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Bird watcher
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Discussion Starter · #1 ·
Can the lot of you that have these type brakes on your bike do me a favor? I'm trying to find the optimum distance between mounting studs for this brake to work correctly. Could you please measure the center to center width from stud to stud on some of the bikes use have? I want to get an average so I know what to use. I am especially interested in the older bikes that may have used wider rims like the Araya 7x. If you have this one can you note it? Thanks, Chris
 

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I measured 11 different bikes. They were all between 81mm and 92mm. Even the three Potts frames had different distances. The optimal distance will also take into consideration, the rim width. The bike with the 7X rims was 86mm.
 

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Neo-Retro Forever
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The Suntour XC-Power setup manual I have here (nos, printed in Japan, Oct, 1984) lists the optimum distance as 88-90mm, as spec'd for rims ranging from 27 to 35mm.

The "allowable" distance 84 - 92mm.

Here is the actual wording -

"The mounting bosses for the XC brake are to be brazed onto the front fork at a distance of 310mm above the center of the front dropouts (when using a 26x1.75 rim) The mounting bosses for the seatstays should be brazed onto the stays at a distance of 312.5 to 314mm from the center of the rear dropouts. The distance between the bosses on the stays or the fork blades is to be between 84 and 94mm. The ideal is 88 to 90mm"
 

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Certified Bike Junkie
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richieb said:
"The mounting bosses for the XC brake are to be brazed onto the front fork at a distance of 310mm above the center of the front dropouts (when using a 26x1.75 rim) The mounting bosses for the seatstays should be brazed onto the stays at a distance of 312.5 to 314mm from the center of the rear dropouts."
Why the difference in the axle to post distance front to rear?
 

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Neo-Retro Forever
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muddybuddy said:
Why the difference in the axle to post distance front to rear?
I have no idea...it probably takes into consideration the different angle that the rim actually passes the stays/fork and how that effects the actual rim diameter at that point...and likely the difference in effective distance the tubing diameter will offset the studs...

That's my best guess...
 

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Hit The Road Cyclery
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muddybuddy said:
Why the difference in the axle to post distance front to rear?
Richie is right. The braking surface of the rim is nearly perpendicular to the chainstay where it crosses the center of the brake pads, while on the fork it is at a visible angle of several degrees due to the offset of the fork tips relative to the brake pads. It's easier to see if you measure one of your own bikes, but hopefully this makes sense.
 
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