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BMX Parts: Strength, Flex, Stiffness

Images by Seventies Distribution


I was talking with Steve Jones from Dirt Magazine recently about metal. Not metal music, metal for bike parts. In mountain biking, frame material and design techniques are absolutely crucial for determining the feel of the bike – for instance, he explained how a steel frame with more flex will give the rider more grip when compared to something much stiffer like a carbon frame, as the wheels can adapt to the terrain more easily. Something like that.

OK, this isn’t totally relevant to BMX but it got me thinking. Flex is a definite consideration for riders in other areas of cycling, but seemingly not so much in BMX. Most BMX riders tend to buy parts based on strength, weight, brand preference or just appearance, but I know flex and stiffness must come into it for some people. I, for one, have noticed significant differences in stiffness between sets of bars, but didn’t really know why or how these elements can be controlled.

To find out more, I quizzed two top product designers and one pro rider…


AGGIE LAU

Seventies Distribution


– As a product designer, what kinds of stresses on parts do you have to consider?

“I start with thinking about how parts fit together, whether anything stretches or compresses any other parts – for example how a stem attaches to forks, if it’s a typical pinch bolt clamp system then you think about how it will stretch and bend slightly on the sides before it has even been ridden, then you have to think about if you add any recess or cut outs and how this will react to any actual riding forces.

Also, with the bike landing from 360s and whips, that can cause parts to flex more than just the typical front to back and side to side machine tests that are carried out in the factory.”

– How does flex play a role in the design of frames, forks and bars? 

“A certain amount of flex can act like a damper and reduce the amount of force being applied to a critical area such as a weld, lowering the likeliness of failure (though this is dependent on material). It is possible to design a part to be ‘too strong’ and actually have this work against the part – putting too much strain on joints or clamps etc.

“360s and tailwhips can cause parts to flex more than just the typical front to back and side to side machine tests that are carried out in the factory.”

Another thing to consider with flex is clearance, for example on a set of cranks you can expect there to still be a small amount of flex, so you can’t have the chainstays of a frame come out too far, otherwise the end of the crank can hit the frame when riding even if the gap looks perfect off the bike.”


– What would happen if there wasn’t enough flex? What would happen if there was too much? How do you control this?

“If a part was really rigid and had no give at all then it can transfer riding forces into other areas and cause them to fail or it may snap or crack itself. During the lightweight craze there were some frames which had really thin tubing to get the weight much lower, then to counter the amount of increased flex and how easily these could be dented, these frames were then post weld heat treated and a lot of these frames ended up with too little flex. That, combined with the thinner material, cracked more easily compared to regular frames.

On the other hand if a part has too much flex it could also be dangerous, for example if a stem flexes too much it could cause the bars to slip suddenly on a harsh landing. There are many ways to control this factor, if it is a frame or some kind of tubular construction then you could increase the wall thickness of the tubing in areas, increase the diameter of the tubing or both.

You can also heat treat to either harden or soften the material, this can have quite a big effect, un-heat treated bars tend to bend much more easily than heat treated bars. You can also control flex with just changes to the shape of a part with recesses, cutouts or adding ribs and by even choosing different productions methods such as forging something over CNC machining it.”


– Do you think certain riders experience flex in their bikes more than others?

“I think there are certain types of riding that cause more flex in a bike than others but then it comes down to how the individual interprets this or if they even notice it. For example someone that does big gaps or drops is going to put more strain on a bike but also at the same time they may be more used to how a bike feels in these conditions, they’ve probably learned to help absorb these kinds of impacts with their body so whilst the bike flexes more they might perceive this less? I also think that some people have some degree of preference without realising it – you can see this by the various different tyre pressures which sort of make a bike feel stiffer or softer.”

– Are there any specific technologies you use that have a direct effect on stiffness? Such as ovalized tubes, gussets, etc. Could you explain them?

“We use different processes depending on the part, for frames we tend to tailor the thickness and diameter of each tube as necessary, taking into account not just flex but also how it’s ridden – the chainstays are thicker compared to the seatstays because even though the CS tube length is shorter (and therefore stiffer already) it’s more likely to get damaged from grinds and stuff. Frames are mostly triangle structures which are pretty good shapes when it comes down to flexing/stiffness (you also see this triangle formation in other things like bridges etc.) and usually just benefit from a little more support in the corners.

“You can also control flex with changes to the shape of a part with recesses, cutouts or adding ribs.”

This is where having headtube gussets comes in, they help to prevent the bars and forks which act like a lever from flexing the headtube too much, which could cause the frame to crack. Parts like bars, forks or cranks have more of a singular formation with less corners/welds and so benefit better from adding heat treatment to prevent bending and reduce the amount of unwanted flex. Cranks can also really benefit from different tube shapes with ovalized or rounded square shapes being used to make the crank stiffer as well resist twisting.

As mentioned earlier, parts can be stiffened by choosing different production methods – some parts can be forged which basically compresses the material while shaping it, making the part denser – crudely you have a part that is effectively thicker even though it’s the same size as if it was made by CNC, you also have better orientation of grain structure which is a big advantage. Though typically forging is more expensive and the strength/stiffness increase may not be necessary (also the weight increases).”


 


GREG ILLINGWORTH


– You’re one of the few riders I’ve seen really consider and talk about the amount of flex in BMX parts. You mentioned your bars having the right amount of flex… What’s the deal?

“The drawings and designs only showed the measurements and geometry, not the flex. I had always noticed bars flexing slightly through the years of trying different pairs out, so I knew they would flex a little. As bars have progressively increased in size so too has the amount the bars flex.”

– How does flex play a part in your riding?

“Well as far as the bars go, I think it soaks up some of the impact when I land harder. Perhaps it soaks up a little energy when I’m pumping too but that would be a tiny amount as most of the force when you pump would transfer from my legs to the frame and wheels.

“I notice a difference in flex the most when I build up a new frame after riding one for a long time.”

I notice flex the most when I build up a new frame after riding one for a long time. If I ride a frame for a year and then build up a new one with the same parts, I can immediately feel that the new frame is more rigid and therefore a bit faster and more responsive.”

– Do you think most riders don’t actually think about flex?

“Yes, I’d say the majority of riders don’t think about it too much… and probably rightfully so. I don’t think it would be worth nitpicking over unless you are really riding on the limit of a bike’s abilities. What I mean by that is, when you are doing massive drops or riding ramps really fast and pumping super hard. For the majority of people who ride BMX it wouldn’t factor in too much.”

– Do you think certain riders experience flex in their bikes more than others?

“Yes, depending on how much you are pushing the bike and what you are riding. I think you’d feel it most riding ramps at higher speeds.”

– Do you know what would happen if there wasn’t enough flex? And what would happen / how it would feel if there was too much?

“If there were no flex in any parts I think two things would happen. One would be that parts would break more often and the other would be that our wrists and ankles would take more impact and probably hurt more.

If there was too much flex in frames and bars then the bikes feel softer to ride, in turn make them slower and less responsive. I suppose having more flex might open them up to bending more.”


 


GEORGE FRENCH

GSPORT


– As a product designer, what kinds of stresses on parts do you have to consider?

“All of them. We have to consider everything; from that huge sudden impact when something goes horribly wrong and a rider stops very fast into the face of a jump or at the base of a flight of steps; to the million little cycles from pedaling or bunnyhopping a curb…and of course everything inbetween. Tailwhip and 360 landings are a particular issue that a lot of brands don’t consider, judging by the designs that are available. It is important to look at instances where the bike lands at a strange angle rather than just seeing loads on the plane of the bike.”

– How does flex play a role in the design of frames, forks and bars?

“The road bike industry loves to use the phrase ‘laterally stiff and vertically compliant’ but then they try to claim that for everything. The truth is that torsional stiffness generally needs to be maximised, whereas for other modes there is a sweet spot. If it is too stiff then you make the bike feel ‘dead’, but not stiff enough and the bike rides like a piece of brie.”

– What would happen if there wasn’t enough flex? What would happen if there was too much? How do you control this?

“Too little flex puts the loads through the roof, when you land hard you have energy to get rid of and flex is how you do it. So you can have a thick section flex a tiny bit (but the thick section can cope with the loads) but the components like pedals and bars have to apply that load, so they can suffer as a result; plus of course, your wrists and ankles have to apply it in the first place, so they can suffer too!

“If it is too stiff then you make the bike feel ‘dead’, but not stiff enough and the bike rides like a piece of brie.”

Too much flex (or a lack of stiffness) starts to promote fatigue, but it also feels crappy and wastes your energy. There is only so much that a material can flex elastically before it begins to fail, and in the case of a material like aluminium, that has no fatigue limit, every single load is slowly breaking the part. Part failure is obviously something that we want to avoid as far as possible, so fatigue is a huge consideration and way too complex to cover here. But feel is important too. If the bike is flexing too much then every bunnyhop will be that much lower, every pedal stoke will be that much harder, and every pump will gain that much less speed as you waste energy fatiguing (breaking) the bike.


Taken to the extreme, too much flex is just the bike bending, permanently, and it isn’t at all uncommon. Look at many used bikes and they will often be bent. You can see cranks much closer to one chainstay than the other, or the tyre closer to the left chainstay and right seatstay, all warnings that the frame (and/or crank) might be bent. Sometimes you can just eyeball along the bike and see that the front wheel and back wheel simply don’t line up!”

– Do you think certain riders experience flex in their bikes more than others?

“Sure. It’s pretty obvious that if you weigh 50kgs or 150kgs its going to make a huge difference to how stiff the bike feels, however tyre choice and pressure is a huge part of the ‘feel’ and luckily it’s also a part that you can fine tune very easily.

“Sometimes you can just eyeball along the bike and see that the front wheel and back wheel simply don’t line up!”

We used to have no choice on tyre width and put our tyres up to 120psi and just leave them there, but the surface you are riding and your weight should dictate it. Wider tyres allow you to run higher pressures while feeling softer on landings for example. A perfectly smooth vert ramp will work great with tyres maxed out, but a softer tyre will actually roll faster over a slightly bumpy surface like rough tarmac.”

– Do you think most people don’t actually consider flex?

“Probably not. It would be a huge job to try to explain all this in marketing, so as designers and engineers our job is just to try to make it as ‘good’ as we can and rely on reputation and word-of-mouth that the results of our work will speak for themselves.

It’s very easy for a new bike company to sell a cheap bike with a great spec list and if riders only look at that spec list then they will sell quite a few, but the customer is inevitably disappointed when it doesn’t ride as well as the brands that have a reputation for making stuff that rides nicely.”

– What specific technologies you use that have a direct effect on stiffness? Could you explain them?

“Strangely almost none. Stiffness is amazingly constant. Many brands will often try to claim increased stiffness from things like heat-treating, when in reality these things actually have virtually no effect. All that really matters is geometry. Not the geometry as in the head tube angle and BB height, but as in the layout, diameter, wall thickness and lengths of the tubes. A millimetre on a tube diameter, a tenth or even a hundredth of a millimetre on wall thickness, a bit longer section of butting, the spacing of the top and down tubes, these are the things that really matter, especially in the rear triangle where chainstay bends and bridge designs can have a huge influence.

Having said this, butting the tubes is a great way to go. Many see it as an expensive luxury but it’s a great way to control the amount of flex, but more importantly where that flex happens. It keeps it away from the vulnerable weld areas and places it in long volumes of material that can share the job, and therefore not suffer from the consequences of concentrated loads.”


 

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