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Calculate clamp force on bike handlebar 1

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Gafferino

Mechanical
Jan 22, 2015
8
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 below for an example.

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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
 
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Gafferino said:
Hi Dougt115

Thanks for the reply. Just to confirm your equation do you mean:

F = T/(0.2xD)

Dougt115 is quoting the standard screw torque equation T=KDF, where K is the friction factor, usually set to 0.2. My textbook (V.M.Faires) shows 0.15 if the joint is lubricated. This applies to standard bolted joints. Corypad is suggesting that your joint is very much more complicated than that. He wrote quite a few of the bolting FAQs in Forum725.

--
JHG
 
Yep sorry I had reworked the T=KDF Eqn to get F = T/KD above
 
I gave the simple equation because it is simple and the task was to show the relationship between the number of bolts and the clamping force. This equation does it very clearly.

Adding an interface material can work but it will have to be harder than either of the other two materials. The shear forces in the material will turn any softer material into a pseudo lubricant and increase the slipping issue. (I did this as a kid to my dirt bike, quite a surprise when my handle bars rotated 90 degrees forward and left me on my face eating dirt. My friends thought it was funny, thankfully before video cameras were around.)

If an interface material, sleeve, is used you could consider adding bite by knurling the inside and outside of it. (Note, too much bite may weaken the parts by adding stress concentration points to the neck and handle bar.)

Second option is to increase the area where the neck grips the handle bars. This would increase the surface area and in turn increase the friction. You would need to increase the torque on the bolt to maintain the same forces on the surfaces.
 
what does the load in the bolt tell you about the torque on the shaft ? Yes, due to the bolt load there is some clamp up applied to the shaft, some resistance to torque applied via the handle bars; but how to relate these ? btw, the T used in the P equation isn't the "torque applied via the handle bars".

The bolt preload (well, really its deflection) applies some pressure to the shaft, which courtesy of friction between the shaft and the clamp, resists torque applied by the handle bar. no?

another day in paradise, or is paradise one day closer ?
 
How much torque is the handlebar allowed to put on the joint before the handlebar moves?
How much load can the bolt take before it fails?
What is the diameter of the handlebar?
What is the free diameter of the stem?
What is the friction coefficient between the handlebar material and the stem material?

I think the limit for torque is mu-D-2*bolt tension. The factor of 2 is because the bolt tension is balanced by the load in the hinge.
 
1gibson is on the right track.

You have a clamping force vs. rotational friction issue. If you increase the clamping force, the friction increases. If you increase the roughness of the handlebar section under the clamping section, it will also increase the friction...same result.

 
Hi Gafferino,

Do you have an actual assembly to evaluate in person?

If so:
- Is there a gap between the faces of the clamp when the screw is fully tightened?
- Or, do the faces of the clamp butt when the screw is tightened?
- Very important, Are there gaps between the handlebar OD and the clamp when the screw is fully tightened?

regards,

Dan T
 
Has there been a tolerance study done on the parts?

If your company can afford to spend the big bucks for Chinese fabrication, surely they can afford the tolerance study.

Also, when calculating the clamping force ask yourself which dimensional tolerance to apply for a worst-case calculation. And think about how completely unknown the coefficients that you are using really are. The only reliable way to analyze this is to conduct experiments with real parts that cover the gamut of dimensional stack-ups, fastener tolerances and applied torque tolerance.
 
Ultimately what matters is the friction moment existing at the clamped interface. It does not really matter how many bolts you use. What matters is the stiffness of the mating parts and how much clamping force is produced at the interface. If the handlebar tube is thin and lacks radial stiffness at the location it is clamped, you will never be able to tighten the clamp screws enough to prevent the handlebar from slipping.
 
Hi Gafferino

Well the problems been well defined by the members here and I agree with most of what's been said but can I ask what was the original torque or preload figure given for tightening the screw?
If there was no information given for the screw tightness then, the people making these components have a point, even if you prove that a certain bolt preload which assembly can withstand and shows the handle bars can't slip, if this information wasn't on a drawing or in a instruction manual then the design department has failed to give all the information for a satisfactory assembly.
Also the problem is more complicated than just running numbers because friction is extremely variable, so even when you obtain the right figure for the clamp how are you going to define it so it works consistently?
The only way I know to get a consistent joint is by practical testing and in addition studying tolerance build up, surface finishes etc.

I'm not trying to criticise here but pointing out when you obtain the clamping force and say to the manufacturer "you need to tighten to this figure" then the response might be "why were we not given this information at the start"
 
the problem is the design and manufacture are separated by several timezones, continents, and cultures !

if designed properly, the bolt should be able to hold the handle bar shaft as tight as you'd want ... it is a pretty typical design. so part of the question is "is it well designed?" eg tolerance studies, clearances, etc.

the other part of the question is "is it well manufactured?" (of course they're saying it is !)

if you're being told "no matter how much we tighten the bolt, it doesn't hold the handle bars" then either
1) it isn't made according to your design, or
2) the design is flawed in some unexpected manner (maybe a flange is bending, without tightening around the shaft?).

i'd've thought that a skype session would help understand the problem. if not, have them send you the parts to verify assembly.

another day in paradise, or is paradise one day closer ?
 
3) They got grease or oil on the surfaces. Maybe retained in the tapped hole even after attempted cleaning, and pushed through to the surfaces as the bolt is installed.
 
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