choddo

Hi all

I’ve taken off this old square taper prowheel crank



This is the new Claris RS200 crank - you can see the chainrings are mounted outboard of the crank, compared to the old one.



I’m guessing I don’t solve this by having the front mech waving out to the right but need a shorter axle? It’s a 68x119 on there now.

Is there a way to figure out what length axle I should replace it with? The fd cage almost seems to be aligned with the small ring when it’s shifted to the large. At the same time, it doesn’t look like I have a lot of margin to play with between the crank and the BB shell. This isn’t a road vs MTB crank design incompatibility is it?

Iride01 

The two cranks might be made for bikes with differing chain lines. What is the rear drop out spacing on that bike and how many cogs on the rear? That is what will determine the chain line or what the crank needs to be.

Though with square tapers, there wasn't much standardization for how cranks fit to both the bike and the square taper back when they came out. Even if you have the correct square taper standard, then depending on the crank set brand, they may have designed if to be on a different length BB. So if you need to get the same chain line, measure difference you see in the pics and that is how much shorter on that side your spindle needs to be. But it almost looks like that crank might not work with your width of BB shell.

wheelreason

Calls for a 110 spindel for most bikes. And yes, there might be an issue putting a road crank on a MTB, but one can usually make it work with enough duct tape and bubble gum...

Bill Kapaun

Apparently that crank requires a 110mm Bottom Bracket.

You need to look up the crank to find the correct BB length.
Incorrect length and the FDER will not be in the correct position.

choddo

Thanks all. 9mm, if that’s all coming off the right end(!) looks like it might do it. Cheap enough to be worth a try anyway!

The frame’s a road frame. Old 7 speed freewheel on the back that weighs about the same as a steam engine.

Love how fast you get help and advice on here.

Bill Kapaun

If you mean cutting off the end of the spindle, that won't do anything.
The TAPER will prevent it from making any difference.

choddo

No I meant if the delta between the 119 and 110 axles was all on the drive side. Which I’m sure it’s not. 4.5mm shorter on each side? Never considered it before as I have never had to swap out a square taper crankset.

maddog34

i build up bare frames fairly regularly... i classify cranks in three categories... innies, outies, and flatties... your newcrank in an outty...the three most common road Bottom brackets are 118, 113, and 110...
i'd try the 110.
"Q-Factor" refers to the Pedal mounting width compared to any given frame..... that is not a chainline, which is what you have imaged.

From Wikipedia, the free encyclopediaThe Q factor is an important factor affecting how close the pedals are laterally, and therefore the affects the spread of the riders legsThe Q factor or quack factor[1][2] of a bicycle is the distance between the pedal attachment points on the crank arms, when measured parallel to the bottom bracket axle.[3] It may also be referred to as the "tread" of the crankset.

i prefer a q-factor that has the pedal ends of the cranks at least 1/4" away from the chainstays... 1/2" clearance is best.

KCT1986

Here is the specs of common cart. BBs. This is from Tange, Shimano is similar/same for most lengths.




Per Shimano, their 110mm should give you about a 43.5mm chainline with your crank, standard for road bikes.

choddo

That’s really interesting! Thanks.

dedhed

Says 110mm for a 43.5mm chainline.

​​​​​​https://bike.shimano.com/en-EU/produ.../FC-RS200.html

Bill Kapaun

For $13, throw on the correct BB-
https://www.amazon.com/SHIMANO-Squar...BwE&th=1&psc=1

choddo

Yeah, that’s the one I’ve ordered after the earlier comments - 68x110 version

choddo

Put the old bb & crank back on and measured it. Looks like that was pretty close to 43.5 (I measured it at 44) so hopefully I’m in business!

Kontact

There really aren't a lot of different chainlines. Road doubles are all the same.

Spindles can be asymmetric or symmetric. Modern cranks are usually the latter.

79pmooney

[QUOTE=Kontact;23006670]There really aren't a lot of different chainlines. Road doubles are all the same.

Spindles can be asymmetric or symmetric. Modern cranks are usually the latter.[/QUOTE]

For me, sadly. My knees want the lowest Q-factors I can get. Narrow, close left cranks are improved Qs for me. Someone above talked of wanting 1/4" clearance to the chainstays, that he likes 1/2" better. I want just enough to keep the paint on. (I've got a few Phil Wood BBs because they are happy to make them narrow.)

And easy way to get exactly the chainline you want with an unknown crankset (assuming JIS taper) - buy a wide $30 or less Shimano BB. Mount your cranks. Measure up the chainline and left crank clearance. Decide what you really want. Now, do the calcs to see what your dream BB spindle would be to get that chainline and left crank location. Order that from Phil Wood (plus the installation tool unless you have access to one). Phil and Shimano both build accurately to the JIS standard. Much of the rest of the cycling world? Not always. (I said this method is easy, I did not say it was cheap. Phil Wood is expensive and will last a lifetime.)

dedhed

Also says 153.7mm Q factor.

Kontact

I don't know what any of that has to do with my post. I was talking about what you'll see looking at spec'd parts.

But if you want low Q cranks, the easiest thing is to find cranks designed for low Q, rather than go on this weird fishing expedition with multiple BBs.


This sort of thread is always a little bizarre. All cranks came designed for a certain spindle or spindles. You can look many of them up, so that should be the first answer to any question about how to correctly mount a crank.

Kontact

Which ain't small.

choddo

Just wanted to say thanks for the advice. New BB is in and everything fits perfectly.