EDITORIAL - DOES THE FUTURE HAVE FEWER GEARS PT VII
Building A Better Kids' Mountain Bike Drivetrain
Bettering BOX 8-spd
I'll say this about the BOX 4 rear derailleur that came on my kid's bike; that thing has taken a right mauling. In seven months it's survived three shifter cables, two derailleur-hanger straightenings, countless direct hits, and more than a few incidences of being used as a combination brake & anchor to instantly arrest my child's forward momentum. Given how much spec this BOX 8-spd-with-clutch drivetrain is getting on grom-friendly rigs, I'm a bit surprised they didn't make the rear derailleur silver (so it doesn't look like crap after the first two real rides) but for the price, it's hard to argue with performance.
The clutch keeps things plenty quiet at 20" wheel speeds, she hasn't dropped a single chain, and the shifting action is light enough for a six-year-old's thumbs to drive the derailleur through most of the gears in the heat of battle. For riding on North Shore single track, the gear my daughter climbs in is the lowest option and in addition to a positively awful chainline, shifting up to the 42t big cog can be hard on the road and often proves impossible on the trail.
Unfortunately, this discourages her from shifting out of the 42t, which all but makes her Early Rider an over-complicated single speed with an awfully-angled chain anytime we're hitting the trails. I also notice that when we're out on the road, my daughter never uses anything close to the combination of her 30t chainring and the 11t small cog. Time to jump on the Sheldon Brown Gear Calculator, crack open my box of outdated spares, and math out a better way to turn 127mm cranks offroad.
My goal is to still deliver the low gearing that my daughter needs to climb...
...reuse old spares, like this 9-spd cassette, as much as possible...
...and get her bike's shifting performance to the point she's comfortable dropping into a higher gear here.
It's a long story, but in my box of bits I happen to have a chunk of a Shimano 9-spd M770 cassette. It's six cogs on a single aluminum carrier with a range that jumps nicely from a 17t to 34t cog (17, 20, 23, 26, 30, 34) and with a 12t lock-ring and a couple of featherlight aluminum spacers from a single-speed kit a project is born.
One major goal is to keep the price down, although I will spring for an aluminum narrow-wide ring over a steel one, and that absolutely means sticking with the BOX shifter and derailleur. Luckily the spacing difference between 8-spd and 9-spd, especially the aggregate change over six cogs, is minimal so I'm unconcerned about mixing-and-matching here. Since the BOX 4 derailleur will easily shift bigger cogs than the stock 42t, and since I'm stepping down to a 34t cog, I can move the cassette quite a bit outboard, as with my 14-spd project. Sadly, the chainline isn't going to be as good as I'd hoped because of interference with the low-limit screw at the higher-end of the range, but I'm happy.
One note for anyone with a box of parts and a BOX 4 derailleur following along at home, the derailleur set screws use a fine thread-pitch and a much longer-than-stock screw is required. I ended up using a Magura pad retention bolt, which ships with every set of their individual-four-piston pads, and then cut it to size. Same thread-pitch and easy customization for the win.
Chainline before is brutal! This is the gear my daughter uses most of the time on North Shore single track.
Chainline after isn't perfect. But, hopefully with the much improved shifting more ratios are in play and it's still better!
But Weight!
Improved chainline and much better shifting in the gears my kid most uses on the trail are my key objectives but the wow factor here is certainly the weight savings. I am saving more than half the weight of the stock chainring and cassette that came on her 20" bike.
To be precise, I'm reducing these two components by 336-grams just for the cassette and ring and that's not looking at the three links of the chain I removed. I would say it works okay with the stock 8-spd chain, but I'll change it out for a much nicer, fairly fresh chain off my wife's old bike which is too short for anything we're riding now. I knew I kept it around for a reason.
318-grams for my fresh North Shore Billet 26t SRAM ring and Shimano 6-speed cassette chunk.
654-grams for the stock steel narrow-wide ring and BOX 4 8-speed cassette
30t steel. Yes, best practice wise I should have cleaned this before weighing...
...but it's really not a contest with the North Shore Billet SRAM ring.
Don't have a box of old parts with plenty of life left? Post a wanted ad and convert some quality beer into recycled bike parts!
To be fair, it's a basic steel cassette with an 11-42t range. The weight isn't unexpected at all.
I could go with a much less expensive steel 26t ring - from Race Face for example - and there's also the potential to butcher the stock cassette to drop grams without any cost beyond time, but here I'm reusing some old parts that are a sunk cost and for which I don't have another use. In terms of the chainring, the North Shore Billet (NSB) rings hold up really well, have great chain retention, are made in BC, and yes I am getting sucked in by the weight savings for my tiny person and the fact they look sweet.
The shifting immediately improved as well with the cassette and chain combining to deliver much smoother and crisper jumps from cog to cog.
More before & after images. I'm very happy with how it turned out both in terms of performance and appearance.
With the same derailleur, bottom bracket, and shifter the drivetrain manages to feel smoother in general and shifting is improved significantly.
Universally Improved
The shifting is smoother and lighter between the higher-end chain, XT chainring, and NSB ring and that's with no change to the BOX 4 rear shifter or derailleur. I'm genuinely impressed with how quickly and easily it came together with no customization aside from the low-limit screw on the rear mech.
I occasionally felt a modicum of guilt previously, knowing my kid could shift on the road but was finding it very challenging to pull off the jump to-and-from the 42t cog on the trail. What started as a plan to just drop on a smaller front chainring - and gain some usable ratios - morphed into a custom six-speed transmission delivering the low-gear she wants on climbs and the higher gear she needs to hit the local trails with plenty of options in between.
I also have the option to add in more high gears as needed if the 26t chainring and 17t cog prove too light for pedaling on the pavement. Maybe I could dig up a 12t high cog and a 15t to fill in the gap? For now, I think the current range is going to be perfect.
Replacing the low limit screw is necessary to make this project work. I used the pad-fixing screw from a set of Magura brake pads and cut it to the length I needed.
Even with the same rider, shifter, and derailleur, the shifting is immediately improved thanks to better chainline, components, and ditching the jump to the 42t.
Some of my home bicycle projects don't turn out this well and I'm excited to see if after a few rides my child starts to shift more. My next projects are a front & rear tubeless tire conversion so we can cheat a bit more on air pressure and I'm also going to make new shims for her brake levers to eliminate some of the play that's resulted from crashing the bike a lot of times.
I'm always curious what projects other parents are undertaking to maximize the performance of their kids' bikes - especially when it comes to min-maxing the investment in something they crash regularly and grow out of right in front of you - so please let us know in the comments below.
Comments
YDiv
3 years ago
Always amazed to see the ingenuity you have when it comes to optimizing bikes, Andrew. Great read every time.
Choosing the chainring is currently a personal dilemma. There are some dirt cheap 30T 6mm offset steel chainrings available, but it's hard to pick between that and a lighter weight (albeit more costly) one. The chainline is just disgusting for 12spd drivetrains, especially if you like to earn your descents on the Shore and you don't have tree trunks for legs.
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Andrew Major
3 years ago
Thank you!
The chainline on my manual-2x7 on the Banshee has totally soured me on the nasty chain bends of the low gears on 12spd drivetrains.
I’m still waiting for floating chainrings to become a thing although I’m guessing 13spd will just move the 10t outboard (see space made by SRAM universal derailleur hanger).
Maybe chains that Pivot on the horizontal and vertical planes? Those would be excitingly expensive if possible!
I’m happy to pedal the weight of a steel ring - see my love of Wolf Tooth stainless but the inexpensive RaceFace ones are good too - but for this bike grams count when I can cut half the weight out of the cassette/ring and improve performance.
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Sanesh Iyer
3 years ago
SRAM has a cool pivoting ring patent
If you have a 4 bolt crankset, floating ring doesn't seem that hard to do. Youd need some custom chainring nuts and bolts
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Andrew Major
3 years ago
At some point (13spd, 14spd?) it seems that some kind of floating ring or spider will become necessary. I think it makes more sense to have the ring floating on a big spline as opposed to small individual chainring bolts? A big spline like you get with CINCH just extended and then have a crank/ring interface that's replaceable and has low friction.
I mean, the whole thing makes me shiver but my guess is that it will eventually become necessary.
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Sanesh Iyer
3 years ago
Oh totally. There are way better ways to do a floating ring, my point is just that it's something that you/we could test out in a diy manner with a 4bolt crankset and some clever hardware.
I don't think a floating ring is a horrible idea. There's something to be said for always having straight chainlines.
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Cooper Quinn
3 years ago
Did you say its excessive and complicated, but might save 2.8 watts?
*CeramicSpeed has entered the chat*
Andrew Major
3 years ago
@Sanesh without a super wide chainring with some kind of bushing interface I don’t see how it doesn’t flex and creak like an ARRRRGGGGGHHHH
@Cooper who said anything about watts? We’re talking about making 14-spd, 15-spd, & 16-spd chainlines possible for Joey McNextbestthing.
fartymarty
3 years ago
Andrew, perfect timing. I'm just in the process of doing a bike swap with my daughters (6 and 9). Oldest is getting a new XS 27.5 Rockhopper and youngest gets the oldest 24" Frog resprayed to the colour of her choice.
The Rochhopper comes with a 3x drive train thats getting swapped out for 1x Square taper 150mm cranks. Next step will be short stem, proper rise bars, cassettes, adding a dropper and proper tyres / tubeless. The forks probably also need looking at and rebuilding.
I did similar on the 24" Frog (and 20" Frog that's just been sold) and it changed the bike for the better.
It will be interesting to see where others are going with this.
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Edit - we picked up the Rockhopper in the weekend. Have swapped 3x cranks for 1x and bars. The Suntour XCM fork is crap (and heavy) - I can barely get it to move with my 95kg so my 9YO has no chance. Am looking to take out one spring and even put a lighter spring in and add a ton of Judy butter to see if I can get it moving. Otherwise it maybe swapped out to my Krampus rigid fork which is similar length. The chainline is crap (but it does change into the 36 easily) so i've got a narrower BB on the way. It amazes me how "generic" off-the-peg bikes are even in smaller sizes. It does look nice tho.
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Andrew Major
3 years ago
Cool! Send me some pics of the progress if you think of it.
Dropper post eh? I go back and forth (not an issue for a while for me here). I think a nice QR (Chromag, Wolf Tooth) is in the future for this bike.
What dropper are you going with? 70mm XC/Gravel model?
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fartymarty
3 years ago
Will upload some pics once done.
Yeah - I promised her a dropper on her next bike. I'm constanly adjusting her seat height during rides so thought a dropper would be the way forward. I was going to use my old 125mm x 27.2 Thomson off my Krampus and shim it to suit. Then put a new 125 x 27.2 PNW internally routed on the Krampus. The Thomson is good when it works but you need to send it away for servicing - PNW are way more user friendly.
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Andrew Major
3 years ago
My brother has an old Thomson post (non-stealth) and is debating whether it's worth fixing as a spare or destined to the recycling bin. Do you know what they charge for a full re-and-re? Plus shipping to the USA HQ I guess?
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fartymarty
3 years ago
The UK based Thomson service centre (Upgrade Bikes) charges about £60 for a rebuild and about £120 if it needs a cartridge rebuild. I've only ever needed the rebuild. At £120 it's nearly cheaper to buy a new post but I like to keep things working if possible. I did see a tutorial on linenon how to rebuild yourself so may have a go at some point. Should be easy with the people you know.
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Mammal
3 years ago
With the original MSRP prices those posts were going for, the fact that they are difficult/expensive to find service for is just insane. It's a bit of a hind-sight situation now, with durable posts going for 1/2 the price they used to, but the Thompson posts were certainly at the premium end of the scale and the value proposition didn't age well.
mrbrett
3 years ago
I like it! The super short chainstays on a 20" bike seem to magnify any chainline issues you may be having.
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Andrew Major
3 years ago
Thanks! Absolutely, the short ACTUAL chainstay length plays havoc here. Totally gapped on that. I always think of them as being proportionately long - a good example of chasing that short person bike fit.
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Poz
3 years ago
Great article Andrew.
I upgraded my sons Bighit Grom from 165 cranks with 36 ring and 12-32 cassette and knackered x5 shifter. To a lbs bin 155 30 black spire ring, 12-36 back and 9speed xt shifter. Night and day difference for him.
This is why I can never bring myself to get rid of old parts. And when I do I find myself in need of those bits eventually.
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Andrew Major
3 years ago
Thank you!
Yeah, I live in a small space with two other humans-with-many-things so I cull stuff regularly but I’ll hide any bike part with life left somewhere - and the habit has paid off both in terms of min-maxing our fleet and projects like this one.
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BkrAdam
3 years ago
Lately I've been buying $10 stems to try and get the bike fit dialed.
In the past I modified the air shaft on a Spinner 20" suspension to get the air compression ratio closer to 2.5:1. The stock compression ratio was over 4:1. The fork would either not mover or blow through the travel. This was a few years ago before other moderately inexpensive 20" forks existed. I also built a tubeless rear wheel using atomlab rims and bontrager/Dt240 hub that I scored for $50. I converted a 10 speed cassette to 8 speed to improve the chain line.
I'm at the point now where I am saving the wife's old bike for the oldest to grow into. That's a year or two away.
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Andrew Major
3 years ago
We stayed rigid for 20" just for the lighter weight, lack of complexity, and just how much unserviceable garbage is out there. For 24" we'll be running a lowered 26" fork. I have a really nice custom Fox 32 from ~2012 that's fresh.
I feel like everything gets easier the closer they get to riding our size of bikes. Including hand-me-downs.
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Poz
3 years ago
The last couple years with brands putting actual bike parts on kids bikes has been a boon (31.8 bars and stems, mtb cranks, and the option for real rubber). Let’s us tailor the bike as the kids grow.
I too have a selection of bargain bin stems to put on the kids bikes. 10mm makes a bigger difference in reach to a 4 footer than a 6 footer.
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Andrew Major
3 years ago
Totally. The same goes for playing with bar width.
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Sun Hester
3 years ago
I love this stuff and I've got my own 'just turned 9' daughter's 24" Ripcord dialed. Got it to 22.1#s with pedals as well!
Had to bribe her with trips to the pet store to get her to practice shifting on pavement and now she's transferred that very well to the trails. We are working on the attack position now.
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Andrew Major
3 years ago
I’m very laissez-faire in terms of her learning on her in (as I did) but certainly trying to make the opportunities available with good shifting and good bike geo. Replacing the stock cable/housing helped too.
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Andrew Major
3 years ago
This comment has been removed.
SteveR
3 years ago
Following. My 5 year old granddaughter is rapidly growing and progressing, and will be ready to move up from her 16" single speed Cleary by summers end, if not sooner. I recall messing with the gearing on her mother's first bikes back in the 90's to get more kid-friendly ratios. Keep that grom content coming!
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Andrew Major
3 years ago
I loved a lot of things about my daughter's belt-drive 16" wheeled bike. The one thing I really didn't love for where we live is not being easily/cheaply able to change to lower gearing. Lesson learned and now the big things for me are cassette hubs instead of freewheels (it's remarkable - and disappointing - how many 20" and 24" bike use freewheels still even ones that cost solid a lot of $$$ and where most their competition is running cassette at the same price point) and cranksets that use really common chainring interfaces.
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Bikeryder85
3 years ago
Love the nerdery Andrew! If only I kept that box of 8sp stuff from '98...
As an aside...I know you did a post on the bike (here or meat engines) but I would like to ask how it is working out...I may have a grom on my hands by summer's end.
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Andrew Major
3 years ago
Thanks! You definitely should have kept that 8sp stuff!
-
How having a grom is working out? I genuinely love being a dad. I have been very privileged to have done it as my main gig since my daughter was very small -thanks to friends who’ve found me part time employment around my kids schedule (like NSMB!) and a very hard working partner who is willing to sacrifice on having lots of new stuff so we can live within our means. I’ve enjoyed every age/stage so far (the “terrible twos” and “f***ing fours” included).
We’ve had two fantastic teachers in a row so even with all the challenges of the pandemic our school experience has been great (thank you amazing teachers!!!)
I’ve written a few pieces about riding with her over at my journal, MEATEngines. My kid just started mountain biking in September (aged six) and caught the bug so hard that she was the one driving us to go night riding after school (even in the pissing rain) all winter. Which has been awesome. I don’t progress much anymore no matter how much I try so watching her roll her 20” bike around the Shore getting better every ride is so cool.
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Marty Zaleski
3 years ago
Interesting. My 6-year-old has the 24" version of this Early Rider bike. Came with an NX 1x11 drivetrain. That stock bit of kit seems perfect for him; I haven't been tempted to swap it. Loving how light this bike is. Now, about those pinner rims and the lack of a water bottle mount and dropper post...
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Andrew Major
3 years ago
SRAM is probably the best choice for kids - money aside - because of the light shifting action v. usable clutch. Certainly NX is easier to shift than Box 4. The big jump to the 42t with this cassette (which is fine with bigger/stronger hands) was the biggest issue though. Well, that and the awful chainline.
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cheapondirt
3 years ago
Tangentially unrelated question. You said the word chainline...
Looking at how to improve chainline on my own bike. It's 32x11-46. Swapping the 11t (which I never use) to the inboard position as a spacer - is that a thing people do? Or would a smaller chainring be a good choice despite not needing a lower gear?
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Andrew Major
3 years ago
It's totally related... to the next addition of Does The Future Have Fewer Gears... as that's where I've gone with the Banshee.
Money no object, smaller ring and smaller cassette block spaced outboard provides lower weight (and on an FS bike that's lower unsprung weight) is, I think, the way to go. I'm running a 26t on the crank with 11-36t in the back.
Ditching the 11t doesn't get you far with most cassettes as it actually overhangs the driver. You can ditch the 11t and move the cassette outboard a bit (you also need a 12t-size lockring for it to work properly) or ditch the next cog or two (12t, 13t) and space the whole cassette out to the 11t. It's just a matter of what your derailleur can handle in terms of cog size at a given location along the driver.
Hope that's helpful!
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cheapondirt
3 years ago
Yes, that is really helpful. I didn't know about the different lockring and didn't consider the overhang. Still might have to try it out!
So the Banshee is back to 1x? Don't give it all away now, but 26x11 seems low.
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Andrew Major
3 years ago
26tx11t isn't much lower than my current high gear on the 2x7 but yes, it sounds very low. Gotta try it. It's a work in progress, and then I need to ride it but yes my next ride on the Banshee will be 1x but with the teeny, tiny ring.
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cheapondirt
3 years ago
Moved the 13t or whatever the smallest "normal" cog is to behind the 46. Don't know what I was expecting, but shifting across the new gap is unremarkable and the chainline still has a lot of room for improvement. Obvious low hanging fruit would be to space the chainring in, but my frame's integrated chainguide prevents that.
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Andrew Major
3 years ago
Every step towards straighter counts. Does derailleur still shift the 46t with no issue?
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cheapondirt
3 years ago
Yes. Maybe not quite as smooth, but it was never smooth. Big jump from 37 to 46
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Skeen
3 years ago
This article and the ensuing conversation are awesome! Not applicable for my 2 year old and his strider yet, but gets my gears turning for future builds and continues to justify my big ol box of spare parts (drive train components and lots of other stuff), i hate throwing out old parts that still have life left, even if its a chain stretched to 0.5. More importantly: good chain lines, shift action, and gear ratios for my up and coming grom. Thanks for the insight Andrew.
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Andrew Major
3 years ago
The chain on this setup was a spare with life left that was just too short for anything except my single speed. It's a good reminder that I take to heart when I am selling old parts that I better be damn sure it's not something I'd turn around and use.
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papa44
3 years ago
While not exactly diy I do have shortened zee cranks for my 10 year olds bike which along with a set of old yeti arc carbon bars probably saves a good few kilos. It’s crazy how heavy they make kids bikes
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Andrew Major
3 years ago
Interesting, I would have assumed the Zee's were hollow at whatever point you would have shortened them too?! How long did you go with?
I've seen Tiagra cranks shortened a few times as well as old XTR M950/M952 arms that have been shortened. Also some older solid RaceFace arms. I'd be very curious to see photos! Cool project.
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papa44
3 years ago
They are solid, at least at the pedal end, and you are able to shorten them to 152mm. I can’t remember why that exact number but there is an engineering company in the UK that will drill and thread them for you. I have to say it’s such a huge improvement for a kid it’s worth every penny
I don’t think that image thing worked so here’s a link:
https://imgur.com/O9ESFUF
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Andrew Major
3 years ago
I'm deciding how brave I am. I have a drill press and I have pedal taps. It's not that hard to track down a used set of cranks...
HAHAHAHAHA. Thanks, I think?!
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Geof Harries
3 years ago
Our 10-year old has inherited what used to be my wife's, then our older son's 2012 Devinci Dexter XP (example) in size small. He likes how squishy it is and how he can run directly into curbs and it bounces up and over.
Right now the Dexter has a 2x9 drivetrain that I'm thinking I'll replace with a lightly used NX 11-speed derailleur, Gripshift shifter and a 30T RaceFace narrow-wide ring combo that I have kicking around.
Do you have any suggestions for the cassette size?
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Andrew Major
3 years ago
With a 26" wheel and a 30t ring you're going to want at least a 42t low cog if you're riding Shore-style terrain with steep climbs and probably larger if your NX derailleur will clean it.
Or you could always do a manual 2x if your crank is a 2x crankset. Then there's an option of a 22t or 24t grannie when the trails get really steep.
We're a ways off, but when my grom is ~10 I can definitely forsee a 26t front ring with an 11-42t cassette or even an 11-46t.
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fartymarty
3 years ago
Mammal - the only reason I bought a Thomson post was it was one of the only 27.2 posts at the time - it was that or a KS which had a crap reputation. From memory I got it for £280 on sale so not as expensive as the insanely expensive £420 retail price. The UK rep did replace it for me after about a year as it kept developing rotation play- which they said it shouldn't.
Now there is a lot more choice in 27.2 so will probably retire it to the kids bikes and fet something like a PNW that works and is user serviceable.
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Mike Kittmer
3 years ago
Second MTB for my little guy now. 2016 24” RM Vertex Jr. tore down for full rebuild including the Suntour XCR (bushings, wipers, air piston replacement). Has hand me down Racefsce Next bars and lock on grips and new Oddessey BMX pedals. He’s loving riding way more now! Is climbing up Knox Mtn here in Kelowna with me now and has conquered the main DH trail twice now. Going to be a fun summer :)
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Andrew Major
3 years ago
Very awesome!
How does your little ripper do water? You're carrying it? Hydration pack?
My daughter's 20" has bottle mounts and I have an awesome Lezyne side-loader cage but getting a small enough bottle to clear for easy in-and-out access is still proving troublesome.
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ZigaK
3 years ago
Thank you for this.
My 7yr has a Norco 4.3+ and had bashed his alivio 8spd derailleur so much that it could not shift in the first gear (34 tooth cog), because it would get caught in the spokes. Even when new, it would throw the chain off if pedaled backwards because of the bad chainline in the first gear.
Reading the article gave me an idea: 7yr doesn't use the gear 8 (or 7 (or 6)), so why not just take the smallest cog off and put a spacer in the back. I had some old 9sp cassette spacers saved up for some reason, and two of them are just right. The existing lockring is also good for locking the 12 tooth cog (or 13, i don't know) on the freewheel. I had to adjust the limit screw for the first gear. I left the B screw as it was.
The chainline for the easiest gear, which 7yr uses most of the time, is now good enough it can be pedaled backwards without problems.
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Andrew Major
3 years ago
This made my afternoon! Thank you for sharing and so stoked this piece inspired you to make bikes better for your kid!
Drivetrains that derail while backpedaling are ridiculous, and even more so when it's the gear that is getting used all the time. So stoked.
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RudyWayne
1 year, 3 months ago
This is exactly what I was looking for as my daughters new 20" bike has a horrible chainline in the lower gears and you can't roll the thing backwards more than a foot before it runs the chain down the cassette and gets all bound up. The only thing I don't quite understand is what happens to the "extra" positions in the shifter? My daughters bike is a Shimano 1x10 so if I find a 10 speed cassette that has say 2 loose cogs and I leave them out, find a spacer and install it behind the cassette, how does the shifter behave? What will my daughters experience with this modified drivetrain be? Am I trading one problem/difficulty for another? For all the talk of making bikes specifically designed for kids (low weight, grip diameters, crank lengths, Q factors, etc.) why install a drivetrain that is so inefficient and that can contribute to a poor experience? Is it just the result of being at the mercy of what adult drivetrain components are available? What is the ultimate solution here short of custom everything?
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