safety margin in bike frame strength

I know there are a few frame designers and engineers on here, so I thought this might be an interesting question:

taking the headtube as a prime example – how much of a safety margin is built into the strength of a frame?

With the new regulations and testing for the EU, perhaps this is even clearer.

An example:

A frame is built for a 130mm fork, but sold to the general public, let's say without an upper weight limit. This assumes that a rider could be anywhere between 50 and 140 kg. So if an average rider is 75kg, a rider of 140kg puts 87% more strain on the head tube.

Compare this to putting in a much bigger fork, say 180mm in the frame designed for 130mm. If the 130mm fork was 500mm axle to crown, and the 180mm fork is 550mm, then this is only a 10% increase.

So by this logic, even a 100kg rider could put a 180mm fork in their 130mm frame and 'only' put 43% more strain than the average 75kg/130mm fork option. So well within the safety margin of 87% for a very big rider.

Does this mean that it's almost always going to be ok to put bigger forks in frames that aren't specifically as light as possible? (assuming you don't mind any geometry changes.)

I guess every company will have their own rationale for defining the limitations on their design, and even then this will be based on very much simplified testing or analysis with lots of margin allowed for abuse, shock loading, fatigue, material variability, tolerances, heat treatment variability, welding etc etc etc – the list is endless.

Add to that how the bike has been scoped – is it a lightweight carbon frame that is fairly homogeneous or is it a burley alu frame with complex welds.

Basically, without knowing what the designer has accounted for, you can't really be sure, however, it is a good bet that there is scope for using the frame outside of those boundaries – albeit at risk.

That's why I mentioned the new EU regulations stuff. I can't remember the exact details, but didn't people on here mention that some frames had to have headtubes redesigned?

So if all frames have to now meet the same standard for strength at the headtube, what is an EU standard max rider weight?

I would imagine it the amount of strength designed in might have more to do with the number sold vs number returned broken equation.

You're making a critical error in your reasoning there. Even though the additional 50mm of travel on the forks only raise the length by an overall 10% they increase the angle, relatively speaking, substantially. Thus generating a greater load on the head tube, not just the 10% you've calculated. Another factor would be with the increased suspension length you're likely to be hitting much bigger drops & obstacles which will also be generating much larger loads on the frame.

"If the 130mm fork was 500mm axle to crown, and the 180mm fork is 550mm"
Not all 130mm forks are that low
A Revelation at 130mm (or Pike) is 508mm
A Fox Talas 36 at 130mm for eg is 515mm

Nor 180mm forks as low as 550mm either
A 180mm Totem/Domain is 565mm for eg

You seem to be trying to make it all far too simple
If you genuinely want to replace a 130mm fork with a 180mm one I suspect you have completely the wrong frame already, or would do with a 180mm fork, not just in terms of whether or not you'd rip off the headtube or not riding the same terrain but with a bigger fork. Massively raised BB height, shortened effective top tube, shortened effective seatstay length, increased head angle unloaded (but similar bottomed out head angle), increased seat angle forcing the rider way over the rear wheel, etc etc ..

Yep, you've assumed that a heavier rider will exert a linear increase in forces & the same for the increase in fork length.
Are the relationships linear? Perhaps they obey some kind of square law?

Also, as mentioned above I have always assumed that the limits on fork length was more to do with the presumed use of the bike after it has been fitted with a longer fork, rather than the literal increase in length.
Stick an 80mm SID on a frame and the chances are you aren't going to be doing huge jumps/drops & general downhill tomfoolery. Stick a 160mm Bomber on it & the chances are that your intention is to slam that frame around a lot harder.

I agree with the change in intended use, but that's essentially part of my original question.

If I have an 'all-mountain' frame (6" travel, so probably into the burlier realms of all-mountain) that is ok for decent size drops etc. at 130mm fork, is the safety margin so big that similar drops with a bigger fork aren't actually a lot more stressful?

Starting out with an all-mountain full suss rather than an xc race hardtail obviously influences the intended use.

In fact though, if all frames now have to pass the new regulations, isn't the head tube on a basic hardtail effectively tested to be as strong as a freeride bike?

there is no magical factor of safety figure

its all far more complex than you are trying to make it

that's why I asked really, to get some more detailed info on the complexity…

regarding the fork length, surely force=newtons x length if I remember correctly, so assuming same newtons (mass of rider) it will be a linear relationship with length of the fork?

would just be interested in some real enginering input. I'm not afraid of a few equations!

I still come back to the CEN frame testing stuff. If every frame has to now pass a test with a certain force applied on the headtube, is this area becoming as standard 'very strong' for all types of frame?

I asked the same question a couple of days ago, as both Brant and Cy have commented that they have had to put considerable design effort into passing CEN tests. Their old frame designs were hardly fragile, so I wondered if anyone thought that CEN was really a bit OTT. It'd be ineresting to know if the new regs were compulsory, and how they were derived, and who was consulted etc.

I think the trouble some test from CEN was fatigue over loading cycles.

And isn't there an argument that more suspension actually lowers dynamic loading compared to a rigid fork?

thanks the00, those are exactly the things I was wondering about!

Just cos you've got a 10in cock doesn't mean she'll enjoy it any more.

(added later)

Sorry – that's not really very helpful. my point which I hope you get is that there really isn't , in many cases, much point in sticking a significantly longer fork in any frame. Anything significantly longer than the frame was designed for, whilst it will give you the slack head angle that is so "this week" won't give you a long top tube to use with a short stem, won't give you a steep seat angle to keep it climbing, won't give you a decent BB drop to make it feel nice, won't give you a torsionally stiff front end to support the loads that you should be putting through the thing if you're riding it hard.

All those other things are just as important as head angle alone.

thanks for the input Brant

To clarify, the question came about because I got a big fork cheap in a sale, and planned to follow up with a frame. In the meantime I've tried it on an existing bike, and find that it actually rides quite well (dh/fr, never expected it to be a trail bike with a 180mm fork).

What I was wondering after a couple of similar discussions on here, is can any figures be put on the extra force on the frame, and because of that, how overbuilt standard frames are these days?

Would installing a headset with a deeper insert into the frame help spread the strain across the head tube as well?

There can

b

nt appear to be so. Wonder what the % failure rate any particular company has set as an acceptable rate?

I have only had 1 day out destruction testing on my Blue Pig which is built to the CEN regs and designed by ^^^^^^^^^^^^you know who.

I know a young DHer who has just broken his Intense ???. He is small and pretty skinny so was not exerting much force in the way of weight. He does however post regular top 5 placings. CRC are doing him a good replacement deal btw 🙂

In the meantime I've tried it on an existing bike, and find that it actually rides quite well (dh/fr, never expected it to be a trail bike with a 180mm fork).

What you running it on right now?

marin rocksprings with TARA (at 150mm travel setting)

Certainly looks like there's enough metal in that front end to hang onto things.

thanks again Brant. So far my only test was that I've not died yet (alps and little slope style park).

If you have any figures/methods about strength and leverage for frames I would certainly be interested from an academic point of view…