View Full Version : stress testing frame & fork in a home brew rig with known load & measured deflection

02-16-2016, 3:06 PM
So, I'm put a new front end on my bike, home built, kinda funky design, wanted to test of safety. Not a chopper (although the steering angle is radically different from stock...) but similar safety concerns could apply when building a frame, raking a neck or using a custom fork. The test I'm showing here is really only needed if you have a front brake (and a good one at that) but I plan to do a similar test for bump loads. Hopefully it gives people inspiration on how to push thier shit farther, better, relatively safely. We may not all be board certified welders, but you don't have to be if you know your stuff can stand up to certain proven loads.

Brake load test completed! 1,350 lbs simulated (1.8g) brake load reached w/ less than 1/2" deflection!


Arm with weight suspended (yes, that is an anvil, 300lbs) has a game scale (goes to 550lbs) to measure load. Arm length horizontal to ground from lower ball joint is three times distance from contact patch to lower ball joint, so load on arm x 3 is the simulated brake load.

Dial indicators are mounted to measure fork deflection towards rear of bike. One measures deflection of fork relative to the jig holding the frame, the other measures fork deflection relative to the point where the front end is bolted to the frame.


In addition to the 300 lbs anvil, there's a lead ingot the size of a brick, a 2 foot chunk of train rail, and a slab of steel the size of a carton of cigarettes. Makes 450 lbs total, for a simulated braking load of 1350 lbs. That's a brake force of 1.8g for the 500 lbs bike, 200lbs rider, and 50 lbs of cargo. I doubt the bike will be capable of pulling over .9g without the front tire breaking loose, but I wanted a x2 safety margin to give a cushion against fatigue failure.


The dial guages indicate 0.46" fork deflection relative to fork mount, and a bit over 1" total deflection. For a 1.8g brake force, that seems pretty damn stiff; in actual riding, the deflection will be much less, likely on par with what you would expect from a good modern usd sport bike fork.


While setting up the jig, I put my 200lbs body weight on the arm end. I had forgotten to install the axle pinch bolt, which caused this.


Fortunately I was able to beat it back into shape on the anvil. I'll be welding all the way around the axle insert (it was only welded on the inside bottom) so that this part isn't entirely dependent on the strenght of the pinch bolt to resist deformation. On the other hand, lots of bikes only use pinch bolts, and they use bolts a lot smaller than the half inch grade 8 monster mine has.

Bonus un-edited vids of test conclusion and silliness -

load rig swing - YouTube (http://www.youtube.com/watch?v=fAcxyTr-90Q)

SECApocalypse brake load test - YouTube (http://www.youtube.com/watch?v=HRaQYG3Zu_I)

02-16-2016, 8:34 PM
Load testing and bend testing are the way to KNOW what works.

Nice anvil too.

02-17-2016, 9:07 AM
Right, even if I had a 3d model with finite element analysis etc, I wouldn't trust my fabrication to match calculated figures. On-off builds always have some unknown elements (fatigue limit being a big one) but loading them can help spot obvious design / assembly flaws (like my pinch bolt issue).

The anvil belongs to our shop manager, I think he picked it up from a farm. Was a bitch just getting it on the jack to lift it up for hanging, the stump probably adds 100 lbs.