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Discussion Starter · #1 ·
Just thought I'd share this Hotrock I've put together for my wee lad who has just turned 5.

He loves his trials and rides an Oset, but his standard Hotrock wouldn't go over the rockery!!! So, I've geared it down (23t front, 20t rear), fitted 160mm hydraulic discs front and rear and added some knobblies to make a mini trials bike.

Inspired by Demo9's build I saw on here!
 

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Very nice. I just got a 16" for my 4 year old. I doubt he will see much rockery with it though as he is still a little shakey on it. Hoping to get him on some flat trails this spring to boost his confidence.

Sent from my ADR6425LVW using Tapatalk 2
 

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What tires are those and do they come in a 20"? The knobbiest 16-ers I was able to find were the standard Kendas (which worked well and were cheap so I'm not complaining).
 

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Awesome build. My son Gavin wants a trials bike as well. We have a Hotrock that is ready for the conversion. Love your disc tab mount.
 

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Discussion Starter · #7 ·
Hi guys

Sorry for slow reply, been a long Xmas day which followed a late night wrapping!

The tyres are from Oset, they have 2.4, 2.5 and 2.7 width. They also have 20", which I think are by Maxxis. Details can be found here Home » OSET Bikes.

I saw the A2Z, but I had a bit of 6mm alloy in the garage and so just made a template and then cut it out with the jigsaw etc. I wanted the caliper to just slide with the disc to adjust the chain, which I don't think the A2Z does? That and I am likely too tight to pay when it only took an hour to make the bracket! The front was similar, in fact you just use the bracket on the front caliper as template to weld on the fork. I can't confess to welding the bracket on though, as my welding skills are limited to steel. I thought the brackets might be a bit of a pain to make, but actually they were quite straightforward, and work well. If anybody wants any bigger pics of the brackets PM me and I'll happily send.

Not happy with the cranks though. I ran out of time pre Xmas and so just used the standards cranks, which also weigh a lot. I think I need a crank arm about 115mm (compared to the current 92mm?). Anybody got any ideas? Am thinking of buying a couple of trials crank arms, cut and radius them and heli-coiling. Something like these Kooka Trials crankset 175mm 22t integrated ring. Anybody got any other suggestions?!
 

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Here is my version of the Hotrock Trials bike. I used a 20" Echo trials fork, cut down Truvativ cranks and a 22t chainring. I had to space out the cranks with a 113mm bottom bracket.
I put a 18t on the rear hub. I am not sure what the gearing is suppose to be on a trials bike, so we will see how this works.
 

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MIOM/Demo9 could do with a bit of advice if you could
My Daughter has just turned 4 and has a hotrock 12 & 16

She's doesn't like the BMX type bars nor the coaster brake.
She can ride the 16" but finds the 12" easier to pedal because is slightly smaller.
I'm tempted to try and swap the 16 fork to the 12 and fit low rise bars just to see how it fits whilst she grows a few more cm.
If this works then ill fit 20" forks to the 16". Would love to go the spinner route but no one seems to import them in the UK and the cost is high.
Demo9 love the Lil Shreders and that's a great idea that they run 16 & 20 on the same frame.
LBS has some 115 sinz cranks in which I'm hoping with the right chainring and cog will give her a bit more leg power.


Brakes, I don't think she'll be on the 12 for very long so would like to fit discs to the 16.
Can you post some close ups so I can see how you've mounted the brake, and can you also let me know what hubs you are using.

Thanks
Lee
 

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Well stripped both bikes today to see if the 16" forks will fit on the 12"
It would have looked ok and allowed me to switch to flat bars on the 12" but only realised late on that the 16" has a 1 1/8 steerer whilst the 12" has a 1" duhh!
May see if I can pick up a set of 16" forks with a 1" from fleabay for the short term whilst I obtain all the other bits for the 16" build.

Bit more research and I now know Demo has used the Choppersrus disc conversion to mount the discs F&R, but still need to figure out hub options
 

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@Griffter18 I used a Trials bike fork for the front. I found this on Ebay Bloodline 20" Black Fork Disc Brake Tab BMX Trials | eBay

It will only work if your frame has a 15" long top tube. If its the 13" top tube it will rake out the front too much.

As far as hubs go, any 28 hole disc hubs will work for the front. I made my own rear disc hubs by shortening the the free hub body and axle to create the 110mm spacing to work in the rear. The main problem with all the trials hubs is they are 32 hole. All the 16" available are either 28 hole or 36 hole.

Choppersrus has this hub Std Rear Hub - FW/Disc BLK 36 - Cycles U.S. LLC/Choppers U.S.

Its heavy but its cheap. You will then need to source a 36 hole 16" 305 rim. It does say its a 112mm spacing but it will work.
 

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Discussion Starter · #15 ·
MIOM
Who makes the chainring you used with the stock cranks? I assume is hex interface?
Hi.

It's a Federal 7075 Light Sprocket - 23T. I know have picked up a set of Onza trials crank for £5 locally, complete with sprocket. Think they are 160mm, so when I get time I'm going to cut, radius and heli-coil them. The standard crank is very heavy.
 

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Discussion Starter · #16 ·
MIOM/Demo9 could do with a bit of advice if you could
My Daughter has just turned 4 and has a hotrock 12 & 16

She's doesn't like the BMX type bars nor the coaster brake.
She can ride the 16" but finds the 12" easier to pedal because is slightly smaller.
I'm tempted to try and swap the 16 fork to the 12 and fit low rise bars just to see how it fits whilst she grows a few more cm.
If this works then ill fit 20" forks to the 16". Would love to go the spinner route but no one seems to import them in the UK and the cost is high.
Demo9 love the Lil Shreders and that's a great idea that they run 16 & 20 on the same frame.
LBS has some 115 sinz cranks in which I'm hoping with the right chainring and cog will give her a bit more leg power.


Brakes, I don't think she'll be on the 12 for very long so would like to fit discs to the 16.
Can you post some close ups so I can see how you've mounted the brake, and can you also let me know what hubs you are using.

Thanks
Lee
I'll take some pics and post them Lee. The brakes are actually oustanding I have to say. After a few months on it now I haven't touched them, and the reach adjustment is incredible. I'm, sure you could spend a fortune and save a little weight on brakes, but at £70 for the complete setup they're a bargain!
 

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Thanks Mark.

Dropped an email to OSET regarding the wheels & tyres.
Not sure if original 16" tyres will fit the rims?

Looks like OSET also do mechanical disc brakes cheaper than Avid so Ive enquired about them as well.

Rear disc calliper mount, I've download your bike photo and think I can see what you've done.

Last question (hopefully), you've used a 20t rear cog. How does it fit on (screw/bolt)?
 

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Discussion Starter · #18 ·
Thanks Mark.

Dropped an email to OSET regarding the wheels & tyres.
Not sure if original 16" tyres will fit the rims?

Looks like OSET also do mechanical disc brakes cheaper than Avid so Ive enquired about them as well.

Rear disc calliper mount, I've download your bike photo and think I can see what you've done.

Last question (hopefully), you've used a 20t rear cog. How does it fit on (screw/bolt)?
I just used the narrow Oset wheels, they will take a standard tyre IT hink, but I put on the narrow knobbly tyres from Oset as well as they were quite cheap.

Just a screw on cog. The axle has quite a lot of movement potential on it as it comes with spacers. From memory we had to move the rim about 5mm to centralise it in the frame, but it was done by re-dishing using the current spokes, as there was enough spoke on them to move them that little bit.

Now I've got to start on a 20" MTB build, anyone any suggestions?! Thinking of starting with a Marin Hidden Canyon.
 

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Im hoping I can miss the 20" stage out and go straight to 24"
Hopefully keeping her on the 16" a bit longer will help develop her skills more (lighter smaller bike).
Time will tell :)
 

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Mark, when you say screw on cog do you mean a screw on freewheel cog?
If you could post a photo it would really help before I drop the cash on the wheels.
Thanks
 
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