Extra short cranks - bodger's delight

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syklist
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Extra short cranks - bodger's delight

Postby syklist » 9 Jun 2019, 7:54am

Our two year old was determined to sit on the trailer bike after our five year old started using their own 24" wheeled bike. The standard Burley cranks were way too long and I couldn't find anyone selling short 110 mm cranks. So after a test run with wooden blocks screwed round the pedals, (to make sure the aforementioned 2 year old was OK on the trailer bike) I decided to go for my plan B.

I took an old cheap welded MTB triple, ground the teeth off the outer ring to turn it into a chainguard, ground off the inner ring before chopping the cranks in two. The bottom bit of the cranks were then turned 90' and bolted to the rest of the crank. I had to angle grind a fair bit of aluminium off to get the crank sections to sit properly. The effective crank length is around 110mm (pedal centre to BB centre) and child no. 2 is now able to reach the pedals.

bodged_cranks.jpeg


The cranks seem to be holding up fine on our normal run to the day care and back. They easily take the weight of the child when mounting and dismounting. The big test will be a two week tour of North Jutland where we hope to be able to cycle 20-25 km per day but we will see what happens when we get there. We plan to take a kiddie trailer as well in case child no. 2 gets tired.
So long and thanks for all the fish...

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gaz
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Re: Extra short cranks - bodger's delight

Postby gaz » 9 Jun 2019, 9:28am

Don't tell Mr Shimano or he'll reinvent the L-Crank for grown ups :wink: .
There'll be tarmac over, the white cliffs of Dover ...

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syklist
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Re: Extra short cranks - bodger's delight

Postby syklist » 9 Jun 2019, 9:36am

gaz wrote:Don't tell Mr Shimano or he'll reinvent the L-Crank for grown ups :wink: .

Fascinating!
So long and thanks for all the fish...

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NATURAL ANKLING
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Re: Extra short cranks - bodger's delight

Postby NATURAL ANKLING » 9 Jun 2019, 9:39am

Hi,
Okay, well I see a problem where the cranks are joined.
The mechanics there aren't particularly very clever, I.E.the lever holding the pedal is long compared With the distance between the two joining bolts.
If you had used some steel cranks, same method you had used, and welded then it would've been more reliable.
But if you're going to weld them then just keep the cranks straight and join two ends together.
Your modification might as well hold up, but I'm just a little worried, I think the obvious thing is probably that they will come loose first.
Not simply break.

Good luck.
NA Thinks Just End 2 End Return + Bivvy
You'll Still Find Me At The Top Of A Hill
Please forgive the poor Grammar I blame it on my mobile and phat thinkers.

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syklist
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Re: Extra short cranks - bodger's delight

Postby syklist » 9 Jun 2019, 10:15am

NATURAL ANKLING wrote:Hi,
Okay, well I see a problem where the cranks are joined.
The mechanics there aren't particularly very clever, I.E.the lever holding the pedal is long compared With the distance between the two joining bolts.
If you had used some steel cranks, same method you had used, and welded then it would've been more reliable.
But if you're going to weld them then just keep the cranks straight and join two ends together.
Your modification might as well hold up, but I'm just a little worried, I think the obvious thing is probably that they will come loose first.
Not simply break.

Good luck.

Like I said, it was a bodge. ;) The choice was between trying to find 110 mm square taper cranks, or buying "proper" crank shorteners like SJS sell or paying even more for similar things here.

The cranks weigh more than a straight 110 mm crank would of course but otherwise behave like a straight crank. What is important is the shortest distance between pedal and BB centres. Our two year old won't be able to put any significant force on the cranks when cycling, the biggest strain is during mounting and dismounting (+-12kg for a second?). The bolts have nyloc nuts on the back and are checked regularly.

Steel was not an option as I only had aluminium crank sets in stock. I will save my welding skills for modifications to a pair of cheap steel crank shorteners so I can bodge them to work with the Burley aluminium cranks as and when longer cranks are needed.
So long and thanks for all the fish...

Brucey
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Re: Extra short cranks - bodger's delight

Postby Brucey » 12 Jun 2019, 8:55am

if it works it works. The strength required for a two-year old is not that much; however an older, stronger, child would soon challenge that arrangement I think.

FWIW a good method for getting shorter cranks (that also means you don't have to buy new cranks every five minutes as the child grows) is to drill and tap new holes in longer cranks;

Image

The extra holes can be drilled and tapped with the 1/2" thread that is used for children's pedals. The cranks won't be as strong as normal but this doesn't completely weaken them either, and several holes can be drilled along the length of each crank to allow for growth.

Tap sets are about £10-£12 on ebay for a set of three (left or right handed). You need a set of each to do both cranks of course.

cheers
-------------------------------------------------------------------------------
~~~~~~~~~~~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~

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syklist
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Re: Extra short cranks - bodger's delight

Postby syklist » 12 Jun 2019, 12:30pm

Thanks for the Ebay tip, I try to buy stuff here first or from other sources that aren't Ebay but had no idea where to start looking for tap sets here. I also needed to get the job done quickly so our 2 year old could get used to cycling on the trailer bike. It was a bit of a rush job as we hadn't expected that child no. 1 would stop using the trailer bike so suddenly (and want to cycle their own bike on tour) nor that child no. 2 would claim the trailer bike for themselves. :) Most things coming from abroad get stuck in customs for a few days and hence takes a week to get here which would have added to the delay.

We will use these cranks for the up coming tour of Denmark, then until the first snow comes for the daily run to day care. Then the trailer bike won't be used until March. I expect to have a better solution in place for the 2020 season. I have plenty of "scrap" cranksets so a tap set will probably be on my Winter Solstice present list.
So long and thanks for all the fish...

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syklist
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Re: Extra short cranks - bodger's delight

Postby syklist » 6 Dec 2020, 4:52pm

Brucey wrote:if it works it works. The strength required for a two-year old is not that much; however an older, stronger, child would soon challenge that arrangement I think.

The bodger's delight cranks survived this year's tour from Svenseid to Dalen via the Telemark canal route including a diversion down to Treungen and back. Child nr two definitely helped me a lot this year pushing on the pedals when asked. Your friendly bike route planner will tell you how much up and down is involved in that route. So the engineering I used to create these cranks was sufficient for a larger stronger child helping in hilly terrain.

The trailer bike had been used most days in the cycling season going to and from daycare between the two summer holidays excluding the first lockdown period. I checked the tightness of the bolts before this years tour and they were fine.

Child nr 2 started cycling without stabilisers when we got back from the cycletour on a 16" wheeled bike before growing enough in August alone to be able to progress to a 20" wheeled bike. As the 20" bike has much longer cranks I will put the original Burley cranks back on the trailerbike during the winter layoff. Although I suspect we won't be needing a trailerbike next year.
So long and thanks for all the fish...

KTHSullivan
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Re: Extra short cranks - bodger's delight

Postby KTHSullivan » 10 Dec 2020, 1:40pm

I do like a good bodge, which often provides a viable solution to a fundamental engineering problem; big thumbs up for the above.
Just remember, when you’re over the hill, you begin to pick up speed. :lol:

Carlton green
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Re: Extra short cranks - bodger's delight

Postby Carlton green » 10 Dec 2020, 2:26pm

NATURAL ANKLING wrote:Hi,
Okay, well I see a problem where the cranks are joined.
The mechanics there aren't particularly very clever, I.E.the lever holding the pedal is long compared With the distance between the two joining bolts.
If you had used some steel cranks, same method you had used, and welded then it would've been more reliable.
But if you're going to weld them then just keep the cranks straight and join two ends together.
Your modification might as well hold up, but I'm just a little worried, I think the obvious thing is probably that they will come loose first.
Not simply break.

Good luck.


I preferred Brucey’s idea of taped holes but sometimes you have to work with what you have. IMHO a neat idea from the OP, but I can also see the ‘flaw’. The simple way around that ‘flaw’, or to at least reduce the issue, is simply to minimise the length of the section threaded for the pedal (‘obviously’ as a result the length of the arm connected to the BB axle would need to increased slightly). A more closely fitting half lap joint with the bolt holes diagonally opposed would have been nice too, but it's easy to pass negative comment from the comfort of my arm chair and with a vision of what’s already been achieved. Well done to the OP on a workable low cost solution.

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syklist
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Re: Extra short cranks - bodger's delight

Postby syklist » 12 Jan 2021, 5:05pm

Thanks for the "upvotes".

I think I will keep my bodged cranks for posterity and hang them on a wall somewhere when I take them off the trailer bike. Fond memories of two great cycle tours. Child number two is already saying that they want to do this year's tour on their own bike so I think the Burley will be hanging on a wall soon too.

I am hoping that we will all have been vaccinated by the summer and can travel to the flat bits of Denmark without risking a bout of the dreaded lurgy. Otherwise it might be interesting with a 4 year old going on 5 touring on their own bike for the first time in hilly Telemark. :D
So long and thanks for all the fish...