Torque spec for fork pinch bolts

[retiredgentleman]

I have a small job on the go to replace my headlight and headlight ears. I removed the forks and now I'm wondering about the proper torque for the lower fork pinch bolts.

My Haynes manual says 8 ft-lbs for the upper pinch bolts and 14.5 ft-lbs for the lower pinch bolts. These bolts are all the same size at 6 mm, so I agree with the 8 for the upper, but the 14.5 seems to be rather high for the lower. What torque are you lads using for the lower pinch bolts?

I see that for the XS650D they specified 6 to 9 ft-lbs, and then for the XS650E they moved up to 11 ft-lbs, finally going to 14.5 for the later years.

 

[weekendrider]

7.23 and 14.5 are what my manuals say RG.

 

[5twins]

The bolts are actually M8s, not 6s, so the 14.5 is not rather high, the 6 to 9 is rather low. I think the upper is so low because you're pinching an alloy triple clamp. Higher torques probably wouldn't hurt the bolt but might break the tree.

 

[pamcopete]

Well, theories abound. Here's mine.

Although we think of the triple trees, especially the more robust lower tree, as being stiff and inflexible, in fact they do bend imperceptibly from the up and down movement of the pavement and potholes that is transfered through the fork tubes to the pinch clamps.

The lower triple is the more robust of the two and it is designed to carry all the weight. The lower pinch bolts are torqued to prevent any upward movement of the fork tube in the clamp.

The upper triple is there to maintain alignment. As the lower triple flexes in response to the road, it bends imperceptibly and pushes the fork tube into the upper pinch clamp. Because the upper pinch bolt is just a bit more than finger tight, the fork tube will actually move upward in the clamp. However, there is not as much force on the downward return movement of the tube, so not all of this movement is reversed. The result is that the tube ratchets itself into the upper pinch clamps up to the point where the small amount of torque stops any further movement. This action results in the triple trees tightening together and maintaining the correct pressure on the stem bearings. All of which results in a more stable ride.

 

[retiredgentleman]

The bolts are actually M8s, not 6s, so the 14.5 is not rather high, the 6 to 9 is rather low. I think the upper is so low because you're pinching an alloy triple clamp. Higher torques probably wouldn't hurt the bolt but might break the tree.

Thanks 5twins, I guess I need to clean my glasses. Being M8s, then I'm OK with the 14.5 ft-lbs for the lower bolts.

Good explanation Pamcopete.