Are freewheels really that prone and susceptible to bent axles?

Question

I'm 90kg (198pounds) and I have a 27.5 hard tail mtb 9 speed freewheel and it is quick release. I only use my mtb as a commuter on roads and never on trail. I run my tires at 35-40psi.

Ever since i bought the bike, the stock axle lasted about 1000-1500 kilometers before my rear wheel started rubbing against my frame. I got it checked out and replaced at my local bike shop cus it was indeed a bent axle. After they had replaced it and replaced my bearings, it only lasted 200km befofe it got bent again. I went back had them replace it with a new one again and same thing happened, only lasted 200km. I tried getting it replaced at another bike shop near me and this time it ony lasted 100km before it got bent.

Is this because of my weight? Or because my tires are too hard? Or are freewheels just prone to bent axles especially when you have more gears?

I have read somewhere that the greater the speed (7-9speed) the more it is prone to having a bent axle if it is using a freewheel.

I do plan on upgrading to a cassette type hub. I just fear that i may still bend my rear axle even after upgrading my hubs. That's why I want to know if there are other factors causing my axle to bend and will it it still occur if I upgrade to a cassette type hub due to my weight or tire pressure (if those are factors causing or contributing to a bent axle)

Sorry for the long post. Thanks in advance of your answers.

Answer 1

It almost certainly won't happen with a cassette hub. It can happen on standard axle cassette hubs, but it takes a lot, and there's basically always a dropout alignment problem at work in addition to heavy loads over time when it does.

Eight and nine speed standard axle (M10 or 3/8") freewheel hubs are the definition of a cynical design. They work terribly and exist only because hubs are fairly reliably overlooked by buyers of entry-level bikes. They're an utterly crass means for product managers to jam more bullet point type features into a given price point. They are that bad. Light riders are spared some of the pain, but what you're dealing with is common for heavier riders who wind up with them.

Five and six speed standard axle freewheel hubs put much less leverage on the axle where it protrudes from the locknut, and have a pretty minor version of the problem, even for a somewhat heavy rider. Seven is kind of in the middle but is not that problematic for most.

You could put in a solid Wheels Mfg chromoly axle, which means switching to nuts instead of QR, and it will mitigate the issues a lot. It's unfortunate if you haven't had that presented to you as an option. I don't necessarily recommend it though, compared to just putting the money towards a new wheel and cassette.

Answer 2

Is this because of my weight? Or because my tires are too hard? Or are freewheels just prone to bent axles especially when you have more gears?

All of these.

Weight is the cause of broken bicycle components. If you imagine that you never put any weight on your bike (i.e. never ride your bike), it won't fail. The more weight you put on it, the more likely it is to fail.

Those having a higher weight should ideally use wide tires pumped to lower pressures so that the tires cushion the peak loads on the bike. I ignore that advice: I weight more than you do (110 kg), I use 28mm tires pumped to 7 bar, and I have found that wheels having less than 36 spokes don't last for me -- their spoke nipples start to unscrew.

However, the true reason of bent axles is a fundamental flaw in some axles, especially those used in freewheel systems as opposed to freehub systems. The right side bearing is too far from the end of the axle.

Shimano fixed this issue with the introduction of the freehub / cassette system, where the right side bearing is very close to the end of the axle.

Many hub manufacturers copied the Shimano freehub / cassette system. Not all put the right side bearing at the end of the axle (possibly due to wanting to avoid infringing some patents).

When I used to weigh about 70 kg, I bent a cassette hub axle. It was a no name brand, "Black Comp". The cause was that even though it was a cassette / freehub system, the right side bearing was not at the end of the axle.

So if you buy a freehub / cassette system to replace the freewheel system, be sure to purchase a Shimano one. If not, do at least verify that the right side bearing is at the end of the axle.