BMX or MTB cranks?

PB Forum :: Dirt Jumping & Street
BMX or MTB cranks?
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flag am-shaegar
Posted: Apr 14, 2016 at 8:06 Quote
Howdy!

I'm a new user, but have been browsing this forums for a while now Big Grin

I have a small problem.

Currently I have a dirtjump bike with bmx cranks, bmx BB but with external bearings, made to fit mtb frames.
The cranks have a 19mm 48 spline chromoly spindle and a chainring of 28t, because I have a driver of 9t, everything brand new...made the bike myself and my local bike shop Big Grin

The problem is that ... the chainring appears to be off-center ... on both axes!
So I have a point where the chain is stretched to the maximum, and another in which it hits my frame because of how loose it is.

I went to fix this at my local bike shop, but they couldn’t do too much and the problem began to worsen Frown
The chainring is perfectly round and flat, so I know it's not from it.
And the spline seems alright, but it appears to move a little inside the BB..
Now, I do not know if the problem are the cranks (xposure infinity) or the BB (Octane One External) ... but I am thinking about changing them both, with something better.

And now comes my question ... should I go for another BMX crank and BB ... or should I get a MTB set like the Hope Spiderless cranks?
What do you think?

Thank you in advance.
flag Metro-Gnome
Posted: Apr 14, 2016 at 9:43 Quote
This is a common 'non-issue' with bmx cranks and bolt on chainrings. It's totally fine, just make sure the chain is tight, but not SO tight that you're messing the bearings of your bb up. That's usually the thing. Spline-drive rings don't have that tight spot like bolt on chainrings.

If that isn't what you want to hear/the actual problem, I'd consider getting a normal (internal bearing) Euro bb. It's quite possible that the external bearing cups are too farr apart and the splines are wiggling around inside the bb.

If you change the bb, don't change the cranks. If you have to replace anything specifically, get a longer spindle, as I think that's your problem.

That's all the help I can give you without pics of what's really wrong. Can you take a few so we cans ee where the problem is? Maybe take your cranks out and measure the spindle...
flag am-shaegar
Posted: Apr 14, 2016 at 12:22 Quote
Metro-Gnome wrote:
This is a common 'non-issue' with bmx cranks and bolt on chainrings. It's totally fine, just make sure the chain is tight, but not SO tight that you're messing the bearings of your bb up. That's usually the thing. Spline-drive rings don't have that tight spot like bolt on chainrings.

If that isn't what you want to hear/the actual problem, I'd consider getting a normal (internal bearing) Euro bb. It's quite possible that the external bearing cups are too farr apart and the splines are wiggling around inside the bb.

If you change the bb, don't change the cranks. If you have to replace anything specifically, get a longer spindle, as I think that's your problem.

That's all the help I can give you without pics of what's really wrong. Can you take a few so we cans ee where the problem is? Maybe take your cranks out and measure the spindle...

Mate, you are spot on!
You just saved me a lot of bucks!

That really is the problem, the spindle is 14cm, my frame is 68mm + the external bearings, it goes to around 93mm
So instead of having the flush part of the spindle on the bearings, I have the splinted part on them, and that leaves room for wiggle.

So yep, I need a longer spindle and I might ditch the external bearings Smile )

Many many thanks Sir, your help is much appreciated! ^_^


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flag Metro-Gnome
Posted: Apr 14, 2016 at 13:59 Quote
Glad I could help. I just pictured the problem in my head, and that was all I could think of.
flag cmc4130
Posted: Apr 14, 2016 at 17:19 Quote
http://forums.mtbr.com/urban-dj-park/bmx-vs-mtn-cranks-ss-need-help-709686.html

Here is a visual depiction of why a person may need a 6 inch spindle if he is putting a BMX crankset on an MTB.

Profile BMX gives the option of buying a 6 inch spindle. Most BMX cranksets use something a little shorter, like 5 5/8" or 5.75".

The issue is not so much the width of the bottom bracket shell, in this case 73mm on the Black Market Mob vs. 68mm on a typical bmx. The issue is the chainstay width.

Because Mtb's have wider spaced rear hubs, they often have chainstays that are set wider. Therefore the crank arms have to be spaced wider to get clearance. If you run a typical BMX spindle, you won't be able to space the crank arms wide enough (unless they're barely hanging on) to get clearance with the chainstays.

Unsecure image, only https images allowed: http://i255.photobucket.com/albums/hh138/upsetbmx/Black-Market-Mob-Chainstay-width-6-inch-spindle_zpsxjxt95ou.jpg
Chainstay clearance. This is why you need the 6 inch spindle, rather than typical 5.5 or 5.75" spindle.
Unsecure image, only https images allowed: http://i255.photobucket.com/albums/hh138/upsetbmx/Chainstay-width-5.25_Black_Market_Mob_zpssihbqjp2.jpg
Note how much narrower the back end of a 24" BMX is:
Unsecure image, only https images allowed: http://i255.photobucket.com/albums/hh138/upsetbmx/Liquid-Feedback-bmx-24_chainstay-width_zpsb4wqivje.jpg
BMX internal euro bottom bracket from Odyssey:
Unsecure image, only https images allowed: http://i255.photobucket.com/albums/hh138/upsetbmx/Odyssey-BMX-euro-bb-on-Black-Market-Mob_zps0hlop0zx.jpg
flag am-shaegar
Posted: Apr 16, 2016 at 5:07 Quote
Many thanks guys!

Since I can't find Profile parts in my country, I went and ordered a Dartmoor Flare which has a 6 inch spindle, and a spline-drive ring from Primo.

But right now I read that the spindle is welded to the right arm :O
So...I don't think I'll be able to use a spline-drive ring Frown am I right? Frown

Unsecure image, only https images allowed: http://www.hardtailnation.co.uk/wp-content/uploads/2009/10/flare-Medium.jpg

Or could I switch the arms, so that I have the left one in the right side?
flag Aaronz25RS
Posted: Apr 16, 2016 at 7:09 Quote
You "may" not have had to do that.. much depends on the spacers..

You don't "need" the cone spacers if the crank arms will clear the frame and sometimes only a thin washer to space the chainwheel off the BB is all that's needed.. there's a lot of adjustment there
flag am-shaegar
Posted: Apr 16, 2016 at 8:23 Quote
Aaronz25RS wrote:
You "may" not have had to do that.. much depends on the spacers..

You don't "need" the cone spacers if the crank arms will clear the frame and sometimes only a thin washer to space the chainwheel off the BB is all that's needed.. there's a lot of adjustment there

I mean that since the arm is welded to the spindle...there are no splines to attach the chain-ring Frown
So I think that these cranks are not compatible with spline-drive chainrings Frown

I've sent an e-mail to Dartmoor to ask them...and now I'm waiting for their reply...
flag cmc4130
Posted: Apr 16, 2016 at 8:33 Quote
am-shaegar wrote:
Many thanks guys!

Since I can't find Profile parts in my country, I went and ordered a Dartmoor Flare which has a 6 inch spindle, and a spline-drive ring from Primo.

But right now I read that the spindle is welded to the right arm :O
So...I don't think I'll be able to use a spline-drive ring Frown am I right? Frown

Or could I switch the arms, so that I have the left one in the right side?


If the arm is welded to the spindle, this makes the crankset a "2 piece" design. Or, Dartmoor is actually calling them 2.5 piece http://dartmoor-bikes.com/hardware/cranks/flare . I don't think you can turn them around and put the left crank arm on the right side etc. The crank left pedal threading is threaded reverse so that your pedal doesn't unscrew while riding. This why Left Side Drive cranksets in bmx are actually threaded the other way, not just regular cranks on the opposite side.

Spline drive sprockets (they're not called "rings" when they're solid and go on the spindle) are really not that important--they're not any better or worse than bolt-on.

And are you sure it's a 6 inch spindle from Dartmoor? I didn't see that written anywhere on their website.

Unsecure image, only https images allowed: http://www.profileracing.com/estore/images/product-images/401/tigdhbbspindles_430.jpg
But, the main thing is, you might need an INTERNAL euro bottom bracket. The external one is causing you chainline problems, right?

http://www.winstanleysbmx.com/product/3729/Odyssey_Euro_Bottom_Bracket

If you do get a new bottom bracket, note that you'll need a tube spacer for a 73mm bottom bracket shell. The one that comes with the bmx euro bottom bracket is for a 68mm bb shell.

Normally a 48mm tube spacer is used with 68mm bottom bracket shell frame and a 53mm tube spacer is used with 73mm bottom bracket shell frame.
Unsecure image, only https images allowed: http://www.profileracing.com/estore/images/product-images/63/tubespacers_5193_900.jpg
http://www.profileracing.com/estore/images/product-images/63/tubespacers_5193_900.jpg
flag am-shaegar
Posted: Apr 16, 2016 at 12:12 Quote
Thanks for bringing me back on track, mate!

Ok, so, I found this spindle:
http://www.chainreactioncycles.com/ro/en/deity-components-vendetta-3-0-48t-crmo-spindle-2015/rp-prod52480

Which is 165 mm and just perfect for my external BB and I can still use a spline drive sprocket Big Grin

Now...I just gotta tell the bike shop to cancel my order, and I feel sorry for that, they are a really nice, local shop and every order helps them Frown
Any advice here? Smile )
flag am-shaegar
Posted: Apr 16, 2016 at 15:14 Quote
Aaronz25RS wrote:
Hate to ask, but do you have the 19 to 22mm chainwheel spacer in place?

https://www.google.com/search?q=bicycle+19+to+22+mm+chainring+spacer&client=ms-android-att-us&biw=360&bih=559&prmd=sivn&source=lnms&tbm=isch&sa=X&ved=0ahUKEwj1lJS1j5TMAhVklYMKHfM9BxEQ_AUIBygC#imgrc=AcclDrk-JMe4MM%3A

Yes, I have the right adaptor Big Grin
I had to change it once, because it got bent and the splines left a quite big mark on it...but I thought that was normal Smile )
flag Aaronz25RS
Posted: Apr 16, 2016 at 17:00 Quote
Ok just making sure, you said the chain had quite a bit of "hop" to it..
flag rusty-shackleford
Posted: Apr 16, 2016 at 17:11 Quote
http://www.profileracing.com/

order directly from their site, they take paypal.
flag cmc4130
Posted: Apr 17, 2016 at 14:04 Quote
am-shaegar wrote:
Thanks for bringing me back on track, mate!

Ok, so, I found this spindle:
http://www.chainreactioncycles.com/ro/en/deity-components-vendetta-3-0-48t-crmo-spindle-2015/rp-prod52480

Which is 165 mm and just perfect for my external BB and I can still use a spline drive sprocket Big Grin

Now...I just gotta tell the bike shop to cancel my order, and I feel sorry for that, they are a really nice, local shop and every order helps them Frown
Any advice here? Smile )


Note this:
https://www.danscomp.com/products/456019/Profile_External_Euro_BB_Kit.html

It says the external (outboard) euro bottom bracket is compatible only with 6 inch spindles.

Profile Racing’s How To’s: Outboard Euro BB Installation
http://www.profileracing.com/estore/product.php?productId=359
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