sysrq

How severe would be the stress on the frame and fork during a crash when losing traction on a front wheel due to braking on a slippery grass at around 40km/h? The frame is made out of 858 heat treated tubing and some kind of Reynolds fork. Also the frame had some centrally located 6kg load with around 1.5kg at the back. Couldn't spot any bends but the frame seems to be less compliant than before, although still rides without any shimmy during high speed descents compared to budget aluminium one.

unterhausen

Did you just fall over? Bikes don't like the front wheel being stopped short, but otherwise they survive most crashes without any damage

guy153

If it hasn't physically changed shape it should be fine. Check for ripples on the down-tube close to the head-tube. You can get a bit of deformation there sometimes that you don't always notice, and it would have a bigger effect on the handling as it would alter the trail.

sysrq

Just went over the bars with only minor facial bruising thanks to cushioning of the grass. The bike stopped as soon as tires lost contact with the grass and toppled on one side.

Cynikal 

My steel CX bike after a decade of racing (and a lot of falling) would tell you it's fine.

pwyg

Unless there is stress to the frame this will show with either dents or stress risers in the tubes. If you see these the frame was impacted hard enough to damage the steel. For dents in steel it varies on the hardness whether the tube will crack over time. 853 is a hard steel so a dent that penetrates 25% of the way in to the tube will most certainly fail with use overtime and how you ride.

sysrq

Not sure if frame can be impacted that much while falling over on one side in a grass with a softish soil. Theoretically the front tire should have taken up all the side loading while falling over. It was a Thorm Audax Mk3R frame.

Nessism

After a good bit of use over 15 years or so of riding and a few crashes I undertook a project to replace the down tube on one of my homebuilt frames. The bike rode fine for the most part but the down tube was dented and there was some rust in various places. Anyway, after cutting the down tube in half the frame immediately sprung significantly out of plane. I was surprised since the frame rode decently before the rebuild. Clearly, there was a lot of residual stress in the frame. Lesson learned.

pwyg

Almost no frames built will have no stress in the frame. It is virtually impossible the way frames are built these days. The only way to do so it to increase the time to build the frame which is simply a no go for builders. Anyway mist builder simply ignore this stress, do not know any better or treat it as a non issue.

unterhausen

Yes, I'm going to continue to treat it as a non-issue. It's not like the frame is going to fail due to residual stresses. And the tubes themselves have residual stresses. Although I am not sure how much stress my frames really have given my experience with couplers. Generally if I cut a frame apart, I would expect the top tube/down tube to get closer together if there is going to be any deflection. Side to side deflection seems unusual.

But all warranties are voided if you cut the bike in half.

I don't think the OP has anything to worry about given his description of the fall. Bikes see worse than that in use and nothing happens.

Andrew R Stewart 

The OP could have the frame and fork checked for alignment by someone with experience to be sure no deformation happened. Frames and forks can certainly become twisted or bent but show little outward damage. Andy

sysrq

It has been said that forks have to be designed to bend before frames. Since Thorn Audax Mk3r is said to be designed for light loads then it wouldn't hurt to check the frame.

Andrew R Stewart 

"It has been said that forks have to be designed to bend before frames" sysrq

I sure wouldn't want to depend on that assumption This is an idea that has been going around for decades. These days there are production/market zone fork (and frame) strength/deformation standards that are way greater then what most steel forks can meet. Back in the day certain Euro brands were known for "soft" forks, till they found the way onto mixte frames. Then these same forks tended to bend after the poorly designed to resist frontal impact bending frames did. So it depends... Andy

guy153

What's the reason for this? To make them more economical to replace, like a sacrificial component?

The problem is a fork is more safety-critical so there might/should be some temptation to overbuild it.

unterhausen

It does seem like usually either the fork bends, or the frame bends. Both only bend if the rider really slammed into something.