Calculate clamp force on bike handlebar 1

[Gafferino]

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.

http://i.ebayimg.com/00/s/MTE5OFgxNjAw/z/rdgAAOxyrYFR2wbP/$(KGrHqF,!k8FHQwpchTzBR2wbP!Now~~60_35.JPG

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

 

[rb1957]

handle bars slipping forward ? yes, this is a common design (using one bolt to clamp the handle bars in place). in my experience the common "failure" is the handle bars slipping and rotating ('cause they're not clamped sufficiently) ... solution tighten the bolt more (what, more?). the design of the bracket should provide a round seat for the handle bars ... if they are sliding forward then the seat is too shallow ... the opening (that the bolt clamps across) should be less than 180deg.

another day in paradise, or is paradise one day closer ?

 

[GregLocock]

...but of course with carbon handlebars designed by amateurs if you tighten them up then you crush the tube.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[rb1957]

designed by amateurs, tightened by wrestlers ?

like the old sketch "who decides the preload on a bolt ?"

another day in paradise, or is paradise one day closer ?

 

[CoryPad]

Pinch bolt designs like this are difficult to analyze. I have seen the use of a triple integral to determine surface pressure and resulting forces. Usually joints like these are tested physically.

 

[Dougt115]

Force generated by the bolt is (Torque)/.2/(diameter of bolt)

For multiple bolts multiply the force by the number of bolts. Generally more bolts are used so that a lower torque value can be used to get the same force.

Using the same torque settings 2 bolts deliver twice the clamping pressure of one bolt and 4 will deliver 4 times the pressure. Likewise 4 bolts at 1/4 the torque will give the same clamping pressure as one bolt at full torque. Also note that switching to a smaller diameter bolt will decrease your clamping pressure too for the same torque setting.

Clamping pressure is just the force divided by the area of the handle bars under the neck.

But your issue is how much torque can the handle bars take before they turn. This is a function of the friction between the handle bars and the neck, and the force applied. Also taking into account material selection and surface finishes. (Weaker materials will shear under lower loads, ridges provide added friction.)

Hope this helped. And good luck.

 

[Gafferino]

Hi Dougt115

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

F = T/(0.2xD)

I have attached a proper picture of the design.

http://files.engineering.com/getfil...40-a831-c97d9ced9a3e&file=Handlebar_Clamp.jpg

 

[rb1957]

i think your "forward" movement of the handle bars is them rotating in this clamp. i think your problem is more about the enforced contact displacmenet around the handle bar shaft than it is the bolt load. sure, the bolt load affcts the problem, but how do you get from bolt load to resisting torque ?

i'd look into the detail design of the clamp ... when the clamp is closed is the opening slightly less than the diameter of the bar ? is there a good amount of friction between the handle bars and the clamp ?

if you want to control the handle bar position, you could put flats on the handle bar and pads on the clamp, and trap it forever.

you could add a small rubber piece that'd compress between the handle bars and the clamp.

another day in paradise, or is paradise one day closer ?

 

[Tmoose]

what does the poor 7 mm bolt/screw thread into?
How tight are you tightening the screw?
How is the screw protected from or accommodate bending? Is the 90 degree lower "lip" expected to both conform to the handlebar and align with the screw axis?

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

Are there gaps between the handlebar OD and the clamp when the screw is fully tightened?

It is not unheard of to knurl handlebars where they will be clamped.

http://www.porkchopbmx.com/images/HANDLEBAR-ART-KK-SQUARE-KNURLED-NICKEL-5.jpg

 

[Gafferino]

Hi Tmoose

The issues you mention are the issues I feel are at the source of this problem. However I am trying to ascertain how I can calculate if 1 bolt is enough to hold the handle bar in place if all the things you mention are correct. I am being told that 1 bolt isnt enough and I simply dont believe this (if all other factors are as they should be).

To one of your points how would you protect a bolt from bending?

Thanks

 

[1gibson]

Can you tighten the one bolt enough to crush the tube or deform the clamp? If so then it is more than enough.

I'm not sure why you are looking at bolting instead of increasing friction.

 

[Gafferino]

@ 1Gibson

This is a product being made in China so I dont have it physically here unfortunately! All of the above suggestions and what you have suggested are correct and steps I will be taking but I want to prove the design with numbers first if I can.

 

[rb1957]

i think your problem is geometry, unless you know that tightening the bolt clamps the handle bar before the faces of the clamp contact (which they probably shouldn't).

you could serate the inner face of the clamp, to improve it's "bite" onto the shaft. Of course, the "ultimate" fixing for the shaft in the clamp would be a cross pin.

what materials are we talking ? steel ? carbon ?? plastic ???

another day in paradise, or is paradise one day closer ?

 

[Gafferino]

We are talking stainless steel.

 

[rb1957]

ok, nice and ductile. how about the geometry questions ?

another day in paradise, or is paradise one day closer ?

 

[Gafferino]

Nope I am not sure on the geometry - in theory its all fine i.e. the drawings are all correct but they are producing may not be. The problem is they are blaming the design (the use of 1 bolt) and I believe it is a geometry problem during manufacturing/assembly so am trying to prove mathematically that 1 bolt is enough so I can press them on the other issues.

 

[rb1957]

ah, the plot sickens ...

you've got your design on hand, so you can verify that the clamp does (by dsign) clamp over the shaft.

The bolt (something like a 1/4") is plenty to bend the flanges of your clamp. you can stiffen these with radius/throat washers if needed.

It sounds like you need a Skype session with them to see how the clamp tightens, maybe it's deforming in an unexpected way?

Maybe wrapping some dense rubber around the shaft will improve the clamp-up ?

another day in paradise, or is paradise one day closer ?

 

[1gibson]

I think you could use some sort of simple keyed feature. Maybe drill a hole in the frame, and use something like this " O= " that goes in between the faces of the clamp (adjust clamp design if required) with the bolt going through the center.

Think of a small eye bolt, but it only needs a straight pin, no threads.

You might be able to use plastic. Cheaper than an extra bolt. A little more complicated to assemble, but is that really factory assembled? How do you ship the bikes?


Anyway, to answer your original question, calculate the pullout strength of that bolt into the clamp. Check to see if that will yield the clamp, or the tube. If it can, then one bolt can work, it's just a matter of determining the torque that keeps it from sliding. To do that, you need to know coefficient of friction.

Did the factory get tapping oil on the interface? That would certainly do it.

 

[BrianE22]

There's actually a hinge joint (free to rotate)? A fixed joint requires bending of the flange arms. The hinge joint makes it much easier to calculate the clamp load and friction on the tube.

 

[Gafferino]

its a hinge joint yes.... can you tell me how to calculate the clamp load and friction on the tube?