The left crank has come off of my Pro Form 4.0 RT Stationary Bike. It is about 8-10 years old although up until this year, I would estimate I put only 200-300 km in total on it. This year I have put 800 km on and most of that in the past 6 weeks. I normally ride at a resistance setting of 12 and in recent weeks, I was really pushing my self to pedal harder to get my speed up. Prior to the crank coming off I noticed a repeating clicking (more of a repeating thud and I could feel it when I pedalled). This was only on the left hand side. The right is smooth as silk. To be clear, the problem is where the crank connects to the bike (the pedal connection to the crank is fine)

I eventually tried to see what was wrong and after a lot of attempts tried to tighten the crank with a hexagon ratchet screw driver. The first time, I simply tightened it and I thought everything was good. But very quickly, it got loose and the screw came right out. I put it back in and tightened again and then the same problem occurred again. When I look at the bolt it does seem ok but the Crank end that is held in (where it connects) does seem to have some rounding to it on one of the corners and there are some medal shavings on the floor (though not a alot).

So I am looking for advice on how I could fix this. I would note that I am not mechanically inclined and normally just take these things into the shop. However, in this case, it would be very difficult to get the stationary bike out of the house and I worry if I dismantle it, I won't be able to get it back together. Plain language would be helpful to me as I did not even know the part was called a crank until I starting looking for help. All help is much appreciated. Crank 1 Crank 2Connect to Stationary BikeBolt 1Bolt 2

interface 1 interface 2

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    Welcome to Bicycles Stack Exchange. We see people asking about cranks coming off square taper axles quite frequently here so I've voted to close your question as a duplicate. TLDR the crank is wrecked and needs to be replaced. Once the square taper hole in the crank starts to deform (and yours is quite bad) it's impossible to get them to stay on tight and the problem gets progressively worse. – Argenti Apparatus Oct 6 '20 at 17:32
  • Hi Argenti. Thanks for the suggestion. It does in part but I am still looking for help. Is it clear from the photos that I need to replace the crank arm? Is that all I would have to replace or are other repairs needed? Is this a repair, a raw amateur like me, could probably do at home? Lastly, if needed a new crank arm (or other parts) where would you suggest buying them? I live in Ottawa Canada - am I likely to find a place that would carry these items. Any help you can give me is appreciated. – Jodey Oct 6 '20 at 17:43
  • What you should do is edit your question to emphasize the parts about what to do next to fix the problem. Also please take a new photo of the axle with the square taper part in focus. There may be damage but I can't rule it out. – Argenti Apparatus Oct 6 '20 at 18:13

As mentioned in comments the crank arm is wrecked and needs to be replaced. Once the square taper hole in the crank starts to deform (and yours is quite bad) it's impossible to get them to stay on tight and the problem gets progressively worse.

If the axle square taper is undamaged you should be able to simply purchase replacement crank arms. The square taper interface is standardized. There are a a few complications for someone not used or equipped for working on bikes.

  1. There are two similar square taper interface standards JIS and ISO. The angle is the same but JIS is slightly longer. The vast majority of bike components use JIS, but it's worth checking. A JIS crank will bottom out on an ISO taper.

  2. The Pro Form 4.0 RT stationary bike has all the drive stuff hidden away internally and used two 'left hand' cranks. If you wanted matching cranks you have to buy two left hand ones.

  3. To get the right hand crank off off you need a special crank puller tool. Video shows how they work but there are various types available.

  4. The retailing bolts need to be tightened quite hard (~30NM). You really need a torque wrench to make sure you get them done up tight enough and avoid repeating the problem.

  • So, the axle in question is almost certainly made of steel, and the cranks are most likely aluminum. Steel is much harder than aluminum, so it seems to me that it's pretty likely that the axle is undamaged. In principle, another possibility for the axle is titanium, but that's exceedingly unlikely (those would be a weight weenie sort of item, probably not installed on stationary bikes), and anyway ti is still harder than aluminum. – Weiwen Ng Oct 6 '20 at 19:12
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    @WeiwenNg There's something about the axle that maybe looks like damage on the upper left hand surface - it might just be a mark though. I just noticed the bolt-hole is off-center which might be a contributing factor to the crank coming loose – Argenti Apparatus Oct 6 '20 at 19:55
  • Thanks very much for such a thorough answer Argenti. I have looked online and see that a pair of replacement cranks for the RT 4.0 are about $75 US. From what I gather, the taper interfaces have standards (thanks for the link) but the cranks do not? I will try to add a picture of the interface on my bike. I am trying to decide whether this is worth trying to fix or if I should simply buy something new. I would appreciate your thoughts and will try to add in a few photos of the share taper interface. – Jodey Oct 6 '20 at 21:24
  • @Jodey Both the axle and the cranks conform to a square taper standard, either JIS or ISO, A JIS crank will not work properly on an ISO axle, and vice versa – Argenti Apparatus Oct 6 '20 at 21:47
  • Does OP need specific cranks? $75USD is way too much for the caliber of part you’re looking for. – MaplePanda Oct 6 '20 at 23:37

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