GreenFix

I recently converted an old rigid steel frame to a suspended 9 speed mountain bike. When I was cleaning out my garage this weekend, I came across the rigid fork, and held it up to the frame with the 100mm suspension fork. The unloaded suspension fork was several inches longer than the rigid fork.

Here is my question: Are there any tricks to estimating the head tube angle?

I spent a little time with an angle finder and a level, but It was a framing level, and the whole thing was a little cumbersome. I am plannign on spending more time with it this weekend. I am interested in knowing how much I may have affected the frame geometry, and if I should be worried about the stress on the frame/headtube. The bike is pretty streched out, it is very stable and fast on technical descents, but it is a little sluggish on climbs (it could also be the engine).

I would also like to know at which angle I should begin to worry about fatiguing the headtube?

Thanks in advance for your comments.

Fat Hack

This is my very crude, "backyard" way of doing it, which I've found to be reasonably effective when comparing my measurements with geometry charts on manufacturers' web-sites:

Draw 3 dots on the frame (on the toptube, headtube, and toptube/headtube "corner"), then trace the dots onto a piece of clear perspex, join the dots on the perspex with a ruler, then put a protracter of the lines. Yes, I actually did this, it is NOT a joke

I draw the dots about 4 to 6 inches from the "corner" of the headtube/toptube, and you've obviously gotta try to get the dots right in the center of the tubes.

I repeat: THIS IS NOT A JOKE

roadfix

This is assuming you have a perfectly horizontal top tube...... but most frames don't.


What I would do is to lay your frame down on your driveway with your original rigid fork installed and mark three dots on your driveway. A dot at the front axle, another dot at the rear axle, and another at the center top of your headtube. The two axle dots represent the horizon. Use this horizontal line to measure against almost any angle on your frame, including seat tube angle. Make sure to compensate for fork rake when measuring headtube angle.

Installing a long travel suspension fork will turn your bike turn it into somewhat of a chopper. Very cool...

GreenFix

Thank you for your advice.

It was pouring last night, so I could not go out in the driveway, and I did not have any perspex lying around, but I did have a big sheet of cork board. The suspension fork is already on the frame, and I did not have a lot of time last night to break down the bike. I will say that the pedals and handlebars interfered with the measurements slightly. I placed the bike on its side on the cork board and used my son's sidewalk chalk to mark the locations of the axels, and to outline the head tube and the fork. The head tube angle was between 73 and 75 degrees. I took several measurements to try to be more accurate.

From what I understand this is pretty slack for a cross country mountain bike, which probably explains why it descends so well. It might also explain why it feels sluggish on the climbs.

Al.canoe

My old bike had a non-compact frame. I used the top tube to find a level spot in my shop. I marked the wheel contact points on the floor for future reference. I just use an old Sears Universal Protactor (#3984) on the tube itself.

I used the published numbers for my wife's Trek to check accuracy and it was exactly on.

The base of the protractor is too large to fit head tubes of the smaller bikes. A short spacer with "legs" on the end to bridge the emblem is needed. I can make those.

Al

Phatman

73 is pretty steep (like a road bike), and 75 is REALLY REALLY steep. even road bikes aren't at 75 degrees.

GreenFix

Yeah the more I thought about it, the more I think I did the measurements incorrectly. there is no way the head tube is steeper than it was with the shorter rigid fork. I am going to try to measure it again, and see what I get. I was quite rushed the last time I did it.

I have decided to replace teh frame anyway. I originally built the frame up about a year ago to see if I would like mountain biking. It turns out I do. A lot. I upgraded to a 9 speed drivetrain loast winter, and I have been riding the piss out of it all summer. As my mountain bike skills have improved I have noticed some limitations of the frame, which I think center mostly around the geometry. I am going to replace the frame with an airborne liberator. I'll probably sitck the original rigid fork back on my current frame and turn it into a beater commuter or a single speed for riding this winter.

thanks again for everyone's advice.

GreenFix

Here is an update. My previous measurements were in fact incorrect. I spent more time with the frame, and the cork board, and what I got for measurements on three measurements was 68, 69, and 69 degrees. Thanks again for the advice and for catching my error Phatman.