Help With Replacing Crank Arm

Hello, I’m making this post because my partner has always had a struggle with being comfortable on the bike. Her dad passed down his tri-bike to her, and it’s been modified and she dealt with some discomfort all last year. She finally went to a respectable bike fitter, and the bike feels better than ever.

However, we cannot afford to spend $700 on a bottom bracket and a crank arm. We are college students so we don’t have lots of money to spend on this new hobby. The bike fitter says we need a 155mm crank arm that has a 34/50 chainring (not sure if I said this right), this is the one she recommended: Alloy 24mm Short Crankset-155mm – Speed&Comfort

My question is, is this really what is necessary? Is there not any other way to make this cheaper. Like maybe a slightly bigger crank and raising the seat? Or a used crank? Ultimately, are there any alternatives or is this our reality? Thanks in advance.

Can’t speak to your specific situation but any bike fitter I’ve felt with in the past has been very conscious and understanding of budgetary constraints when considering any changes that may have been required and working that out with the client.

Do you have a post fit dialogue open with your fitter? The first thing I would do would be to raise my concerns and see what they can advise.

Used crankset is the way to go. If you’re lucky you’ll be able to find something suitable and sell your existing crankset for similar money so it’s a cost neutral switch. Though 155mm is a pretty unusual size, which may either work against you (limited supply) or in your favour (limited demand!). What cranks and bottom bracket do you currently have on the bike?

As to whether it’s worth switching, if you trust the bike fitter I’d say yes. Shorter cranks can really help you get more comfortable in an aero position.

Well those cranks use a standard 24mm spindle so really you could find any crankset to fit.

155mm is very short for adult cranks but if you search for children’s/youth cranksets you should be able to find more options. $400 for a crankset is a lot, I mean a lot. A quick search (UK based) for 155mm cranks gives a Hope crankset as the most expensive option at £160 or about $200 but there’s quite a few at half that price so sub $100. You don’t say what sort of BB you have, it will either be BSA or Press-fit, but a reasonable BB will be in the range of $30. The hardest part in fitting will be the BB, the crankset should just slide in after that :wink: but depending on how old the rest of your drivetrain is you may also need a new chain and cassette.

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Used cranks are perfectly fine, its hard to break a crank arm and most people will sell because they have a bike with a different BB or because of scuffmarks etc. 155mm is very short though, so might be hard to find.

Beware of used chainrings, they might be past their useful life and won’t work well with your existing chain and cassette.
You can replace chainrings, the cheap ones on ebay are often fine. Check how many chainring bolts the cranks use (4 or 5), and what the bolt circle diameter (BCD) is, the diameter between two opposite bolts. (You can measure or it might say on the cranks/chainrings). Then search for chainrings with those measurements and the amount of teeth you need.

Why were you told you need a new BB? What is the current BB?

Unless its Hollowtech. That type aside I would expect them to last forever (or at least last me out) if maintained. The chainrings could worn so thats something for the op to look for.

@jasonandersonjr what have you got at the moment ? If they’re vastly different and depending your goals it could be worth it and the aforementioned 2nd hand is the way to go if you need to budget.

What size cranks are currently fitted?

Don’t really want to be second guessing a fitter without seeing your partner or her bike but my initial thought is that a far more common 165mm crank might be more affordable and do the job assuming she is currently running a 172.5 or a 175mm.

I think Shimano 105 goes down to 160mm?

Whether it is necessary it is hard to say. I went from 172.5 mm cranks to 165 mm cranks and it significantly increased comfort for me.

Like the others have said, go for a used crank, although 155 mm cranks are rare. If you have a Shimano groupset, anything from 105 up (with the same number of gears) will work. If you can’t find 155 mm, try 160 mm or 165 mm. The latter should me more common, especially on smaller bikes. The other issue is BB standards.

Have a look at other crankset manufacturers like Rotor or SRAM. Rotor makes excellent cranks and they are modular. So you can replace the crank arms separately from the rest.

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This is a counter-intuitive thing: if you go with a longer crank, the seat will need to be lower, not higher. It helps to imagine you’re looking at the rider with their leg fully extended. Longer crank = lower seat and vice versa.

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Does she even need a triathlon bike, or do you mean a trike? If I never did triathlons, I would never willingly choose to ride a tri-bike. The same can be said for TT bikes.

You should ask your fitter. It should depend on a number of things, leg measurements, hip angles, and range of motion. Longer crank arms mean the leg comes up higher, and if beyond her range of motion, she’s going to deform her body to get the leg up. If your partner has short legs, this may be an issue if using 175mm cranks, for instance.

Some fitters have an adjustable bike, and they can analyze the leg and hip motion with different sized cranks.

That’s usually the best alternative.

There are short crank modifications. I used those for my kids. They are cheaper. Lost of folks use these. Short Bicycle Cranks

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You want your maximum leg extension angle at the bottom to remain approximately the same regardless of crank length. So if you fit longer cranks your leg wants to stretch more so you need to lower your saddle to keep it the same as before the crank change.

So if the fitter set her up to one height on 155mm cranks. She may be comfortable on 165mm crank but after lowering the saddle circa 10mm from what the fitter said.

Picture of existing set up would help

Yes, my partner has had a conversation with them. It seems like the situation was just this crank is necessary for her specific anatomy. At first I was skeptical, but it does seem like the case. I think we are planning on finding a used option. Thanks!

Currently, I am not sure what she has on her bike. Thanks for the reply, I think we are going to go with a used option. Her crank is much longer than 155mm for sure, not sure about the BB.

I think this is what we are finding out. She’s not a fan of the triathlon bike. I think at the time, we jumped into triathlon and didn’t want to spend a lot of money. We are actually now considering a road bike.

Most smaller road bikes use 170 mm or perhaps 165 mm cranks.