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This question bothers me for quite a while and is about bicycle frame construction, specifically a BMX frame and even more specifically it's front triangle. It is a heavy duty frame by definition, and most common point of frame failure in this area, is lowest point around area D - and may happen even if type1 gusset is present. It's usually a crack along weld (which, if unnoticed, may result in tube C snapping off upon next considerable impact and is highly unwanted). gusset comparison I am looking into building a frame with type2 gusset, which appears more reliable overall, but unfortunately lack knowledge to confirm this.

I tend to think additional heat stress from second weld during type1 gusset install does not do good and useful weld surface does not increase a lot.

With type2 gusset, however, useful weld surface increases considerably, also holding all three tubes together, which should relieve stress on area D as well.

However, few reputable people suggest type2 construction would still need type1 gusset between tubes B and C, but cannot back up their argument with real data.

To summarize - by what amount does type1 gusset does actually improve situation here, and would it be any use along with type2 gusset? I would rather increase tube B wall thickness around weld area rather than add type1 gusset, if this proves an issue.

Additional info:

Frame tube material is 4130 chromoly steel, TIG welded. Tubes A and B are usually around 35~38mm outer diameter and have 1.25mm thick wall. Tube C wall is usually 2-3mm thick.

Frame experiences repeated cycles of 'sharp' dynamic loads (i.e. handling rider mass upon landing from height).

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  • $\begingroup$ First, why are these gusseted? How do they fail? Have photos of failures? From what type of load? A crash, or just under normal use? A hard landing?..where is the load applied? Then, you can design the joint to accommodate that loading.. $\endgroup$
    – GisMofx
    Dec 16, 2017 at 3:21
  • $\begingroup$ Is the frame designed to take the landing loads now applied or, just a design (by the victorians way back when) that has just been copied as “this is what we use”.... $\endgroup$
    – Solar Mike
    Dec 16, 2017 at 5:12
  • $\begingroup$ A third type is a straight plate gusset welded in the center between A/B/C filling the corner and running about 50-100 mm aft of the headtube. Sometimes drilled for weight. bmxsociety.com/uploads/monthly_02_2011/… $\endgroup$
    – Criggie
    Dec 16, 2017 at 8:06
  • $\begingroup$ Here's a fourth way to reinforce that area of the frame: i.stack.imgur.com/K3tuj.jpg but it would leave the head tube really short. $\endgroup$
    – Criggie
    Dec 17, 2017 at 9:09
  • $\begingroup$ I would guess the reinforcements in the picture would make type 1 the stronger connection. In all cases the weld heat-affected-zone will reduce the steel tensile to about 100.000 psi. If heat-treated tubes are used , they likely start with a strength of 200.000 psi. Lugged frames have been used to avoid the softening caused by welding, they are brazed at temperatures below the range that will soften the steel. $\endgroup$
    – blacksmith37
    Dec 18, 2017 at 23:03

1 Answer 1

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This answer is from the riding/cycling side, not an engineering view.

If you think of the forces involved on a head tube, then the "static" force ( while it is standing still) is upward through the head tube. The top tube is under compression and the downtube is under tension.

When riding, its roughly the same, but there are sideways forces from steering which are approximately balanced by the rider's leaning. Plus minor oscillations in he force magnitude due to pedalling.

That's all well and good for a nana bike ridden gently, but BMXs have additional forces.

  • Standing to pedal puts an opposite force through the handlebars (pulling on the bars for more power) A BMX rider in a sprint start could be pushing 1000 to 1500 watts through the frame for 10 seconds.
  • Landings - a big increase in the static forces.
  • Potholes and lumps - the wheel will push upward and backward if the bike is ridden into a pothole or a berm/curb/speed-bump etc
  • Crashes - everything happens in a crash, tension compression rotation and vibration.

Another relevant stress situation is a frontal impact. Here's a road bike that has bashed into something with its front wheel (IE, straight line impact with a non-moving object.)

From http://www.ellisbriggscycles.co.uk/buying-a-secondhand-steel-frame/

Note the subtle top tube bend, and the more significant downtube bend. With time, both sites will become a ring of rust. The fork and front wheel may have been a write-off, or they mat have survived largely unscathed - both are possible.

Why? The welding process on steel will harden the area of the weld, and increase its brittleness. So the front section of top/downtube is less flexible, and its more likely to bend at the first soft bit.

The gusset helps stiffen and protect that area of the bike's frame, and the leverage that can be exerted on the first softer section is reduced.

You don't want a bike to be all stiff, because then it would be uncomforable to ride - one of the plusses of a steel bike is that it has give, or flex, to take the buzz out of the road. An aluminium frame doesn't do that as well.

The pictured road bike also has downtube shifter bosses brazed or welded into place, which probably provided a gathering point for stresses and encouraged the larger bend under load.

Also notice this frame is lugged which is the ornate couplers around the tube joints. These do a similar job to the gusset.

As a rider - the last thing you want to happen is have your bike become a spontaneous unicycle. If any major part of it breaks, you're probably going to hit the road. This is to be avoided at all costs, even if it means a small weight penalty. Teeth don't grow back.

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  • $\begingroup$ Curiously, I couldn't find a good photo of a BMX with a bent frame due to frontal impact. That alone implies either "BMXs aren't ridden in ways where frontal impacts are possible (ie tracks and racing vs roads) " or "BMXs don't tend to bend the same way" $\endgroup$
    – Criggie
    Dec 16, 2017 at 8:09
  • $\begingroup$ A lugged frame design is used to avoid the softening affect caused by welding . The lug connections are brazed so the tubing temperature does not get hot enough to soften the steel . $\endgroup$
    – blacksmith37
    Dec 18, 2017 at 23:04

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