Front Disc Brake

[mjperk]

I recently just purchased a cheap Schwinn Varsity 1200 700C bike as a starter to get into cycling. It also just so happens that I am a mechanical engineer and like to tinker a little too much. I've been doing some reading and it seems like a generally tough thing to change a bike over to disc brakes. That being said, I am also stubborn and need a project

I'm just going to do the front brake and am trying to figure out what kind of fork I'll need to find (with tabs) so that I can install the components. From the reading I've done, it seems that Schwinn bikes use uncommon threads or something was different about the headset? I'm not sure and am getting confused.

So my task at hand right now is to find a replacement fork, if possible, that will have tabs for mounting disc calipers on. Can anybody help me with EXACTLY what all specs I need to look for as far as threads (if any), sizes, spacings, etc. of what would essentially be a drop in replacement fork, only with tabs?

I really appreciate all the help I can get!

 

[alfeng]

Schwinn bikes may have used DIFFERENT THREADS at one time, but it could be suggested that it was the other companies which used different threads than Schwinn used ...

Those days are probably in the distant past.

YOUR concern is whether the current front fork has a 1" OD steerer or a 1 1/8" OD steerer ...

If the bike's fork has a 1 1/8" OD steerer, then you just have to buy any fork which has disc tabs which has a 1 1/8" steerer ...

• a RIGID MTB fork for 26" wheels will be your best bet, IMO, to limit the change in the elevation of the head tube from what it is now ...
• that is, a fork with disc brake tabs which is designed for 700c wheels will probably have a greater dropout to shoulder distance.

There are SOME threaded 1 1/8" forks floating out there ... avoid them & ensure that the fork you get uses a threadless headset & stem.

If your frame's head tube can only accept a 1" OD fork & you are committed to installing disc brakes then you can EITHER weld a NEW head tube on the frame OR weld TABS onto your fork as long as your understand that the fork will have to be heavily bolstered to prevent torquing when under the load of braking.

I suppose that ONE other possibility is to replace the steerer on a 1 1/8" OD fork with the steerer from a 1" OD fork ... this will require adding a collar onto which the headset's race will sleeve.

There may be some 1" suspension forks which have disc brake tabs which may-or-may-not be found on bikes sold at big box stores ... however, if they exist then they will probably impose a 3-to-4 lb. weight penalty over the weight of a rigid steel fork.

 

[mjperk]

Thank you that is exactly what I was looking for. Just a little more clarification please--
The tube is 1 1/8" I believe, so no issue there. I'm confused as to why you suggest using an MTB fork for 26"--wouldn't this actually change height more? I'm not sure what is meant by "probably have a greater dropout to shoulder distance." Sorry, I'm pretty green to bike terminology.

I've seen a few road bike forks with tabs for sale--why not just use them? If I could find a 700C road bike fork with a tab, wouldn't this be my best bet, or is the issue just that; finding one?

Lastly, is there a standard axle width? It seems like from what I'm reading 135mm is the standard for discs vs. 130mm for regular brakes.

 

[alfeng]

Originally Posted by mjperk .

The tube is 1 1/8" I believe, so no issue there. I'm confused as to why you suggest using an MTB fork for 26"--wouldn't this actually change height more? I'm not sure what is meant by "probably have a greater dropout to shoulder distance." Sorry, I'm pretty green to bike terminology.

I've seen a few road bike forks with tabs for sale--why not just use them? If I could find a 700C road bike fork with a tab, wouldn't this be my best bet, or is the issue just that; finding one?

Some MTB forks are suspension corrected, and some aren't ...

• if the Rigid 26er MTB fork is suspension corrected, then it will probably change the height more than a 700c fork by about 2", +/- ...
• however, some Rigid 26er MTB forks are not suspension corrected ... my recollection is that a contemporary ROAD fork + the lower portion of the headset is about 15" between the axle & the frame (a hidden headset will be different because the bearings are inside the head tube) ...
  
• for a non-suspension corrected, the combined length of a Rigid MTB fork + the lower portion of the headset is something like 15.5" ...
  
• basically, by my reckoning, a 700c fork designed for CX or Touring is dimensionally similar to a vintage 27" fork and that combined length is about 16" ...
  

The "Stack Height" of different headsets varies (more so with threaded headsets) & there are certainly some other unforeseeable variations which may be due to the whim of the manufacturer or frame builder ...

And, MY memory could be faulty ...

Consequently, ALL OF THOSE APPROXIMATIONS COULD BE WRONG & SHOULD NOT BE CONSIDERED TO BE DEFINITIVE!!!

Lastly, is there a standard axle width? It seems like from what I'm reading 135mm is the standard for discs vs. 130mm for regular brakes.


Basically, for all intents & purposes, you are correct if you are looking at a snapshot in time of what's what in the 'World of Cycling' & limiting that view to off-the-peg MTBs & Road bikes ...

However, the "standards" are different for different eras AND for different types of bikes ...

In the post-War era, ~120mm was (AFAIK) the "standard" width for the rear dropout spacing ... and, I think that 120mm persisted as the rear dropout spacing until the mid-70s.

My recollection is that Touring bikes have had 135mm rear spacing since the mid-80s (possibly, the early 80s) at a time when Road bikes still had 126mm rear dropout spacing.

Until recently, all CX frames had 130mm rear spacing ...

• I have heard rumors of Road bikes going to 135mm for at least a decade ...

But, 130mm as the rear dropout width for Road bikes has managed to persist as the "standard" for the past 12+ years ... but, 135mm could certainly supesede 130mm if someone wins a TdF on a bike with disc brakes (THAT will be subject to approval by the sanctioning bodies, of course, which may never come ... ) OR if Shimano and/or Campagnolo agree to go to a 14-speed Cassette!?!

 

[dabac]

I'd like to suggest a spot of measuring.
With rim brakes, matching fork to an intended wheel size is important, as the brake needs to bolt up at the right spot to reach the rim.
With disc brakes, matching your particular wheel size isn't that interesting any more, as the brake caliper sits at the same spot WRT the wheel axle regardless of wheel size.
You'll need clearance enough to run the intended wheel + tire, and, if you're trying to keep the handling of the bike unaffected you'll want the axle-to-crown measurement(and the rake) to be as similar as possible to the current fork.

But measure first. There are plenty of rigid 26" MTB forks that are roomy enough at the top to take 700C wheels straight off, as long as you pick a fairly narrow tire. With sus-corrected, you might even be looking at an actual increase of axle-to-crown compared to a road bike fork.

Once yoiu know your ATC, and your steerer tube diameter, you can start shopping for forks. You may need a new stem and a new headset too, if your bike currently has a threaded headset.

And axle length/dropout width isn't dependent on brake system.
For front wheels 100 mm is overwhelmingly most common - regardless of brake setup. There are more options on the rear. Currently dominating standards are 135 mm for MTB and 130 mm for road bikes. And 135 can coincide with both rim, and disc brakes.