Global-flat.com Board
English => Parts & Bikes => Topic started by: Feeble on May 03, 2020, 02:10:59 PM
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Hi
I have just bought a complete St Martin Happy Place bike and the crank is slipping when I try to pedal hard. I have never seen a crank like this on a BMX before and have no idea how to fix it... The bike has a free coaster if that makes a difference. But when you pedal hard, the cranks move and the sprocket doesnt. Is it just a case of nipping up the nut that secures the sprocket, or something harder? Would I need a special tool to tighten the nut?
Thanks in advance.
Matt
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Hello. The sprocket is attached to the cranks, so I'm guessing that your freecoaster is not turning when you pedal, correct? I would take off the rear wheel and see how the cranks spin. If they spin freely, check your freecoaster. It may just be too tight, or you could have internal issues with it.
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No, the freecoaster seems to be working fine, and as long as I am pedaling slowly it is all good. But when I push down too hard on the pedals the whole crank moves but the sprocket and chain stay still. ???
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As far as I can remember this crank does not use a normal sprocket bolt, doesn't it? Do you mean this one?
http://www.global-flat.com/parts/part/695So (http://www.global-flat.com/parts/part/695So) maybe the thread of the sprocket is damaged? I think it was the only crank with that system, so a replacement sprocket will be hard to find. If the crank arm thread is damaged I would say time for a new crank and sprocket...
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I cant seem to get that link to load, but you are right, it is a weird set up! The guy did knock £50 off of the price because of it so that will go some way towards a new crank! I was hoping it would be just a matter of tightening something or other... But it's no big deal, I will just get a new crank and sprocket like you said if it cant be fixed! O0
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Daddycool made a typo, here is the corrected link:
http://www.global-flat.com/parts/part/695 (http://www.global-flat.com/parts/part/695)
I tried looking around to see exactly how this setup works but I haven't been able to find anything conclusive. I think there may be a version of this crank that has a bolt that tightens the sprocket to the crank arm. But I don't know if that's how the one you have is designed, I agree with daddy cool but I'm still not sure exactly how your cranks are designed since I haven't seen them up close yet.
For reference, here is a photo of my cranks. These are a common design: 3 piece cranks and the sprocket is attached to the crank arm by a small bolt. In the photo I'm pointing to this bolt with a red pen. Because of this bolt, the sprocket always turns with the crank arm; they are locked together because of the bolt.
(https://i.imgur.com/wnG5fRh.jpg)
Unfortunately I don't know how exactly your cranks and sprocket work together so I'm not sure how to advise you.
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Yes, typo, sorry...The version shown in the link works with such a sprocket:
http://www.global-flat.com/parts/part/393Later (http://www.global-flat.com/parts/part/393Later) there was a version for normal sprocket (also mentioned in the description):
http://www.global-flat.com/parts/part/818I (http://www.global-flat.com/parts/part/818I) think you need to disassemble it to see what is going on...
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Damn it, no typo, I just missed the blank...http://www.global-flat.com/parts/part/393 (http://www.global-flat.com/parts/part/393)
http://www.global-flat.com/parts/part/818 (http://www.global-flat.com/parts/part/818)
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It is definitely the crank that you linked first DaddyCool. Not a traditional crankset at all! There is no bolt on the crank arm, but there is a large round nut between the crank arm and the sprocket so I assume that has to be done up real tight.
I can post a picture this evening of the set up to try and help. In the meantime, this is the bike in question - https://www.ebay.co.uk/itm/202977700862 (https://www.ebay.co.uk/itm/202977700862)
Cheers guys
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Hi again guys. I have come to the conclusion that there should be no way that the sprocket should turn on the crank arms if the threads were actually in tact. Therefore I think I do need a new crank set and sprocket.
Do you know of a crank set that is available in the UK, 160 or 165mm that would accept a 24t sprocket? I was looking at the Primo Stevie 24mm cranks with the 160mm arms. Would it be suitable for my St Martin frame and 24t chainwheel?
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I suggest that you find out if the thread on crank and sprocket is damaged or maybe on the sprocket only. In this case a new sprocket would be the fastest and cheapest solution... But you need this specail kind of sprocket of course.
Any regular crank accepts a 24T sprocket. Why shouldn't it? 23T is the minimum for the standard bolt positon on the crank arm, but some sprockets can even work with this and 22T. Everything smaller needs the bolt position called "nano drive" where the bold is closer to the spindle of the crank.
A 24 mm spindle is rare and some sprocket do not work here because they have a 22 mm hole. Most sprockets have a 24 mm hole and adapters for 22 mm and 19 mm spindles.
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Thanks DaddyCool, that is great info! I will strip the crank down this weekend and see where I stand! Many thanks!
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These cranksets from St. Martin were basically inspired from other bike disciplines, they are closer to MTB bikes and Trial bikes but I believe they use Euro bottom brackets which are screw one instead of the current, more trendy, Spanish BB's or Mid BB's (a mix breed of the former two).
At the end of the day they released some sort of spline drive cranks (the sprocket has threads that exempts the arms from having the bold socket) whose arms look like MTB's and that uses a BMX Euro bottom bracket.
Check if the bottom bracket is the right one, if it is properly screwed on, etc.
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This is exactly the design flaw with these cranks. The sprocket is threaded on and uses a lock ring to attach it. This problem is not uncommon. There is a hack fix but the correct fix is to replace the cranks. Sorry.
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I remembered these cranksets being hugely popular when they were released, they had a ver clean look, considerably light and allowed nano gear ratios down to 16T. That being said, they may be worth something to new school flatland collectors.
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I stripped it down this morning. The thread on the chainwheel is indeed damaged, so I will have to order a new one at the very least. I am not that impressed with the thread on the crank arm in general though as even the locknut is quite a baggy fit, even on the pristine top of the threaded part... Also, it seems to be another mission to find the appropriate 24t sprocket here in the UK.
Am I better off just buying a new crank? I think the 24t St Martin chainwheel has been discontinued... >:(
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Flatland specific parts have very limited availability, usually they are few years on the market and the brands seem to discontinue production very fast, it's a niche market.
Finding a sprocket for that cranksets won't be easy, if you're in the USA try Bmxmuseum website sales section, they usually have pretty unique stuff their.
Summing it up, yeah you are better of selling that cranksets yo someone who needs it for a new school build and get a good old classic bolt drive of spline drive cranksets with an axle that fits EuroBB.
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Thanks my man. Sadly I thought as much! But at least that should be quicker than sorting out the existing crank, though not cheaper! Lol. I'll get something ordered up! O0