# Hotrock 16 Trials Build



## MIOM (Sep 24, 2012)

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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## Chillout (Sep 19, 2010)

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.

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## EZuphill (Nov 21, 2011)

Sweet ride:

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## mtnbkaz (Feb 2, 2004)

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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## Demo9 (Nov 20, 2006)

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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## JonnyScotti (Aug 3, 2012)

That's so freaking awesome! Question about the rear disc mount... Was that something you decided to make? I ask because of this...

Amazon.com: A2Z Rear Disc Brake Mount for Tabless Frame, DM-UNI: Sports & Outdoors

Perhaps the A2Z won't work? None the less, an A+ for ingenuity!


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## MIOM (Sep 24, 2012)

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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## JonnyScotti (Aug 3, 2012)

The only company I know making 115mm cranks is SINZ. 

Thank you for the info on the bracket.


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## Demo9 (Nov 20, 2006)

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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## griffter18 (Jul 3, 2009)

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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## griffter18 (Jul 3, 2009)

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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## jrogs (Sep 2, 2012)

Those are awesome


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## adamm3 (Mar 26, 2007)

MIOM
Who makes the chainring you used with the stock cranks? I assume is hex interface?


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## Demo9 (Nov 20, 2006)

@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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## MIOM (Sep 24, 2012)

adamm3 said:


> 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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## MIOM (Sep 24, 2012)

griffter18 said:


> 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.
> ...


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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## griffter18 (Jul 3, 2009)

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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## MIOM (Sep 24, 2012)

griffter18 said:


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


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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## griffter18 (Jul 3, 2009)

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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## griffter18 (Jul 3, 2009)

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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## MIOM (Sep 24, 2012)

griffter18 said:


> 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


Yes, like this - BMX / SINGLE SPEED FREEWHEEL 16,18,20T | eBay


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## griffter18 (Jul 3, 2009)

*Rear Wheel*

Hi Mark,
Is this what the rear wheel looks like?

Products Hubs Single Speed 6pawl Mini Cassette DMR Bikes


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## griffter18 (Jul 3, 2009)

Just a quick update should anyone else look to undertake this build.
The 16" disc wheels can be obtained from OSET for circa £30 ea.
The rear one is a custom hub which on first inspection has disc mounts on both sides of the wheel.
The discs actually connect to the circular flange not the typical star shape that is on most wheels.
In order to fit a freewheel you need to remove the star shaped mount.
Place the star shape part in a vice and turn the wheel anti clockwise. Its on tight but will release and can then be removed.

If you do use OSET they are also reasonable on other parts ie 160 discs £10ea and mechanical breaks are also cheap compared to MTB prices.


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## MIOM (Sep 24, 2012)

The hubs are by Joytech. Not the best, I stripped and rebuilt them before fittign as they were quite tight!


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## felchi80 (Jun 2, 2014)

Is the OSET rear wheel available anyway online in north america?


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## MIOM (Sep 24, 2012)

felchi80 said:


> Is the OSET rear wheel available anyway online in north america?


Not sure about online, as I'm UK, but there's a dealer network at Dealers » OSET Bikes and the wheel is listed on the US web-site.


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## felchi80 (Jun 2, 2014)

MIOM said:


> Not sure about online, as I'm UK, but there's a dealer network at Dealers » OSET Bikes and the wheel is listed on the US web-site.


 Thanks

I contacted a dealer in Canada. 
He didn't give me a price yet. Said it would take a month to order it....

Maybe I should just lace one up with one of the hubs posted earlier.

Who invented the damn coaster brake... My daughter just learned to ride her Hotrock 16... Coming from a push bike. 
She cannot start by herself because the pedal cannot be positioned. Also she cannot brake!!
This is very bad.... I need hand brakes and a freewheel asap.


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## MIOM (Sep 24, 2012)

felchi80 said:


> Thanks
> 
> I contacted a dealer in Canada.
> He didn't give me a price yet. Said it would take a month to order it....
> ...


I hate the coaster brakes, my boy couldn't get along with his despite being quite a good rider. Am sure someone will say they're for safety due to strength of little hands etc, but you can make any type of brake efficient enough for a kid to pull it. Hydraulic discs though are the best!!


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## felchi80 (Jun 2, 2014)

uh oh... I messed up.
Bought a flip flop hub with 30mm thread instead of standard on one side.... seems like biggest freewheel available for 30mm thread is 15T... Now I need to find a smaller chain ring... I just bought an SRAM crankset with 38T to be shortened. 
Would 15T rear 38T front be too hard for the little one? She does not have much leg power yet.


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## spyguy (Apr 18, 2006)

does anyone know what stem size will fit on the hotrock 16?


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## bikin222 (Apr 23, 2007)

It is a 1 1/8th steer tube. Is that what you mean?


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## spyguy (Apr 18, 2006)

thanks bikin222.
for a freewheel rear disc hub, any one in the US have a recommendation?


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## GrayJay (May 16, 2011)

spyguy said:


> thanks bikin222.
> for a freewheel rear disc hub, any one in the US have a recommendation?


Std Rear Hub - FW/Disc BLK 36 - CYCLES U.S.


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## cromo900 (Jun 12, 2006)

Since my son finally wore out his back tire with all the big coaster brake skids, I figured it was time to add real brakes.

I spoke with Steve at OSET (Contact » OSET Bikes) and within a week have the following:
Front and rear disc wheelset (narrow). The wheels ship complete with a 2.5" front and 2.7" rear tire for $46/ea. I also got a set of their narrower 2.4" tires for $17/ea.

First impressions:

Fit -
The back wheel is not aligned for proper chainline or rotor clearance on the non-drive side. This was easily remedied by swapping a spacer between the cone/locknut from the driveside to non-drive side of the axle. As mentioned previously or in another thread, the drive side 6 bolt adapter will need to be unthreaded/removed. This is easily done with a small vise. The rear wheel did not include axle nuts, but are standard axle size (10x1 I think), so the stock nuts transfer over.
The front hub flanges do not clear the fork legs. If you want to use OSET's front wheel, it seems you will need a fork with wider clearance around the dropout. Now I'm wondering how MIOM got around this&#8230;?

Weight - 
OSET Front wheel @ 712g vs. stock @ 575g (But I won't be using the OSET front wheel due to fit issues). OSET Rear wheel @ 766g vs. stock @ 1176g (-410g!). OSET Tires: 2.4 @ 685g (+/-10g), 2.5 @ 706g, 2.7 @ 865g. Stock tires are @ 350g (+335g per 2.4" tire! :-( ). I think if I were to do it again, I'd just go with the 2.5" tires as they seem nicer; maybe it's just my perception because they are actually branded Kenda vs. something generic. At any rate, the 2.4" tires seated evenly and easily on the OSET rear rim and stock front.

Will be working on an aluminum bracket to mount the caliper up to a 140mm rotor next...


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## felchi80 (Jun 2, 2014)

cromo900 said:


> Since my son finally wore out his back tire with all the big coaster brake skids, I figured it was time to add real brakes.
> 
> I spoke with Steve at OSET (Contact » OSET Bikes) and within a week have the following:
> 1. Front and rear disc wheelset (narrow). The wheels ship complete with a 2.5" front and 2.7" rear tire for $46/ea.
> ...


is that including shipping?


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## cromo900 (Jun 12, 2006)

Shipping was an extra $17 from CO to WA.


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## cromo900 (Jun 12, 2006)

Well, it's all set up and the boy rode it for the first time tonight. Skids are just as bad out of the gate, but I think he will learn to modulate eventually. Disc caliper adapter bracket was a pain to cut out with the hacksaw and I'm not thrilled with the final homemade look of it. I'll find a bandsaw for the next one. A mill would be even better as you could mill down 1/2" aluminum in a way to not need any spacers between the bracket and caliper mount.

At any rate it was a fun little project and relatively easy with the wheel from OSET.


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## griffter18 (Jul 3, 2009)

Don't beat yourself up you've done a good job since it was with a hacksaw.
I was fortunate and had a band saw which made light work of it, I also had a band & disc sander to clear it up.
Paint it to match the frame and it will blend in.

I'm sure with a bit of practice the skids will be reduced. From memory we used a slope where lil1 had to use brake but not lock up.

The good thing is you now have safe braking and they will transfer to the bigger bike much easier when the time comes and you can move the brakes over


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## spyguy (Apr 18, 2006)

so hard finding a 110mm rear free hub for the disc.... any suggestions?


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## Nyquist (May 12, 2005)

Lots of trials hubs are 110mm (or 116mm with those 3mm snail cam things) and are available with disc mounts one side, freewheel the other. 
Take a look here for ideas:
116mm Fixed Hubs

Novatech also make one (bit of a rare beast though).
EDIT - sorry, the Novatech one is 120mm, so would need modification.


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## cakemonster (Sep 30, 2012)

felchi80 said:


> Is the OSET rear wheel available anyway online in north america?


This looks promising....

https://kingcobraofflorida.com/osetbikes/parts.php?category=Wheels

$45 for the narrow rear disk setup, beats the $100 with shipping I was quoted from the UK...


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## cakemonster (Sep 30, 2012)

Nyquist said:


> Lots of trials hubs are 110mm (or 116mm with those 3mm snail cam things) and are available with disc mounts one side, freewheel the other.
> Take a look here for ideas:
> 116mm Fixed Hubs
> 
> ...


Echo TR here and Bonz hub look good, can get 32 hole 16 inch rims from Velocity or Lil Shredder (better option).

That Oset wheel is a cheaper option through and they have a 12.5 for Hotrock 12 conversion!


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## cakemonster (Sep 30, 2012)

cromo900 said:


> Well, it's all set up and the boy rode it for the first time tonight. Skids are just as bad out of the gate, but I think he will learn to modulate eventually. Disc caliper adapter bracket was a pain to cut out with the hacksaw and I'm not thrilled with the final homemade look of it. I'll find a bandsaw for the next one. A mill would be even better as you could mill down 1/2" aluminum in a way to not need any spacers between the bracket and caliper mount.
> 
> At any rate it was a fun little project and relatively easy with the wheel from OSET.
> 
> View attachment 908918


Did you use a template for that adapter that attaches to the internal brake boss on the frame? That seems the most secure of the ones I have seen. How thick was the aluminum, 11/16?


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## griffter18 (Jul 3, 2009)

Cake monster follow this link and look at post #17
http://forums.mtbr.com/families-riding-kids/hotrock-16-build-thank-you-850533.html


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## MIOM (Sep 24, 2012)

cakemonster said:


> Did you use a template for that adapter that attaches to the internal brake boss on the frame? That seems the most secure of the ones I have seen. How thick was the aluminum, 11/16?


Hi. Yes, I just made a template and used some 6mm aluminium plate to make the bracket. If you want me to take a copy of the bracket and e-mail it to you let me now, happy to do so.

Thanks, Mark.


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## cakemonster (Sep 30, 2012)

MIOM said:


> Hi. Yes, I just made a template and used some 6mm aluminium plate to make the bracket. If you want me to take a copy of the bracket and e-mail it to you let me now, happy to do so.
> 
> Thanks, Mark.


Thank You, that would be awesome. How did you create the correct spacing for the disc caliper? Custom spacers? Do you have a picture of the final setup? Thank you.


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## griffter18 (Jul 3, 2009)

Felchi80 heres the link to the OSET USA Distributor list.
Dealers » OSET Bikes

Alternatively look at this thread as there is an alternatives http://forums.mtbr.com/families-riding-kids/16-freewheel-options-ways-do-911575.html

Good luck


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## ihstiv (Apr 6, 2015)

cromo900 said:


> Well, it's all set up and the boy rode it for the first time tonight. Skids are just as bad out of the gate, but I think he will learn to modulate eventually. Disc caliper adapter bracket was a pain to cut out with the hacksaw and I'm not thrilled with the final homemade look of it. I'll find a bandsaw for the next one. A mill would be even better as you could mill down 1/2" aluminum in a way to not need any spacers between the bracket and caliper mount.
> 
> At any rate it was a fun little project and relatively easy with the wheel from OSET.
> 
> View attachment 908918


Thank you for sharing your project here!

I am following in your footsteps w/ an identical build and ordered the OSET rear wheel today.

I'm wondering what I will need for the rear drive gear though? Can I use a standard threaded BMX freewheel like this? 








Thanks again.


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## cromo900 (Jun 12, 2006)

Yes, standard bmx freewheel fits


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## ihstiv (Apr 6, 2015)

cromo900 said:


> Yes, standard bmx freewheel fits


Thanks!

..for anyone else looking at the OSET wheel option, here are a couple pics showing each side of the 16", narrow, OSET rear hub that the distributer sent me.


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## ihstiv (Apr 6, 2015)

@cromo900 - does the star/cog shaped side of the hub unscrew so that I can screw the freewheel on there?

It didn't come off easily so thought I'd ask before I apply more force..

Thanks!


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## griffter18 (Jul 3, 2009)

Yes. See post 23 in this thread.
Put the star shaped part in a vice and turn the wheel. It will then start to come off and you can finish without the vice.
You then replace it with your freewheel.


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## ihstiv (Apr 6, 2015)

griffter18 said:


> Yes. See post 23 in this thread.
> Put the star shaped part in a vice and turn the wheel. It will then start to come off and you can finish without the vice.
> You then replace it with your freewheel.


Aha - missed that post, thank you!!


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