Go Back  Bike Forums > Bike Forums > Bicycle Mechanics
Reload this Page >

Change from 74BCD to 58BCD

Bicycle Mechanics Broken bottom bracket? Tacoed wheel? If you're having problems with your bicycle, or just need help fixing a flat, drop in here for the latest on bicycle mechanics & bicycle maintenance.

Change from 74BCD to 58BCD

Old 09-21-08, 10:35 PM
  #1  
Booger1
Senior Member
Thread Starter
 
Join Date: Sep 2007
Location: Gaseous Cloud around Uranus
Posts: 3,738
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 31 Post(s)
Likes: 0
Liked 0 Times in 0 Posts
Change from 74BCD to 58BCD

I wanted lower gears for my tired old legs but wanted to keep my old crank.So I looked in to the quad ring thing but decided it was too hard to get the rings.I liked the idea though, so I decided to see if I could make one with hand tool in my garage.

Bolted it on and set the spacing.Shifts Great! How bout that,a 20 tooth gear on a 74BCD!

I used a power drill,drills,coping saw, and hand files.With the above tools,I made one of these out of 1/8-6061 tempered aluminum:

http://

Bolt it on,bring on the hills!

http://

http://

http://

Last edited by Booger1; 09-21-08 at 10:39 PM.
Booger1 is offline  
Old 09-22-08, 01:28 AM
  #2  
Chris_W
Likes to Ride Far
 
Chris_W's Avatar
 
Join Date: May 2007
Location: Switzerland
Posts: 2,276

Bikes: road, commuter/tourer, hardtail MTB, touring tandem, cargo, folder

Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 19 Post(s)
Likes: 0
Liked 0 Times in 0 Posts
I don't quite understand how you prevented the bolts and plate of the 74 BCD from interfering with the chain when on the 20T ring. Your third photo kind of shows it but it is too small to see in detail. Could you please elaborate on how you solved this issue?
Chris_W is offline  
Old 09-22-08, 09:35 AM
  #3  
Booger1
Senior Member
Thread Starter
 
Join Date: Sep 2007
Location: Gaseous Cloud around Uranus
Posts: 3,738
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 31 Post(s)
Likes: 0
Liked 0 Times in 0 Posts
The crank had 3/16th spacers between the ring and crank.So I removed the spacers,replaced with the 1/8th thick adapter,put 1/16th spacers between the new gear and adapter,then beveled the the ears to give the chain more side clearance.
Booger1 is offline  
Old 09-29-08, 02:09 AM
  #4  
Chris_W
Likes to Ride Far
 
Chris_W's Avatar
 
Join Date: May 2007
Location: Switzerland
Posts: 2,276

Bikes: road, commuter/tourer, hardtail MTB, touring tandem, cargo, folder

Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 19 Post(s)
Likes: 0
Liked 0 Times in 0 Posts
Interesting, thanks for the clarification. Unfortunately, a lot of triple cranksets don't use any extra spacers for the 74mm BCD bolt-holes. This solution is likely not to work unless you have those spacers.
Chris_W is offline  
Old 09-29-08, 04:36 PM
  #5  
DMF 
Elitist Troglodyte
 
DMF's Avatar
 
Join Date: Jun 2006
Location: Dallas
Posts: 6,926

Bikes: 03 Raleigh Professional (steel)

Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Likes: 0
Liked 0 Times in 0 Posts
What is your original crank? (Cool solution, btw!)

So what's your low gear, like a 20/34? Can you even spin fast enough to stay upright?
__________________
Stupidity got us into this mess - why can't it get us out?

- Will Rogers
DMF is offline  
Old 09-29-08, 05:13 PM
  #6  
melville
Senior Member
 
melville's Avatar
 
Join Date: Jul 2007
Posts: 493
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 5 Post(s)
Likes: 0
Liked 0 Times in 0 Posts
Originally Posted by DMF View Post
What is your original crank? (Cool solution, btw!)

So what's your low gear, like a 20/34? Can you even spin fast enough to stay upright?
Looks like a melt forged SR.

I've run 20/34 on my mountain bike. Just slam it into low gear and tractor my way up nearly anything. Trails. Logging roads. Skidder trails. Even the trees, before they're cut down. And doing this with my kid in the trailer (Go Daddy Go!). Hard to say how slow it is--my old Avocet gets a little flaky below 2.5 mph. It's like a really fast trackstand.

My rule for myself is if I can't ride up it, I shouldn't ride down it. Keeps me out of the hospital. I'd like to ride down a whole buncha crazy stuff, so I've worked on my technique and set up the bike to get up nearly anything.

OP--sweet work with hand tools. Nicely done. I used to work with a guy who was the Michelangelo of the bench grinder. He'd make all sorts of stuff just leaned over the grindstone throwing big sparks. More craft there than you'd think.
melville is offline  
Old 09-29-08, 05:43 PM
  #7  
Booger1
Senior Member
Thread Starter
 
Join Date: Sep 2007
Location: Gaseous Cloud around Uranus
Posts: 3,738
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 31 Post(s)
Likes: 0
Liked 0 Times in 0 Posts
Man! you know your cranks!I don't how you could tell from those pictures.

1978 Forged SR crank,it's on my touring bike,(only bike).I could have taken it to work and done it on a CNC machine,it would look like a jewel.It's a little rough around the edges but the important parts came out right and I wanted to do it at home with stuff most people have around the house.When it was new it had 52-42-32,now 46,34-20. 8 speed freehub in the back with 14-34.

If a person wanted too,you could do this on any crank,but you would have to mill the pads down on some?/most?

Last edited by Booger1; 09-29-08 at 06:05 PM.
Booger1 is offline  

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off


Thread Tools
Search this Thread

Contact Us - Archive - Advertising - Cookie Policy - Privacy Statement - Terms of Service

Copyright 2018 MH Sub I, LLC dba Internet Brands. All rights reserved. Use of this site indicates your consent to the Terms of Use.