700 C Axle Center to Brake Bolt Center Distance / Range

[Michael Gordon]

Hello All,

I'm working on my first project. In my research, I have not been able to find a standard distance / range for a 700 C fork - axle center to brake bolt center. Is there a standard, or are there any general distances / ranges that others use?

Thanks,

Mike Gordon
Highland Park, IL

[Jonathan]

Mike, since I'm in teaching mode today ill say you can figure it out yourself if you know that standard or short reach road brakes have 40-50 mm reach.

[Michael Gordon]

Thank you, Jonathan,

I have been using the dimension of 40-50mm of reach as a guide, and I have come up with about 31mm from the center of the axle to 45mm of reach, and about 35.5mm from the center of the axle to the center of the brake bolt. I'm just looking for confirmation with other out there on whether or not my measurements are on or off.

I always appreciate teaching mode! Thanks for your reply.

Mike Gordon
Highland Park, IL

[Andrew R Stewart]

You could add up the dimensions that comprise the A-BH distance (rim radius+brake reach) but there's a little correction factor what with the rake and the rim's curving up or down from the brake's mounting point (I add 2mm for the rear and sub 2mm for the front). Or just measure a known frame/fork. Andy.

[Michael Gordon]

Correction to my last posting... I pulled a hubble and forgot to move a decimal point. Working off of the 40-50 mm of reach for a typical caliper, I've been measuring 310 mm from the center of the axle to 45mm of reach, and about 355 mm from the center of the axle to the center of the brake bolt.

From the helpful replies I have received so far, it seems as though everyone works off of their own measurements as opposed to an established standard.

Thanks,

Mike Gordon
Highland Park, IL

[Doug Fattic]

Mike,

It is helpful to have more information from you about what kind of tires you plan on using on this fork and for that matter if this is a replacement fork or if you are building a new custom frame. If you are establishing this distance for skinny tires than having the brake shoes be in the middle of your brake slot looks great. However many building steel custom frames today like the option of being able to put on at least 28mm tires and possibly even fenders. In which case you want as much brake clearance as your brakes will allow minus a mm for builder error. Shimano short reach brakes are 39mm to 49mm (Campy is 40/50) so using 48mm as your standard dimension works well if you want to have the option of using 28mm tires. By the way I use 311 as my rim distance so 311 + 48 = 359mm is the center of your axle to the center of your brake hole distance.

However unless you are talking about a performance type of frame (in other words a racing frame even if it isn't going to be raced) it is common for many custom steel frames to be made with what used to be called standard reach brakes which are 47 to 57mm. This distance was defined by the original Campy side pull brakes that came out in '67 or '68 (eventually about 10 years later they made them shorter). These brakes allow you to use 32mm tires with a fender if you put the brake shoes at the bottom of the slot. In this case your distance would be 311 + 56 = 367.

Doug Fattic
Niles, Michigan (about 100 miles east of you)

[Jonathan]

For the most common crowns I go axle to crown race but knowing how to measure it is important if you're trying to maximize clearence or use longer reach brakes.

[Todd Mosley]

Keep in mind that a rim's brake track can affect where the pads will strike the rim. When I swap out my training wheels for my HED Stinger 6's, I usually have to move the pads down a bit. Because the brake arms travel in an arc, wider rims will cause the pad to hit at a different point. If you are setting your brake bridge as high as possible to clear big tires, you're better off testing with a few rim and tire combos instead of going off of calculations.

[Mark Kelly]

Some parts of the answer are obvious, some not so much.

The "reach" of the brakes is measured diagonally from pivot to pad centre, so the dimension from the virtual rim radius to the brake pivot is given by Pythagoras: SQRT (Reach^2 - (Rim width/2)^2). For a typical rim this is reach - 1mm.

For the front wheel the brakes are offset forward from the steering axis and the so is the wheel but they are by different amounts. The brake offset is the distance from the centre of action back to the steering axis, so it's the apparent mount height of the brake (typically 25-30mm for modern brakes) plus the height of the mounting boss from the steerer centre when the fork is laid horizontal (typically about 20mm). Net offset = fork rake - brake offset, note that for many brake / fork combinations this is near enough to zero.

For the rear wheel it's not so simple, it depends on the angle and placement of the chainstays. This is most easily measured off the frame drawing by finding the length of the line from the dropout centre which makes a right angle with the chainstay centreline. Again the net offset will be this dimension minus the brake offset which this time is the centre of action plus half the depth of the brake bridge centre boss.

Once you've got the offset, the distance to the virtual rim radius is given by Pythagoras again: SQRT(actual rim radius^2 -offset^2). The actual rim radius can be taken as half the bead seat diameter for most rims (622mm is the BSD for 700C rims).

If you add these two lengths you get the distance to the virtual offset point which is not exactly the same as the distance from axle centre to brake pivot, though it's good enough for government work. To get the exact distance we need another Pythagorean triangle, so it's SQRT ((reach + virtual radius)^2 + offset^2).

[blasdelf]

It's also possible to place the seatstay bridge so the brake hole isn't perpendicular to the stays themselves.

If you angle the brake up a little bit, the pad slots don't reach as far down — but the brake arch also doesn't protrude as much into the wheel envelope. If you're clever about it you can angle it just right to maximize clearances (Rivendell has done this in recent years).

[blasdelf]

And as always, Shimano's documentation is pretty great:

Road-Caliper-Position-web.jpg

[Will Neide]

Good info!

I just clamped my caliper (mounted on the brake bridge) to the rim with the pads in the middle and marked where to cut. Worked fine. Maybe next time I'll measure. :)