Shortening your cranks

[S13]

Over the past weeks ive been gathering information on how to shorten the cranks on Rocky Mountain Powerplay bikes. I hope this info helps some of you with this quite complicated task.

So in order to get shorter cranks than the stock 170's (or if you are lucky the 165) there are a couple of solutions, both involve getting either a RaceFace Atlas or Turbine crank set.

So to start with you either have one of 2 situations:
1. You already have the bigger 30mm axle cranks installed on your bike (probably Turbine)
2. You have the smaller 24mm axle Ride cranks


The solution for situation 1 is simplest. Buy a set of Atlas cranks in your preferred length. Shortest you can go is 165mm. If you buy the longer 175mm you can drill them down to 155. You can also drill Turbine cranks shorter, if you prefer that route. 170 to 150 is possible and 175 to 155 as well. Turbine is sold out everywhere so might not be the easiest option. Turbine does not come shorter than 170.
After you get your new and optionally machined cranks, fit them with the original spindle that came with your bike. It should be a RF149 spindle (149mm end-to-end spindle length). Also fit the original clutch spider assembly with the 30mm locking ring. Install on bike and you are done!

Now situation 2 is more complicated, because you dont have the parts to use the 30mm cranks, and re-drilling the original Ride cranks is not recommended due to the internal structure. So the only option is to first install a 30mm bottom bracket (for example a BB92 Double Row CINCH 30mm with 92mm shell width). Then you need to buy the correct length 30mm spindle. Rocky Mountain advises the RF149 (149mm end-to-end spindle length), and that is what most people seem to use. You can buy these spindle kits at several web shops. You also need to contact RM or you local RM dealer to get a hold of the 30mm locking ring for the spider clutch. For the remainder, the same applies as for solution nr 1; get a set of Atlas (or Turbine) cranks, optionally drill them to desired length, fit cranks with the new correct sized spindle, install original clutch spider with new 30mm locking ring, and you are good to go.

One point to remark is that you can also buy Atlas cranks in so called "83mm" variation. On the Raceface website this shows as "Spindle Size 151". These cranks come with the 151mm RF151DH spindle already attached to it. Now im not sure these will fit the bike, but the difference is only 2mm compared to the recommended RF149 spindle, so perhaps minor enough that you can adjust the free play with free play adjustment ring, or perhaps a spacer. This would save you the hassle to buy a separate spindle kit for another 100 bucks.

This document from Race Face provides you with information on the available spindle sizes.
Note that it says:
• Spindles with suffix “BM” or “DH” (eg.RF136DH) are compatible with SixC and Atlas cranks only
• Spindles without “BM” or “DH” suffix (eg. RF134) are compatible with Next R, Next SL and Turbine cranks only

So according to the document the Atlas crank should not be compatible with the RF149 spindle, but is compatible with the RF151DH spindle. However on the facebook group Norman mentions he has successfully used the RF149 spindle with his drilled Atlas cranks without any problems.

Atlas cranks drilled from 175 to 155 by Norman:

Turbine cranks drilled from 170 to 150 by Andy:

According to Andy, drilling 175 Turbine to 155 is also possible.

So there you have it. I have not verified any of these solutions, so i cannot guarantee the information provided here is correct. But hopefully it helps you get started.

 

[cody311]

I would like to add some information here as I have no doubt others are having this same issue.

I sent RM this question in regards to options for us:

"I am needing to downsize the 170mm cranks on this Power Play but I am having a difficult time finding the correct axle length within the Cinch lineup of options. I am under the impression that a 149.5mm length is required to accommodate the sprag clutch. With the offerings on the RaceFace website for Atlas cranks in 165mm length but only offering the 151mm axle length, can I just use a 1.5-2mm spacer here?

I am aware that I will need to also get the larger 30mm BB bearings and associated snap ring for the sprag clutch, is this available for purchase on the Rocky website for the 2022 models (the snap ring, I can get 30mm press fit without issue)? I can see a ton of part numbers on the Rocky site but so many of the purchase options do not have a photo so it is hard to understand exactly what it is I am purchasing."

Here was the response from Reed,

"Thanks for reaching out. You're on the right track. The 151mm Spindle length with a small spacer will work great with the spider assembly on the cranks. I'm afraid the 30mm Lockring is not available directly from our site though you can order it through your local Rocky Mountain dealer. The part number you'll need is 1347006. If you provide that to the shop they'll be able to get it ordered for you fairly quickly."


I am currently working with someone to maybe do a run of Atlas cranks drilled and tapped at 155 and 160mm. If there is any interest here, please reach out. I would like to do a run of 4-8 and gauge interest based around that initial investment. Looks like the most difficult part of this will be getting the correct snap ring but I bet an alternative could be found on McMaster or something. I will update here with more information.

 

[S13]

Thank you for the additional information.
Good to know the RF151DH will fit the powerplay bikes.

The part number for the 30mm lock ring matches that what i had already found:

RMB SP Lockring Clutch 30mm Aluminium

Rocky Mountain Spider Bearing Lockring 30mm Aluminium Race Face version

www.zerobikes.co.uk

That webshop also has a nice overview of the spider clutch:


It shows both the small and big lock ring option:
Nr7 is the 30mm lock ring - with partnr 1347006
Nr8 is the 24mm lock ring

 

[cody311]

Interesting. Why are they showing two difference chainring assemblies in this Illustrated Parts Breakdown (IPB)? I was also under the impression it was a snap ring and not this threaded lock ring. Thank you for supplying a link to a suitable replacement. The issue here is that those IPB's seem to be for the 2019-2020 PP's. As the Dyname 4.0 system is a bit different, I would hate to make an incorrect assumption these parts are interchangeable. Do you happen to have any data focused around the new bikes in this capacity?

 

[S13]

What does IPB stand for?

Anyway, the 2 chainrings are different because of the tooth pattern. If you look closely you can see the difference. One is for shimano and the other for SRAM chains if i recall correctly.

Im not aware of any difference between the Dyname 3.0 and 4.0 when it comes to cranks and axles. Ive measured my 2022 altitude axle (that came with Ride cranks and small 24mm lock ring) and it could fit the RF149 perfectly. But hey... still untested... Ive asked my local RM dealer to order the parts but he messed up 2 times now, so he is not eager to help at this point (he kind of ignored all the information i provided). Also, now im travelling again, so i wont be able to buy parts myself until i get back in a couple of months. And then there is the issue of availability of 165mm atlas cranks. Hopefully that will improve in the next months.

If you want to see how it the clutch and lock ring come a part, take a look at this video:

 

[cody311]

What does IPB stand for?

Anyway, the 2 chainrings are different because of the tooth pattern. If you look closely you can see the difference. One is for shimano and the other for SRAM chains if i recall correctly.

Im not aware of any difference between the Dyname 3.0 and 4.0 when it comes to cranks and axles. Ive measured my 2022 altitude axle (that came with Ride cranks and small 24mm lock ring) and it could fit the RF149 perfectly. But hey... still untested... Ive asked my local RM dealer to order the parts but he messed up 2 times now, so he is not eager to help at this point (he kind of ignored all the information i provided). Also, now im travelling again, so i wont be able to buy parts myself until i get back in a couple of months. And then there is the issue of availability of 165mm atlas cranks. Hopefully that will improve in the next months.

Illustrated Parts Breakdown (IPB). I edited my post to include that acronym. My bad.

Do you know if there is an ability to buy the axle and crank arms separately? Knowing that the 151mm axle will work is nice but if the axle and be separated from the crank arms then other options present themselves. To my knowledge only the carbon cranks are 3 piece. I have inquired with RaceFace about this as well but I don't know if I will get a response.

 

[S13]

I dont think you can purchase cranks separately.
But spindles, yes, in any variation you like.

So you can detach the spindle from Atlas cranks (and Turbine) and replace it with a different size if you like.

 

[cody311]

I dont think you can purchase cranks separately.
But spindles, yes, in any variation you like.

So you can detach the spindle from Atlas cranks (and Turbine) and replace it with a different size if you like.

Could you not do something like the Affect R crank arms?

Affect R CINCH

 

[S13]

Nope, wrong spindle size. For these smaller 24mm spindle cranks (Aeffect, Aeffect R, Ride) only the AM83 spindle would fit powerplay bikes, but that is OEM only. This is what Rocky Mountain uses for their Ride cranks, but is not available for us.
24mm cranks always have non removable spindles, so its impossible to change to the right size.

You really have to go to the bigger 30mm cranks, because only then you can get the proper spindle size.

 

[cody311]

Nope, wrong spindle size. For these smaller 24mm spindle cranks (Aeffect, Aeffect R, Ride) only the AM83 spindle would fit powerplay bikes, but that is OEM only. This is what Rocky Mountain uses for their Ride cranks, but is not available for us.
24mm cranks always have non removable spindles, so its impossible to change to the right size.

You really have to go to the bigger 30mm cranks, because only then you can get the proper spindle size.

Gotcha, you did mention something along those lines in the original post. There are just no options for 160mm cranks. Was hoping to find some youth cranks or something. It is nice that you can separate the cranks on the Atlas and Turbine but 165mm still too long for me. Also unfortunate you can't just buy crank arms without the spindle. I don't fully understand RF business model here.

 

[vr6tee]

I'm interested in the 155mm setup.

 

[S13]

Ok, i can now confirm that Atlas cranks in the "83mm" variation (in other words with a RF151DH spindle pre-fitted) will directly fit the Powerplay models.

So basically i first upgraded my bottom bracket to a 30mm variant: BB92 Double Row CINCH 30mm | Bottom Bracket BB| Raceface
I got the 30mm locking ring from Rocky Mountain, part nr 1347006 (€45,-).
Also got the Atlas cranks in 165mm length, "83mm" variant, color black (jeez that was hard to find!).
Then i installed the old spider clutch on to the Atlas cranks with the new locking ring.
Finally installed the cranks in the new bottom bracket and hey presto: you have shorter cranks.

I did not use any spacers (even non that came supplied with the bottom bracket). You are left with a tiny gap of ~1mm on the non drive side which you can easily adjust for with the adjustment collar that comes standard on the Atlas crank.

 

[DrChris]

I'm seriously considering going to the effort of getting some shorter cranks. With my 170mm RF Ride cranks on my A50, I hit my cranks regularly on my normal trails. My 170mm cranks actually measure 190mm to the end of the crank (so there is a lot of extra length sticking out after the pedal spindle location). Can someone measure the actual BB centre to end length of their 165mm Atlas cranks before I commit? I'm hoping it will be <180mm.

Thanks
Dr Chris

 

[S13]

I measured my Atlas 165 cranks at 180mm center-BB to tip.
The difference is quite big when you have them side by side with the Ride 170's.
The Ride 170 is actually a 175mm crank arm where they moved the hole 5mm further inwards. The Atlas doesnt suffer with that problem, its a true 165 crank. So you gain about 10mm in center-BB to tip.

 

[DrChris]

I measured my Atlas 165 cranks at 180mm center-BB to tip.
The difference is quite big when you have them side by side with the Ride 170's.
The Ride 170 is actually a 175mm crank arm where they moved the hole 5mm further inwards. The Atlas doesnt suffer with that problem, its a true 165 crank. So you gain about 10mm in center-BB to tip.

Awesome, thank you!

 

[eTrax]

Thank you for the additional information.
Good to know the RF151DH will fit the powerplay bikes.

The part number for the 30mm lock ring matches that what i had already found:

RMB SP Lockring Clutch 30mm Aluminium

Rocky Mountain Spider Bearing Lockring 30mm Aluminium Race Face version

www.zerobikes.co.uk

That webshop also has a nice overview of the spider clutch:


It shows both the small and big lock ring option:
Nr7 is the 30mm lock ring - with partnr 1347006
Nr8 is the 24mm lock ring

Its crazy how heavy that spider is with bearing, must be like 3lbs. I know its an ebike but holy crap. Like to see a ti version.

 

[darok]

I measured my Atlas 165 cranks at 180mm center-BB to tip.
The difference is quite big when you have them side by side with the Ride 170's.
The Ride 170 is actually a 175mm crank arm where they moved the hole 5mm further inwards. The Atlas doesnt suffer with that problem, its a true 165 crank. So you gain about 10mm in center-BB to tip.

Having the Aeffect Cinch 170mm cranks on the C70, it will be exactly 5mm less with the 165mm Atlas crank from center-BB to tip. Not sure if you had the Ride or the Aeffect cranks but do you think it's worth the swap?
Thanks

 

[A06]

Hey everyone! I joined this forum specifically to discuss this thread. I picked up a 22 Altitude Alloy 30 coil two weeks ago and the pedal strikes are crazy frequent. I need shorter crank arms.

This is the best single source of info on the topic I have found and I wanted to add another potential solution.

Can anyone provide a reason why theses cranks wont work?

Canfield Bikes AM Cranks (NEW! Standard Offset)

Short MTB cranks for sale. 150mm cranks, 155mm cranks, 160mm cranks, 165mm cranks, 170mm cranks. The shortest, toughest, mountain-bike cranks on the market.

canfieldbikes.com

Would need to run their BB which isnt a bad thing as the 24mm size means larger bearings. Also RM does make a SRAM ratchet spider which these cranks are compatible with. Spindle length ranges all the way up to super boost and with shims/spacers anything lesser than that.

I'm fighting the urge to impulse buy these the bb and the sram ratchet spider. Would prefer to talk through it here before being the test subject. 155 seems ideal to me.

Any input appreciated!

 

[S13]

Can anyone provide a reason why theses cranks wont work?

So i dont think these cranks have the Raceface proprietary CINCH system, which is what you would need in order to bolt the spider clutch to the crank.

 

[A06]

So i dont think these cranks have the Raceface proprietary CINCH system, which is what you would need in order to bolt the spider clutch to the crank.

You are correct this crank uses a Sram pattern chainring interface.

I mentioned in my post a new Sram ratchet spider would be needed. RM does make them. I contacted Rocky to confirm that this wasnt a mistake the ebay seller made in the description. RM confirmed this is an actual part they make but few are sold.

Spider Clutch Sram for Models Powerplay Rmgsc Rocky Mountain Transmission 770416290592 | eBay

Find many great new & used options and get the best deals for Spider Clutch Sram for Models Powerplay Rmgsc Rocky Mountain Transmission at the best online prices at eBay! Free shipping for many products!

www.ebay.com

 

[S13]

You are correct this crank uses a Sram pattern chainring interface.

I mentioned in my post a new Sram ratchet spider would be needed. RM does make them. I contacted Rocky to confirm that this wasnt a mistake the ebay seller made in the description. RM confirmed this is an actual part they make but few are sold.

Spider Clutch Sram for Models Powerplay Rmgsc Rocky Mountain Transmission 770416290592 | eBay

Find many great new & used options and get the best deals for Spider Clutch Sram for Models Powerplay Rmgsc Rocky Mountain Transmission at the best online prices at eBay! Free shipping for many products!

www.ebay.com

Well, the SRAM thing has to do with the chainring interface of the spider clutch. Those are the 4 holes that bolt to the chainring. Indeed for Shimano there is a different spider clutch with a slightly different hole pattern. As can be seen on this image (nr 9 vs nr 10):

However this has nothing to do with the cinch interface. The cinch part refers to the large big hole in the center of the clutch bearing. And that has to match with the mounting system on the crank arm.
To give you an idea of what cinch looks like:

As far as i can tell, all RM spider clutches work with Cinch (your ebay posting included) and so you need a crank set that supports that cinch system. And i dont think those Canfield cranks have cinch tbh.

 

[S13]

Ah, ok ive looked it up, those Canfield cranks use the SRAM 3-Bolt Direct-Mount interface. And while it looks somewhat similar to cinch, it works differently. It uses 3 bolts, whereas cinch uses a lock ring. And you need that lock ring to be able to mount the spider clutch. It would never work with the 3 bolt system.

 

[A06]

I would like to talk through this so I want to be clear I'm not arguing the point but rather trying to understand the viability.

My current understanding is cinch is the spline pattern the crank attaches to the chainring but in this application the splines attach to the ratchet spider and the chainring attaches via 4 bolts to the spider.

The Sram pattern is a different spline arrangement thus a different ratchet spider for the chainring. I have two other bikes one with RF the other with Sram cranks and chainrings they are different.

What I dont know yet because I havent taken the PP crank apart yet is what the 30mm/24mm collar attaches to. If that is RF specific then yeah this wont work.

 

[S13]

I would like to talk through this so I want to be clear I'm not arguing the point but rather trying to understand the viability.

My current understanding is cinch is the spline pattern the crank attaches to the chainring but in this application the splines attach to the ratchet spider and the chainring attaches via 4 bolts to the spider.

The Sram pattern is a different spline arrangement thus a different ratchet spider for the chainring. I have two other bikes one with RF the other with Sram cranks and chainrings they are different.

What I dont know yet because I havent taken the PP crank apart yet is what the 30mm/24mm collar attaches to. If that is RF specific then yeah this wont work.

You are correct.
And yes, those 30mm / 24mm collars are the locking rings (nr 7 and nr 8 in the diagram) that are designed to interface with the race face cinch system only. So there is no way of getting around that.

 

[A06]

BUMMER! But hey thanks for the clarification. Looks like easiest pathe is sending to bikesmith to redrill and converting the 24mm to 30mm BB.

 

[A06]

You are correct.
And yes, those 30mm / 24mm collars are the locking rings (nr 7 and nr 8 in the diagram) that are designed to interface with the race face cinch system only. So there is no way of getting around that.

So I thought about this a lot last night and something just doesn't add up. Specifically why would RM ever make such a spider considering that RF as far as I can tell never made cranks with a Sram spline interface and Sram never made cranks with a RF spline or RF Cinch locking ring.

Perhaps there was some obscure application or some other answer.

I sent an email to RM this morning referencing this discussion and links to the thread and the Canfield cranks. None of the pictures Ive seen of the Sram splined ratchet spider show the inboard side. What I am really hoping they reply back with is that that spider is secured using the 3 bolt Sram method but with longer bolts and some type of spacer with a 30mm I.D.

I'm probably very wrong in my assumption that this is the case but we wont know if the question is never asked of the source.

I will post their reply upon receipt, they usually reply back within the same day.

A06

 

[A06]

Doh!

OK just figured out what you were saying S13...the spider and chainring are Sram pattern not the inner splines.

Got it. Guess I will try 165's and see if they work better if not will drill them.

 

[S13]

Doh!

OK just figured out what you were saying S13...the spider and chainring are Sram pattern not the inner splines.

Got it. Guess I will try 165's and see if they work better if not will drill them.

The 165 is a fair improvement, so try it and perhaps you'll like it.

 

[A06]

The 165 is a fair improvement, so try it and perhaps you'll like it.

Gonna go that route and drill down if the pedal strike problem persist.

 

[A06]

Bit the bullet ordered, the 165 Atlas with 30x 83mm spindle, dual bearing 30mm BB, 30mm lock ring and switching away from magpeds going back to Oneup pedals as the magpeds are rather thick.