Results 1 to 9 of 9
  1. #1
    mtbr member
    Reputation:
    Join Date
    May 2011
    Posts
    1,055

    SX 26" Trek 4500 kid build

    I recently finished my latest bike build for my 9-year old daughter. I previously built-up a sub-20 pound 24" bike for her year a year ago; Trek MT220 24" mod build
    The 24" wheel MT220 worked great and she could probably still fit on it for another year with a longer stem but the 24" bike now fits her tall 7-year old younger sister even better so I am moving the 9 year old up to her first 26" wheel size MTB.

    As a start the build, I bought this approx 15 year old Trek 4500 (and a wheelset) from a local bike co-op for $50.
    SX 26" Trek 4500 kid build-img_7552.jpg
    I chose this frame for it's extreme small size, the seattube length is just 12", she has no problem with the standover height and the ETT from seatpost to center of headtube is 52cm, only slightly longer than her last 24" wheel bike. The frame is likely just straight gauge (non-butted) aluminum and weights in at 1800gr, not particularly lightweight but it will do. I've pretty well resigned myself to not obsessing and making this another weight-weenie build. I have a hoard of older take-off and swap-meet components on-hand to use so this build was largely an attempt at utilizing the available parts I already had in order to put together a reasonably nice bike on a shoestring budget.

    First order of business was shortening down a nice older forged specialized (sugino) crankset from 175mm to 155mm.
    SX 26" Trek 4500 kid build-img_8292.jpg
    SX 26" Trek 4500 kid build-img_8297.jpg
    I used a 1/2" drillbit aligned to an old pedal spindle threaded into the 175mm length hole to get the new hole drilled plumb, then used 9/16" L/R pedal taps at the bike co-op to thread the holes, hacksaw to remove the old pedal end and then grinder and files to smooth out the new shortened end. End result was a nice forged 4-bolt 104mm BCD square taper crank, 610gr complete with 3X rings. I still may later change out the large ring for a bashguard, even on smooth road she just never goes fast enough to need the 42T.

    SX 26" Trek 4500 kid build-img_8344.jpg
    Obligatory side-on garage door shot of the completed bike. In comparison to the 26" tires, it gives a good perspective on just how compact this frame is. The Treks original rims were decently lightweight and in good shape so I re-used them and re-built as a 12/18 spoke wheelset using a better quality cassette hubset and a nice lightweight XT casette w/ lightweight aluminum spider to save weight.

    SX 26" Trek 4500 kid build-img_8345.jpg
    Front suspension provided from a 1994 RS Mag 21 fork. I bought this fork brand new back in the day and rode it myself for many years, maintained, tuned and modified it for "long travel" (60mm). The mag21 is noodly for an adult rider but should be sufficiently stiff for a 75 pound girl. Air shock is extremely easy to adjust for her weight and the hydraulic oil dampening is reasonably sophisticated and tunable. This mag21 weighs around 1450gr so is reasonably lightweight. Best of all, it was entirely paid for 20 years ago and I was never going to be riding it again so good to see it put back to use.
    The Mag-21 had a cable stop for cantilever brakes so I couldn't resist using a set of pink anodized empella frogleg CX brakes that I had picked up from a swap meet a few years ago, the brakes are very lightweight and daughter highly approves of the color.

    Complete bike initially tips the scale at 23 pounds. Changing to some lightweight tires and tubes could probably be justified to loose a bit more weight. I ran the spreadsheet weight calculations and concluded that with another $700 of new weight weenie components I could get the bike down below 20 pounds but considering that I spent less than $100 on it at this point, we will likely just enjoy and ride it as-is.
    Last edited by GrayJay; 05-20-2015 at 12:13 AM.

  2. #2
    mtbr member
    Reputation:
    Join Date
    Oct 2012
    Posts
    324
    Nice. Both of your builds appear to be top notch. I wish I had the time, knowledge and resources to do more of this stuff.

    How hard was it to drill and tap the new holes for the pedals? As you might remember from my thread on the 24 Raleigh, I was looking at the pineridge cranks for my kid. But I keep wondering if I should trying to just shorten my cranks instead.

    Nevermind. I called a local bike shop and found they will shorten my cranks for around $20. I don't see me messing with it if they will do it for that price.
    Last edited by Fargo1; 04-06-2015 at 01:20 PM.

  3. #3
    Who are the brain police?
    Reputation: Locoman's Avatar
    Join Date
    Jan 2004
    Posts
    10,106
    Quote Originally Posted by GrayJay View Post
    I recently finished my latest bike build for my 9-year old daughter. I had built-up a sub-20 pound 24" bike for year a year ago; Trek MT220 24" mod build
    The 24" wheel MT220 worked great this past year and she could probably still fit on it for another year with a longer stem but it fits her growing 7-year old tall younger sister even better so I am moving the 9 year old up our first kids 26" wheel size MTB.

    As a start the build, I bought this approx 15 year old Trek 4500 (and a wheelset) from a local bike co-op for $50.
    Click image for larger version. 

Name:	IMG_7552.jpg 
Views:	371 
Size:	110.5 KB 
ID:	978878
    I chose this frame for it's extreme small size, the seattube length is just 12", she has no problem with the standover height and the ETT from seatpost to center of headtube is 52cm, only slightly longer than her last 24" wheel bike. The frame is likely just straight gauge (non-butted) aluminum and weights in at 1800gr, not particularly lightweight but it will do. I've pretty well resigned myself to not obsessing and making this another completely weight-weenie build. I have a hoard of older take-off and swap-meet components to select from so this build was largely an attempt at utilizing the parts I already had available to put together a reasonably nice bike on a shoestring budget.

    First order of business was shortening down a nice older forged specialized (sugino) crankset from 175mm to 155mm.
    Click image for larger version. 

Name:	IMG_8292.jpg 
Views:	338 
Size:	93.4 KB 
ID:	978879
    Click image for larger version. 

Name:	IMG_8297.jpg 
Views:	263 
Size:	120.9 KB 
ID:	978880
    I used a 1/2" drillbit aligned to an old pedal spindle threaded into the 175mm length hole to get the new hole drilled plumb, then used 9/16" L/R pedal taps at the bike co-op to thread the holes, hacksaw to remove the old pedal end and then grinder and files to smooth out the new shortened end. End result was a nice forged 4-bolt 104mm BCD square taper crank, 610gr complete with 3X rings. I still may later change out the large ring for a bashguard, even on smooth road she just never goes fast enough to need the 42T.

    Click image for larger version. 

Name:	IMG_8344.jpg 
Views:	1665 
Size:	92.5 KB 
ID:	978881
    Obligatory side-on garage door shot of the completed bike. In comparison to the 26" tires, it gives a good perspective on just how compact this frame is. The Treks original rims were decently lightweight and in good shape so I re-used them and re-built as a 12/18 spoke wheelset using a better quality cassette hubset and a nice lightweight XT casette w/ lightweight aluminum spider to save weight.

    Click image for larger version. 

Name:	IMG_8345.jpg 
Views:	308 
Size:	106.5 KB 
ID:	978882
    Front suspension provided from a 1994 RS Mag 21 fork. I bought this fork brand new back in the day and rode it myself for many years, maintained, tuned and modified it for "long travel" (60mm). The mag21 is noodly for an adult rider but should be sufficiently stiff for a 75 pound girl. Air shock is extremely easy to adjust for her weight and the hydraulic oil dampening is reasonably sophisticated and tunable. This mag21 weighs around 1450gr so is reasonably lightweight. Best of all, it was entirely paid for 20 years ago and I was never going to be riding it again so good to see it put back to use.
    The Mag-21 had a cable stop for cantilever brakes so I couldn't resist using a set of pink anodized empella frogleg CX brakes that I had picked up from a swap meet a few years ago, the brakes are very lightweight and daughter highly approves of the color.

    Complete bike initially tips the scale at 23 pounds. Changing to some lightweight tires and tubes could probably be justified to loose a bit more weight. I ran the spreadsheet weight calculations and concluded that with another $700 of new weight weenie components I could get the bike down below 20 pounds but considering that I spent less than $100 on it at this point, we will likely just enjoy and ride it as-is.
    Bravo! Your kid are lucky to have you--
    The Who - Glittering Girl
    Ween - The Grobe
    Yellowman - Strong Me Strong
    all your base are belong to us

  4. #4
    mtbr member
    Reputation:
    Join Date
    Jun 2011
    Posts
    340
    Nice work. Keep us posted how your kid likes it on the trail.

  5. #5
    mtbr member
    Reputation:
    Join Date
    Feb 2010
    Posts
    1,774
    Great job.

    Quote Originally Posted by GrayJay View Post
    ...I used a 1/2" drillbit aligned to an old pedal spindle threaded into the 175mm length hole to get the new hole drilled plumb...
    I actually use a dummy BB to get the alignment right. Those speci cranks are probably high enough quality that the old pedal threads will be drilled right, but on some of the cheaper cranks its better to use the BB axis as the reference point.

  6. #6
    mtbr member
    Reputation:
    Join Date
    May 2011
    Posts
    1,055
    Quote Originally Posted by Fargo1 View Post
    How hard was it to drill and tap the new holes for the pedals? As you might remember from my thread on the 24 Raleigh, I was looking at the pineridge cranks for my kid. But I keep wondering if I should trying to just shorten my cranks instead.

    Nevermind. I called a local bike shop and found they will shorten my cranks for around $20. I don't see me messing with it if they will do it for that price.
    Getting your cranks shortened for $20 is a great deal if they can do a good job. The trailcraft crank also looks like an good option on on 104 BCD crank, provided the 152mm length is suitable for the rider...

    To agree with Tig, to be 100% technically correct, you should align the new crank hole with the bottom bracket spindle as it is entirely possible that the original pedal hole was not precisely aligned with the square taper hole and you would just be replicating the imprecision if you only align to the old pedal hole. Tig also had a info page on crank shortening, well worth reading if you try; FAQLoad - Crank arm shortening
    On a previous attempt at crank shortening, I cobbled together a crude support/jig to drill the new crank hole on a friends drill press. It was challenging to accurately position and securely hold the crank arms plumb to the drill press in order to get a new hole made, particularly for the non-drive side crank. My pseudo-jig wobbled and slipped as I drilled resulting in a slightly mis-aligned hole when I trusted the alignment to the drill press & jig. Mis-aligned pedal holes will make the pedal oscillate out of plane with the crank and feels really weird, undesirable. To use a drill press and do it right, you would need to construct a very stable and accurate jig, probably worth researching further for a good design.

    For this attempt, I somewhat cheated and primarily used the old pedal hole to reference the drill bit alignment. The human eye is surprising good at judging if two lines are parallel. I first took the spindles out of an old set of junker pedals and threaded them into the original crank pedal holes from the inboard side of the crank so that I could visually check the spindle alignment against a bottom bracket spindle tightened into the crank. Satisfied that the old pedal holes and the crank spindle were reasonable strait to each other, I moved the spindles to the outboard side of each pedal. I just used a hand-held drill that I carefully positioned referenced to the old pedal spindle to make the new holes plumb. It is possible to position the drill by looking at the pedal spindle, sighting along two different axis to get them aligned on each axis and carefully watching the drill as I went. I first started the new holes from a mark that I center-punched onto the cranks to locate the holes on the centerline and at same length on each crank. Using a hand-held drill, I carefully made a series of increasing size pilot holes, checking the resulting alignment each time before I enlarged the hole and adjusting as needed to get plumb alignment until I finished with a 1/2" bit.

    Buying a set of pedal taps is about $30/set so a bit cost prohibitive to purchase if you are only doing one or two sets of pedals, better to find a shop or co-op that will loan them. Once the new holes are tapped, file the outboard face of the crank arm flat so that the spindle flange contacts evenly then use the spindles to check an gauge the pedal spindle alignment with the bottom bracket spindle. If there is any perceptible mis-alignment of the new pedal spindle hole, minor corrections can still be made by clamping the old original end of the crank tightly in a vice, put a cheater pipe over the bottom bracket spindle and torque on the crank arm to nudge into alignment. Significant bending of aluminum crank arms will likely cause them to crack sooner or later so be sparing and only using bending to correct very minor final alignment issues. I would probably be more reluctant to cold-set a crank arm for an adult weight rider but this was common practice decades ago and bikeshop tool suppliers even offered medieval looking crank-arm bending tools, I doubt that the lawyers would approve of the practice these days.

    Once any final alignment is complete, proceed with hacksawing off the sacrificial vice-scarred tip of the crank arm. I shortened a 175mm crank by 20mm down to 155mm, It would not be possible to shorten by much less than 20mm as the new hole would be too close to the old hole. Not all cranks are amenable to being shortened, you need an area where you can produce a flat face for the spindle lip to contact the crank and hollow cranks (such as newer shimano hollowtech) would also be questionable depending on length.

    It took several hours of work for me to shorten both cranks, definitely a labor of love that would likely be cost prohibitive to hire out unless working with a shop that already has an alignment jig and tooling setup. Still, for a DIY project where time is no object, it is viable and opens up possibility for re-using old cranks. Most kid-length cranks are BMX based 110mm 5-bolt circle, nice to have option for making shorter 104mm 4-bolt cranks via the DIY route for a better selection of chain ring sizes.

    My daughter has been riding the "new" bike for couple of weeks now and it is working great, frame size looks perfect for her.

  7. #7
    mtbr member
    Reputation:
    Join Date
    Oct 2012
    Posts
    324
    Thanks for the indepth writeup. Sounds like tedious work. I think I'll have the shop do it or buy the Trailcraft cranks. I'm a litte concerned that shortening my 165mm to 143+/- will be too short and require raising the seat to much to get proper leg extension. On the other hand, I'm not sure its worth $80 to only shorten the crank by 1/2". My kid might just get a different gear set on his freewheel or have to ride it as it is. But your builds are definetly inspiring.

  8. #8
    mtbr member
    Reputation:
    Join Date
    May 2011
    Posts
    1,055
    Quote Originally Posted by Fargo1 View Post
    Thanks for the indepth writeup. Sounds like tedious work. I think I'll have the shop do it or buy the Trailcraft cranks. I'm a litte concerned that shortening my 165mm to 143+/- will be too short and require raising the seat to much to get proper leg extension. On the other hand, I'm not sure its worth $80 to only shorten the crank by 1/2". My kid might just get a different gear set on his freewheel or have to ride it as it is. But your builds are definetly inspiring.
    Buying the 152mm pinecraft cranks probably makes good sense in your situation - they will be the right length for a 24" bike rider now, then with your older kid outgrows the bike you can leave it as a rideable bike to pass along the then next kid and eventually re-sell when you have no further use for it. Save the 165mm cranks a couple of years until your older kid soon grow into them, use them on the next size bike.

    Alternately, 175mm cranks are probably most common size for adult MTB's, keep searching co-op, swap meets or craigslist for a set of cheap set of 175mm's to shorten, save the 165mm set which will soon be useful.
    Last edited by GrayJay; 04-16-2015 at 11:36 PM.

  9. #9
    mtbr member
    Reputation:
    Join Date
    Oct 2012
    Posts
    324
    Quote Originally Posted by GrayJay View Post
    Buying the 152mm pinecraft cranks probably makes good sense in your situation - they will be the right length for a 24" bike rider now, then with your older kid outgrows the bike you can leave them together with the bike to pass along the then next kid
    Yeah I thought about that. The problem is the next kid will be ready next summer. He's only about an inch shorter than his older brother. So I will likely be needing 2 24"s at the same time. However, I have noticed that most of the 26ers have 170mm cranks. Which seems like it would still be too long even in 2 years. So I might be able to move the Pinecraft cranks up to the 26er when that time comes. Thanks for the thoughts though. Sorry to hijack your thread.

Similar Threads

  1. Help Deciding! Trek Wahoo 29er vs. Trek 4500 Disc MTB
    By Zantikus in forum Bike and Frame discussion
    Replies: 2
    Last Post: 07-06-2013, 08:17 AM
  2. Trek 4500 Disc Vs Trek Marlin?
    By RideRed12 in forum Trek
    Replies: 10
    Last Post: 03-06-2012, 09:08 PM
  3. Replies: 9
    Last Post: 08-22-2011, 08:05 PM
  4. Choice: '09 Trek 4500 or '04 Trek 4300 w/ upgrades
    By tardo in forum Bike and Frame discussion
    Replies: 0
    Last Post: 03-16-2011, 07:30 PM

Members who have read this thread: 14

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •