Forces applied on the bolts in the bike's hinge

[kielbasa]

TL;DR Summary

Forces applied on the bolts in the bike's hinge in the alluminium frame. Steel grade of the bolts is needed.

Hi,
I recently bought a folding bike and it turned out over time that its weak point is the main hinge where two bolts (vertically placed) are located.
I've had the lower bolt break twice during a ride, leaving a part of it stuck inside the hinge thread. It's clear that the forces applied were too much for the low-grade steel bolt to handle. Anyway - since I don't want to spend more money on service bolts which are obviously bad quality, I'd like to use ready-made ones but higher steel grade and modify them to my needs by a turner.

My main question is, how can I estimate the maximum forces applied during a ride, for instance when getting off a 10cm curb? I think in these circumstances the hinge is most vulerable to destruction (maybe there are some others?)
I primarily use this bike for city riding, not for off-road or mountain biking and I'm trying to figure out the appropriate steel grade for the bolts that will be sturdy enough for this purpose so I can use them instead. Good quality bolts are made of 10.9 graded steel, which means they can withstand around 1000mPa / approximately 100kg per mm2 of the screw section. This translates to a yield strength of around 1600kg for an M6 bolt. This seems like a lot, but I'm not sure if it's enough. I can't quite estimate the forces acting on the hinge. Any ideas on how to do it ? It looks like the forces are still significant enough to break the service bolts that are sold in shops, and I'm certain they're not made of plasticine...
My weight is around 105kg.
Any ideas? thanks!

 

[jrmichler]

The peak load from driving off a curb can vary by an order of magnitude depending on how you drive off the curb. All you really need to know is that the factory bolts are not strong enough for your weight, road conditions, and riding style. If you replace with bolts that are sufficiently stronger than the factory bolts, the next weakest part will break.

A realistic approach is to:

1) Look at your riding style. Do you move your body so as to minimize the forces on the bicycle frame? Do you actively avoid hard bumps?

2) Change the bolts to the strongest that you can find, which would probably be metric class 12.9.

 

[jack action]

Good quality bolts are made of 10.9 graded steel, which means they can withstand around 1000mPa / approximately 100kg per mm2 of the screw section. This translates to a yield strength of around 1600kg for an M6 bolt. This seems like a lot, but I'm not sure if it's enough.

Why not use bolts of 12.9-graded steel and call it a day? They are the highest-grade bolts with 1200 MPa and probably not much more expensive than the 10.9. If they still break, nothing else will help except redesigning the bike frame or hinge.

 

[berkeman]

In addition to using the strongest bolts possible, you might look into whether there are wider tires available that will fit inside that frame. You could run them with a little less air pressure to give you a bit of a "suspension" effect (since your bike appears to be rigid). What pressure are you running in your tires right now?

If you replace with bolts that are sufficiently stronger than the factory bolts, the next weakest part will break.

^^^^this^^^^