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: .
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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.
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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
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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...