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 Post subject: Orbea MX20 kids bike issue. Is this normal?
PostPosted: Fri May 08, 2015 6:59 pm 
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Posts: 407
Location: Romford
So I've been told there is no problem with my sons bike highest gear and he is not supposed to pedal back on any occasion.

Bike has been for a week with the store and they attempted to bend the cog and tighten the drive side crank arm to rectify the problem.

He fell 3 times because of the chain falling before I took it to the store to be looked at.

So is this normal?Any ideas?



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 Post subject: Re: Orbea MX20 kids bike issue. Is this normal?
PostPosted: Fri May 08, 2015 7:15 pm 
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To me that looks like the rear mech 'b' screw needs adjusting as the top jockey wheel is a bit tight to the sprockets. You should be able to back pedal in any gear when you have a single ring up front.


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 Post subject: Re: Orbea MX20 kids bike issue. Is this normal?
PostPosted: Fri May 08, 2015 7:45 pm 
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Yeah thats not supposed to happen. I had a similar issue and I rectified it by buying another hub. taking off the part where the cassette sits and just replacing that


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 Post subject: Re: Orbea MX20 kids bike issue. Is this normal?
PostPosted: Fri May 08, 2015 9:12 pm 
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Location: Romford
Thanks. I will be looking into it myself tomorrow. I had a quick test on XT 2x10 for comparison and despite you are not supposed to ride on biggest front and rear cogs the chain did not exhibit the same issue.


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 Post subject: Re: Orbea MX20 kids bike issue. Is this normal?
PostPosted: Sat May 09, 2015 3:45 pm 
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B screw and a bit more alignment needed

--------------------------------------
Bike piccys!
http://tinyurl.com/okyhpdg


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 Post subject: Re: Orbea MX20 kids bike issue. Is this normal?
PostPosted: Sat May 09, 2015 7:19 pm 
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Location: Romford
VW-surfbum wrote:
B screw and a bit more alignment needed


Two store mechanics had the bike for a week to fix it and they failed.
I had a good look today and managed to make is sort of ok. But none of the adjustments of the rear derailleur make a difference for this specific problem as the chain is falling only when backpedaling from the top of the big cog.It's the chain-line that need adjusting as the offset between the front chainwheel and rear cog is a 0.5mm too big. I raised it with orbea directly as the store are useless hopefully will get some sort of resolution or will have to consider returning the bike as not fit for purpose. :wtf:


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 Post subject: Orbea MX20 kids bike issue. Is this normal?
PostPosted: Sat May 09, 2015 7:21 pm 
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Location: The land were time forgot
What was the shops or shop?

:lol:


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 Post subject: Re: Orbea MX20 kids bike issue. Is this normal?
PostPosted: Sat May 09, 2015 8:47 pm 
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Posts: 407
Location: Romford
Cyclesurgery Romford mate.


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 Post subject: Re: Orbea MX20 kids bike issue. Is this normal?
PostPosted: Sat May 09, 2015 10:56 pm 
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I know isn't going to be popular . But I buy & repair bikes and have sold a good 60 or so at carboot sales and on ebay.

This is completely normal behavior when pedaling backward on a small child's frame ( which have shorter chain-stays)

Think about normal pedaling, the chain is pulled tight across the top when driving forward and the slack return is guided to the appropriate rear gear as is passes through the derailleur. When pedaling backwards the chain, is meeting the sprocket well before the derailleur and so is free to do what it likes ( in this case it will return to its straightest line due to the links in the chain resisting the bend at either extreme

If its a massive problem consider dialing out this gear entirely, fitting a closer ranged cassette (to give the derailleur more of a chance) or fitting an older chain which wont fight the bend so much. But obviously not pedaling back-wards is still your best choice


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 Post subject: Re: Orbea MX20 kids bike issue. Is this normal?
PostPosted: Sun May 10, 2015 9:57 pm 
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Posts: 407
Location: Romford
MarkL wrote:
I know isn't going to be popular . But I buy & repair bikes and have sold a good 60 or so at carboot sales and on ebay.

This is completely normal behavior when pedaling backward on a small child's frame ( which have shorter chain-stays)

Think about normal pedaling, the chain is pulled tight across the top when driving forward and the slack return is guided to the appropriate rear gear as is passes through the derailleur. When pedaling backwards the chain, is meeting the sprocket well before the derailleur and so is free to do what it likes ( in this case it will return to its straightest line due to the links in the chain resisting the bend at either extreme

If its a massive problem consider dialing out this gear entirely, fitting a closer ranged cassette (to give the derailleur more of a chance) or fitting an older chain which wont fight the bend so much. But obviously not pedaling back-wards is still your best choice


You are right but this is a frame design fault or not suitable components to me.
Try to explain to a 5 year old to adjust his pedal position by lifting the rear tyre and turning the pedals forward :)
I might try different cassette/chain combination depending on what Orbea come back with next week.


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 Post subject: Re: Orbea MX20 kids bike issue. Is this normal?
PostPosted: Mon May 11, 2015 9:39 pm 
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That first gear is certainly the size of a dinner plate so you are probably on the right lines trying a new cassette


If you don't get any joy from Orbea,

May also be worth having a look at the clearance between the cassette and chain stay . It should be minimal in top - there's a chance they have used a full width lock-nut rather than a half width one - in effect moving the cassette across

Similarly the front chain ring should be really close to the chainstay - there's a chance they have assembled the bike with the wrong bottom bracket - spindle length rather than bearing width.
Sounds silly but they do make mistakes - look how many bikes come out of Halfords with the front forks backwards


If you really want to go nuclear ...
You could try bending the chainstay legs apart (equally) and putting another wheel nut on the inside of non-drive side chainstay.
Problem with this is you will also need to dish the wheel more than usual. (About a half turn of each spoke on the non-drive side) to re-centre the rim (and maybe find a longer spindle)
http://www.sheldonbrown.com/frame-spacing.html

Or just return the bike as not fit for purpose ( as you mentioned on OP) and save yourself a load of aggro ....

Happy Trails


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