How bad is switching derailleur gears under load? I have heard several times the wisdom that you should not switch derailleur gears while pedalling hard (&quot;under load&quot;). The &quot;right way&quot;, according to those people, is to pick up speed, stop pedalling, then switch gears, then pedal lightly (without any force actually pulling the bike) until the chain switches, and only then can you resume pedalling with full power.
Otherwise Unspeakably Bad Things are supposed to happen.
My wife however doubts this, and upon a little thought I have to admit, I don't see anything that could go wrong either. But then neither of us are very experienced.
So... how bad is it to switch gears while putting a lot of force on the chain? Is it total bullshit; will the bicycle explode immediately; or something inbetween?
The answer heavily depends on the groupset construction/generation and personal perception of what is &quot;bad&quot;, among other factors. Below are a few aspects that I am aware of.

Switching gears in the traditional bicycle drivetrain assumes there is a momentarily side load on the chain, twisting it and forcing it to jump/fall from one cog to another. The chain is very flexible in one plane, but has limited sideways flex before the metal becomes too stressed. In the worst case, &quot;Unspeakably Bad Things&quot; is one of links of the chain breaking because of that excessive side load. Usually, bad shifting manifests itself as horrible metal clunking and abrupt pedal jerking, but nothing breaks (yet).

Aside from the chain, similar side load is experienced by the cassette and chainrings. Similarly, a potential for bending individual teeth on them exists.

The probability of a mechanical problem during shifting can be increased by several factors. Among them are: front-shifting while loading the pedals, back-shifting while pedaling hard, weak pin in the chain, fast shifting over many gears at once causing the chain to twist too much; misaligned derailleurs; worn drivetrain components; bad luck.

Older (more than ~40 years old) cassettes were made of plain cogs without any features to help shifting. Recent cassettes and chainring have ramps and pins that assist shifting, making the opportunity window for chain overload smaller. Most modern MTB cassette designs were reported to greatly alleviate the problems with shifting under load in either direction, according to many reports I've seen on the net. Even for cheap Deore 12-speed cassettes, it is said to be safe to shift without taking it easy on the pedals.

Your description of events &quot;to pick up speed, stop pedaling, then switch gears, then continuing pedaling&quot; applies to internal gear hubs, but not to the cassettes/derailleur designs. In order to complete the shift, one must keep the chain moving over the cogs, which means pedaling (even if with less effort) during the whole shifting process.


In terms of practical advice, I'd say that there are several situations.

It is usually safer to shift to smaller front or rear cogs while pedaling hard than the other way around.
It is usually safer to shift at the back than at the front under load. The twisting angle at the cassette is usually smaller than it would be at the chainring.
It is recommended to be conscious about shifting when pedaling uphill. The pedaling torque is the highest in such situations. If possible, one should choose the right gear combination just before starting the uphill movement, at the trajectory's lowest point. Making such predictions requires some experience.
Similarly, never shift while standing still. The torque is high at the starting point, the chain may be cross-shifted over multiple cogs, making it more twisted than usual. If you cannot safely start in current gear, it is safer to lift the bike and pedal &quot;it in the air&quot; with your hand to complete the shift.