head and handle bars are made of stainless steel.

[jboggs]

My immediate reaction to your screenshots is that your problem isn't in the bolts. Your problem is in the sheet metal clamp. My experience with these clamps is that they will never grip a cylindrical surface tightly enough to resist any significant torque. That is because they deflect (bend) so easily themselves. You need a stiffer (thicker) clamp. One that will not bend as you tighten the bolts.

[Kelly_Bramble]

Straight Knurled contact surfaces would help prevent slipping as well..

[JAlberts]

FLM, Now that I see your design, I'm inclined to agree with jboggs about the lower thin clamp. I believe that the flexiblity of the portion of the lower clamp not reinforced by its attament gusset is acting like a spring under load to limit the amount of clamping force that your upper rigid hinged clamp can apply to the handlebar.
Also, even if you stiffen the lower clamp, as I stated in my original post and as suggested by Kelly, I recommend you seriously consider adding a short straoight knurled section to the portion of the handlebar inside the clamp to maximize the contact friction of your clamp. Sraight knurling is a very effective way to add friction; and, is a simple and economical lathe procedure that can be quickly applied to the handlebar prior to bending it to shape.