Shortening Alu Cranks (for my kid)

[quade]

I've just ordered a set of these for my 9 year old:
https://www.aliexpress.com/item/1005005335472302.html

I went for the 155mm crank with a 30T chain ring and BB
They do lengths from 110mm upto 160mm

Its only around 550grams so not bad, and cheap at approx $50
Downside is they will take a long time to arrive, I didn't fancy paying the same price again just for fast postage and although they have good reviews, I'm still cautious.

It amazes me they put 175mm cranks on kids bikes, they are simply way too long.
If you want to figure out what crank length your kid should be running there is a rough calculation:

height (in CM) /10 +0.5
That will give you the crank length in CM

so if your kid is 150cm tall
150/10 +0.5 = 15.5 CM (a crank length of 155mm)

Read this article for more info
https://kidsracing.co.uk/blogs/bikes/kids-bike-components-framesets-cranksets

Remember to use a smaller front chain ring, I went with 30 but I deliberated 28
I'll run a 1x12 (11-51) setup on a XS 26er frame, so that should give him plenty of uphill granny gear  range.

The obvious problem is these kids grow quickly at this age, so I need to see him ride as he grows to figure out if I should move to a 160mm in future, one you get to 165mm, there are plenty of main stream crank options available, but under 165mm they are as rare as rocking horse poo
 

[quade]

A quick update, the crankset arrived, the bike shop tried with the existing BB but opted for the one that came with the kit as its "fitted" better, so rather pay a bit extra for BB that can come with the the set

I'm extremely pleased with the result, so would recommend if you can wait the 3 weeks for delivery.

[Bos]

Never saw this thread, but I was looking for this recently. Nice to know those cranks on Ali X is worth while. I recently built a 24'' bike for my sons 9th Birthday. 

Nils has done some which caught my attention on his Instagram page. 

My sons bike was 14kg not 12kg as advertised, so I'm pulling all of my weight weeny learnings out!

His current cranks are not bad, 150mm but square taper with press fitter 36t chainring and some plastic guides. I know they will be 1.3kg as those BB's were especially heavy. I have run out of steam with this a bit as I got distracted with other things but I picked up  an old Deore crankset which I want to convert to 150mm. He pedals really nicely with the 150, so I don't want to go to much shorter. This will also knock 500g off his bike and give him a 32'' narrow wide option. These cranks has a scalloped area but I think I will be able to cut the hole where there is enough material. 

These photos are from Nils's Instagram. I will also attach a diagram with the suggested crank lengths. 

 

 

 

 

[Bos]

I think bike hub should have its own "kids bikes build" thread. Its quite fun as old parts can be picked up pretty cheaply and gains are easy to make as the bar often starts pretty low with these bikes.

I got him a Silverback SKID 24" which was designed by a friend of mine, and the angles are on the more current side. However the website advertised the bike at 12KG, and when I got it, It was 14kg! Considering he's only 28kg, that would be like me riding a 35kg bike! The bike comes specked better than most and was well priced, but I find with the 24'' bikes and pricepoint its a mix of OEM parts

• Suspension Fork- I got a straight steerer to tapered converter from Summit cycles and could fit an old RockShox Recon which I turned into an 80mm fork (saved 500g and I tuned it up nicely for him it now actually works)
• Swapped cable brakes -  to hydro Avid Elixir just a performance thing he can now use 2 fingers to brake and 3 to hold on
• Swapped stem to 60mm Giant contact (100g)
• Cassette - Swapped the 11-32t stock for an old 1x10 -11-42. These are easy to come by now and great upgrades as I find he spins at 110rpm all the time! and he can now do proper climbs no problem. 
• Tiyes - I have done one wheel as this is quite pricy. I thought I wasn't going to have to do tyres until I took one off to fit a new inner tube and found out they came in 970g each for a non tubeless 2.4'' and had a 180g tube in there.. There are not many 24'' options out there, but Kenda makes a tubeless 24'' Booster 2.2' which was 570g tyre and increased the comfort of his ride massively while it knocked off another 500g.

I will do another tyre next month, and the cranks as I have time. I think its around 11.5kg now and after the mods It should be around 10.7kg ish..(without trying to hard)

The weight difference makes a massive difference in the responsiveness and his ability to ride technical trails. 

[Stefanovdm]

Hi all,

 

Thank you for this post and recommending Nils from WCW. 

 

After seeing the other posts, I dropped off my son's cranks to be shortened to 150mm from 170mm with Nils and today he posted the below pictures.

 

We decided to retain the original 170mm holes for whatever reason (I think if he gets to 170mm I would just buy different cranks).

Definitely worth it for the younger one's.

[Bos]

Glad you came right! I still have to drop mine by Nils, and considered leaving them the full length. But did wonder weather they would be strong enough when he gets old 'big' enough to need that length 🤔.  As they go, he seems to like the jumps and downs already. Anyways one would have to wait and see I guess.

[quade]

Good effort guys 

I just returned from the UK and Isla bikes are going under so selling off a bunch of stock on eBay under various sellers 

I picked up a 24" 24H wheelset with stans crest rims

The hubs are horrid, but 2 full wheels were cheaper than 2 rims locally so will order some super light Ali express hubs for peanuts and switch them over 

The weight savings after this point are getting expensive for minimal returns so will have to stop

[quade]

On 6/5/2024 at 4:26 PM, Bos said:

 

 

• Tiyes - I have done one wheel as this is quite pricy. I thought I wasn't going to have to do tyres until I took one off to fit a new inner tube and found out they came in 970g each for a non tubeless 2.4'' and had a 180g tube in there.. There are not many 24'' options out there, but Kenda makes a tubeless 24'' Booster 2.2' which was 570g tyre and increased the comfort of his ride massively while it knocked off another 500g.

I will do another tyre next month, and the cranks as I have time. I think its around 11.5kg now and after the mods It should be around 10.7kg ish..(without trying to hard)

The weight difference makes a massive difference in the responsiveness and his ability to ride technical trails. 

 

 

Where did you get the kenda tyres from and how much are they ?

I'll save a bit of weight on these lighter ones 

[Bos]

10 hours ago, quade said:

Where did you get the kenda tyres from and how much are they ?

I'll save a bit of weight on these lighter ones 

https://dvillecyclery.co.za/product/kenda-booster-pro-tyre-29x220/?gad_source=1&gclid=CjwKCAjwhIS0BhBqEiwADAUhc2g1o9bdljX5ALd91m9hgNfRdf40o0a7e6JP2MIRVERjESTzXF9_ihoC9EIQAvD_BwE

Kenda Booster 24 on special at the moment. I've only bought one and was planning to look for a grippier front and move the one I have to the back. The Boosters are cool but the knobs are fairly short and small. Its as if the tire designers scale the entire design down instead of redesigning it from the ground up? The reason for going grippier is purely to make up for lack of experience in knowing how far to push on different surfaces.

However, so few options in 24". I'm not sure there is much better.

• The Maxis Recon 's also come in 2.4'' but they are identical to the Booster Pro, (I saw some in person) The Vee tire co crown gem 2.2 is slightly narrower but not available anymore.
• Durbanville has 3 options:
• They have Maxxis DHRII 2.3's and Maxxis Snyper 2.0 Tyres. If only the Snyper came in a 2.4'' It would be an easy pic. but 2.0 might be to narrow. So the DHR 2 is an option, but closer to the opposite side of the spectrum for a 9 year old ripper and quite heavy again. I dont want it to be slow.
• I see there's a Hans Damph 2.35 from merlin cycles in the UK, and I'm there next week.

I just need to make my mind up. The child wont even know the difference>

Word of caution with the Booster pro. Use lots of soapy water when mounting. It had a wobble the first time I mounted it and I had to re-mount and pumped to 5 bar to stretch and seat it properly. It now runs true at about 52mm wide. It was 56mm at 3 bar though, but my 25kg laaitie runs slightly lower pressures.

 

[Bos]

11 hours ago, quade said:

Good effort guys 

I just returned from the UK and Isla bikes are going under so selling off a bunch of stock on eBay under various sellers 

I picked up a 24" 24H wheelset with stans crest rims

The hubs are horrid, but 2 full wheels were cheaper than 2 rims locally so will order some super light Ali express hubs for peanuts and switch them over 

The weight savings after this point are getting expensive for minimal returns so will have to stop

Please share a link. I'm out in the UK next week. 

[quade]

1 hour ago, Bos said:

Please share a link. I'm out in the UK next week. 

I bought this wheelset
https://www.ebay.co.uk/itm/256543360845


Here is their shop
https://www.ebay.co.uk/sch/i.html?item=256543360845&rt=nc&_trksid=p4429486.m3561.l170197&_ssn=valentina8484

 

[quade]

1 hour ago, Bos said:

https://dvillecyclery.co.za/product/kenda-booster-pro-tyre-29x220/?gad_source=1&gclid=CjwKCAjwhIS0BhBqEiwADAUhc2g1o9bdljX5ALd91m9hgNfRdf40o0a7e6JP2MIRVERjESTzXF9_ihoC9EIQAvD_BwE

Kenda Booster 24 on special at the moment. I've only bought one and was planning to look for a grippier front and move the one I have to the back. The Boosters are cool but the knobs are fairly short and small. Its as if the tire designers scale the entire design down instead of redesigning it from the ground up? The reason for going grippier is purely to make up for lack of experience in knowing how far to push on different surfaces.

However, so few options in 24". I'm not sure there is much better.

• The Maxis Recon 's also come in 2.4'' but they are identical to the Booster Pro, (I saw some in person) The Vee tire co crown gem 2.2 is slightly narrower but not available anymore.
• Durbanville has 3 options:
• They have Maxxis DHRII 2.3's and Maxxis Snyper 2.0 Tyres. If only the Snyper came in a 2.4'' It would be an easy pic. but 2.0 might be to narrow. So the DHR 2 is an option, but closer to the opposite side of the spectrum for a 9 year old ripper and quite heavy again. I dont want it to be slow.
• I see there's a Hans Damph 2.35 from merlin cycles in the UK, and I'm there next week.

I just need to make my mind up. The child wont even know the difference>

Word of caution with the Booster pro. Use lots of soapy water when mounting. It had a wobble the first time I mounted it and I had to re-mount and pumped to 5 bar to stretch and seat it properly. It now runs true at about 52mm wide. It was 56mm at 3 bar though, but my 25kg laaitie runs slightly lower pressures.

 

thanks
Seems the link is for 29", cant find 24" on their site

[quade]

Looks like Isla bikes have a decent crankset for sale on their parts list
https://www.islabikes.co.uk/shop/creig/creig-24/creig-24-2019-2023/cranks-bb30-sram-pattern-direct-mount-140mm-creig-24-luath-24

Not sure of the weight of these though, but they will have a narrow Q Factor as isla bikes were considered high end kids bikes,

We've had 2 Isla bikes, and for 24" and below they are amazing (woom also great)

[Bos]

49 minutes ago, quade said:

thanks
Seems the link is for 29", cant find 24" on their site

Sorry about that. I obviously missed the 9 part. I was actually looking at the DHR in 24.. 

https://www.olympiccycles.co.za/product/tyre-kenda-24x2-20-booster-sct/?gad_source=1&gclid=CjwKCAjwp4m0BhBAEiwAsdc4aO9hpuKlhjmpBkkGMc4fsX2wrF-PC4MGJGiKnmVYsI-8J1D_l4cWJhoCHYIQAvD_BwE

Olympic has them for R550

[RobynE]

Not tubeless but another option for 24” - skinwall and 30TPI and great price: 

 

https://www.decathlon.co.za/bike-inner-tubes-tyres-wheels/311761-52973-all-terrain-grip-kids-mountain-bike-tyre-24x195.html

 

And they have slime tubes: 

 

https://www.decathlon.co.za/bike-inner-tubes-tyres-wheels/85-76774-24x17-22-self-sealing-bike-inner-tube-schrader.html