Why would a stationary exercise bike be subject to greater stress than a road bike?

My indoor exercise cycle specifically cautions against using pedals which are not designated as being strong enough for indoor cycles. Supposedly, the stresses put on pedals on exercise bikes are greater than those on real bikes.

It is not apparent to me why a stationary bike would necessarily have to cope with more stress than a road bike. Assuming the same rider at the same cadence, the input force at the pedals would seem to be the same. So what might account for greater stress within the pedal or pedal attachment?

Granted, there are numerous other differences between a stationary bike & a road bike. Aside from probably very different construction, the stationary bike has to dissipate all the input energy somehow, probably as heat, which a road bike doesn't really do. But still, I don't see how this affects the pedals, specifically.

A possibility is the assumption that a road bike frame + tires absorbs some amount of the "shock" of riding. But pedals on a road bike also have to cope with bumps & other external shocks, so again its not obvious this would reduce the maximum stress they would experience.

• Would Engineering be a better home for this question? Aug 12, 2020 at 19:33

The power required from the biker is $$P = T\omega$$, where $$T$$ is the torque done on the axis attached to the pedals, and $$\omega$$ is its angular velocity.

I can think of a reason for the remark of the supplier:

A difference in the stationary bike is the absence of a minimum speed to avoid falling. So it is possible to set the torque to high levels (by some type of friction device for example), if the target is to work legs muscles.

That way, the exercise could move from the aerobic (low torque and high speed) to a kind of body building range.

In a bike with several gear ratios, the user could in theory do the same by using the wrong gear ratio while climbing. But I suppose nobody does that.

Most cyclists do not spend all their time on a road bike applying maximum force on the pedals. When cruising at constant speed, the bending moment at the join between the pedal shaft and the crank arm is much lower than when you are straining with all your might to, for example, put the bike quickly into motion from a standstill.

On an exercise bike, the point is to load the rider down as much as he or she can stand, after which the exerciser must then maintain that load for as long as possible. This means that the exercise bike's pedal/crank arm join will be sustaining greater stress levels on average than the road bike will.

Typically, the shaft running through the pedal (that screws into the end of the crank arm) has the inner ball bearing races ground into its circumference, which means that the entire shaft is made out of bearing race steel which is extremely strong and tough material. If it were not, the shaft would bend the first time you put your weight on it.

• Re, "Most cyclists do not spend all their time on a road bike applying maximum force on the pedals." I challenge that assertion. If you're out riding on the open road, what is there to stop you from riding as hard as you like? Of course, there are different reasons for riding out on the road, and some riders might not ride as hard as they would ride a stationary bike, but I'm pretty sure there are others out there on the road who do it just for the exercise. Aug 12, 2020 at 21:52
• One-word answer: Age ;-) Aug 13, 2020 at 2:20