Revised Review: 2010 trek 3900 disc

MSRP (new) $529.99

this bike is made for commuting I have it in size 18,and in orange

the geometry so, not good for Mtb, for me, but for people with short armed and short back, this is not a bad thing, but I am about 180 cm high and i am average sized  everywhere,so  I get a very upright position, But when I was a teen i was not as big as I am now so it was ok.

which is too small of mountain biking  but for commuting its OK, you will sit upright.

the bike is no real mountain bike, frame is way too short and the handlebar is too short too, this make the bike too responsive.

ignore the sticker that say trek mountain bikes, it just refers to the standard for mountain bike frames. it’s not a mountain bike,but it can be with some upgrades.

Seat post parts look like this:

2010 Bontrager seatpost  saddle clamp parts
2010 Bontrager seatpost  saddle clamp parts
2010 Trek 3900 disc 18'' chain drop damage
2010 Trek 3900 disc 18'' Right dropout
2010 Trek 3900 disc 18'' - dropouts- upside down

the frame can withstand mountain biking, but the geometry is horrible for mountain biking.

2010 Trek 3900 disc 18'' - BB shell -chainstays


  • max tyre size is 2.3
Chris King BB 30 & Race Face Turbine_0628
  • slack seat tube angle
  • steep head tube angle
  • horrible wheels, the rear wheel comes with a FH-M475 rear hub which has been troublesome, bending axle, possibly due to bad tolerances, this happen on all of the FH-M475 rear hubs I used.
  • The Shimano Acera M360rear dérailleur is horrible, too soft spring, no clutch. the use of a nut instead of a bolt was a bad choice.
2010 trek 3900 Shimano Acerarear derailleur
2010 trek 3900 rear a look from above
  • The handle bar bends in a crash, a simple side fall will bend it.
  • Steel stem with only two bolts which is also quite heavy & low quality:
my custom 2010 trek 3900
2010 Bontrager Approved 2 bolt stem

You can notice the low quality part I mentioned on the inside of the stem, weird wave pattern instead of a smooth hole, there were no wall there that has been rusted away, it is just made this badly, as you can see there is paint on it,so this means it clearly was like this when new.

2010 Bontrager Approved 2 bolt stem
2010 Bontrager Approved 2 bolt stem
2010 Bontrager Approved 2 bolt stem
  • The fork is a SR Suntour SF9 XCT V2, 100mm travel  does not have the promised 100mm travel, but more like 20mm, the fork had play and it got even more play so when i was braking hard i felt like the fork would snap or fall apart. it also lack of adjust ability.
  • Nylon Wellgo pedals, which were horrible, the plastic snapped on on side, I don’t remember how that happen, and they were very sketchy, foot coming off was a common issues for me. here is a review from Bikechanic for what seems to be the same or similar pedals:

  • The all in one unit Brake & Shifters/triggers: the stock are v brake lever and shifters, shifters are horrible you need to swing the lever way too far to change gear, so changing gear becomes unnecessarily difficult. if you want the shifter to be at a good angle the brake will be at a bad angle and wise versa.
  • Brakes: the bike comes with single piston mechanical brakes from Promax.. The mechanical brakes don’t work well you need to press very hard to brake. these brake are so bad that V-brakes actually are better for braking wise,not wear wise of course. so i upgraded  to hydraulic & shifters/triggers.
  • Saddle– saddle is way too big for me, it wears out fast because of flex in the saddle the plastic under  it that  have screws in it will flex too so the screws will pop out.
  • Overly beefy brake mount, it’s heavy, and wide.
2010 trek 3900 brake upgrade Shimano Acera M39
with upgraded brakes, but they failed.
2010 Trek 3900 disc 18'' - brake caliper mount & dropout
2010 trek 3900 downtube & bottom bracket shell
BB area 0508
  • horrible headset: VP-A68-TK, 1-1/8″ semi-cartridge, with horrible seal, making it a greasy rust water mess.
my custom 2010 trek 3900- Inside the headtube
my custom 2010 trek 3900- Inside the headtube
  • The Bontrager LT-3, 26×2.0 tyres got horrible grip in the wet they are a death trap. on wet asphalt they slip and you will crash, there is not enough space between the knobs and the rubber is too hard and not sticky enough to grab on to the asphalt check out the the review by Bikechanic :

  • 25221077486_cf268d17c3

winter ready & upgraded 2010 Trek 3900 Disc


  • replaceable derailleur hanger, which is a must.
  • decent welds.
  • stiff enough frame for a average rider for at least this size.
  • the brake rotors can handle metal pads.
Rusted Promax 160mm disc
Rusted Promax 160mm disc

Rusted Promax 160mm disc


seatpost swap Trek 3900_1900
Most recent state of the bike, assembled with parts from part I took from other bikes I have, and some parts I had used previously. this bike does not fit me, but I just did this to have a spare bike, I will eventually build a new bike with some of the parts I have on the fuel ex plus some new parts.

waste of money, unless you need a bike fast and it’s the only bike you can then at the point you are i the store.

I only used the 3900 as winter bike, plus the front dérailleur does not work in  below -2 degrees for some reason, maybe due to the wear it has.

the review was originally posted 26 june 2015, but I improved the text and added pictures.

Kids/small person MTB bikes November 2016

Note that this article is work in progress, at least now you know some more brands or bikes.

These bikes are in no particular order.

If any of you have found more bikes or child specific stuff, be so kind to comment below to share with us.

I have no experience with these bikes, So I will say my opinion, and my experiences with the components used,I will not decide for you.

This article will be update when I find more good bikes
order and title of article might change but I publish this right ow so you can see it, sorry if there are mistakes and typos.

there are few kids bike brands that make good bikes for kids,and there are even fewer adult bike brands that make kids good kids bikes,and there are not many kids brand that make good kids bikes for 24 and 20 inch wheel size.

Some facts:

Kids should have as safe,light,strong,and as well performing bikes as adults.

for the kids to enjoy riding the bikes for a long time the bikes must be light.

Fit is very important,Adjustable reach for brake levers, right crank arm length,saddle that fit kids,I have seen kids riding on saddles that are painful to sit on,the shape of the saddle the length of the saddle the height/depth of it,Its all very important,a flexible saddle is a must In my opinion for optimal comfort.

When you want to buy a bike please ignore the silly marketing tricks, like using using A celebrity or pro riders name, or a company’s name..

this is a good example of what I mean: Hyper Cam Zink Edition is a good example:

I see many kids got awful bikes, and jump and ride downstairs jump off walls, and they do it on awful and dangerous bikes. the kids I saw ridden like Seth does in this video.

Shame so many kids ride on bikes that is junk,and they don’t know how dangerous it is,and the parents got no clue.

but in this guide I will show some the best kids bike I have found on the internet.(custom builds only commercial products will be listed here)

In my opinion bikes like these should be illegal to sell, but sadly they are not.

And then there are the fake dirt jump bikes that are marketed as bikes that can withstand abuse. which they can’t do at all, bikes with Sr Suntour xct and xcm forks,the plastic adjustment, pops out, flexing, knocking sounds, or almost no rebound, depending on the model.

Some bikes have warning stickers on that say the opposite of what the ad say.

I see many kids got awful bikes, and jump and ride downstairs jump off walls, and they do it on awful and dangerous bikes. the kids I saw ridden like Seth does in this video.

If you are a beginner and want to do advanced riding I you need to subscribe to this channel.

Most of the worldwide big bicycles brands, only make cheap awful bikes for the kids, or really basic hardtails.

Many of the Kids MTB guides, are misleading, many of the bikes got awful components.

Many of the kids bikes I have seen are absolutely awful junk.

Almost everything is wrong with these 100 dollar bikes.

Just some of the things that are wrong with them:

  • they are very heavy,a too heavy bike can be dangerous, the bike was glued to the ground I could no jump it.
  • suspension that does not do the job,the most common fork on these cheap bikes are ZOOM.
  • mechanical disk brakes that are hard to brake with for kids.
  • saddles that is torture to sit on.
  • tyres with no grip

Example of kids bikes that are marketed as mountain bikes, but they are to they are garbage, they are not made for Drop-Offs and jumps, and shredding in the woods.
this is junk 14 KG for a kids bike is  stupid.

There are many choices on the market right now,

but If you want the best speed bike without spending too much I recommend buying from Direct Sale brands.

This video shows me jumping a 24 inch fat bike that is way too heavy:

The List:

Here are some real mountain bikes for kids. all from XC to freeride,downhill

modern geometry, long reach,long chain stays, long wheelbase, stable.

Lil Shredder Prodigy


Lil Shredder Phenom
24″ WheelsRace face narrow wide ring make sure the chain stays on.(unless you pedal at extreme speed or ride down 40-50 degrees steep strairs) so A chain guide would be nice.
As you can see, Shimano ZEE RD, which can take a beating,short cage is excellent, specially for kid’s bike so now they get clearance too.

Full suspension 26er trail bikes:


Enter a caption

10 speed 1 by drive trail with shimano Deore shadow plus, and Rock Shox fork  & shock & hydraulic mineral oil disk brakes from shimano. I think this is a nice package,but  a chain guide would be nice
this model got 11 speed 11 speed Sram drive trail and high end fork.
This kona has a budget is a budget built, but no clutch on the rear derailleur not good,I think the extra you would pay for a clutch is worth it.
Transition Ripcord,1&op_usm=0,0,0,0&iccEmbed=0&cache=on,on&bgc=247,247,247

2016 Trek Fuel EX Jr



So far this is the only high -end bike for kids from Ttrek.

X-Fusion Performance Suspension is tuned for lighter riders

10×2 setup makes sure that the kid got all the gear range he or she needs. the bike got Deore shifters which are lighter to push than the Deore XT M8000, so shifting should not be an issue for kids, but it would be nice if the bike had 1x drivetrain,  for reduced weight and simplicity. the bike lacks a dropper post, so you will have to get one if you go for this ibke. the adult size bike has issues with hte linkages, the bearings dislocate, chainstays wear out, rocker ink cracks, but I have no experience with the kid’s bike, but it might not be an issue.

The bike has decent seat tube angle so you shoudl be able to put the tip of the saddle over the bottom bracket for good pedalling.
Propain Yuma this bike got some decent components
Propain Frechdax   Its a shame they put Acera RD, would be nice with Shimano Zee instead,but if the kid crash a lot and ruin RD’s this is not a issue.

Shimano Zee gives ground clearance.

This bike got no clutch Rd.


Trailcraft Titanium Pineridge 24

16 inch and below:

The saddle is in child’s size,

Cnoc 16 Pro Series Age 4+ / £799.99

Beinn 20 Pro Series
Age 5+ / £999.99
Creig 26 Pro Series Age 9+ / £1499.99
KANIA PYRO X.13 / B.15 / B.18

Dirt jump bikes:

Kotori – 24″

this is a dirt jump bike with decent specs, but it got no front brake so it’s not street legal,at least not in Norway.Fyoutub


Prevelo bikes:

They make balance bikes, 16, 20, 24 inch wheeled bikes. the bikes are light weight, have short cranks. With head angle of 67, the head angle is nto too far off from what most trail bikes for adults are, but not as slack as the slackest bikes out there. the seat tube angle at 74 is not too crazy slack.

But they don’t offer any full suspension bikes yet.


Flaws I see with this bike are, saddle doesn to seem to be good, there is no replaceable hanger.