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Old 04-06-13, 10:03 AM   #1
corwin1968
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Forward facing dropouts and chainstay length

Do frames with forward facing dropouts essentially have a variable chainstay length? My understanding is that chainstay length is measured from the center of the BB to the center of the rear axle. My droupouts are easily 3cm long so there is quite a bit of fore-and-aft space to move the axle.

My bike will be set up with a rear derailleur and 9-speed cog. Will the LBS have some latitude in deciding where to place the axle? I assume chain length dictates some of this but I don't know how small the increments can be when removing links. Will there be one spot that works, based on chain length, or will there be several spots that work?

I bring this up because I've been riding a bike with 43cm chainstays. This frame is listed as having 43.6 cm chainstays. I've been curious about the effect of longer chainstays so if shoving the axle as far back as possible would give a longer CS length, I might want to experiment with that. Measuring from the BB to rear axle when all the way back in the dropouts is in the 46cm range, although I may not be measuring along the correct axis (ie, my meausurement was along the chainstay, rather than a straight line).
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Old 04-06-13, 10:10 AM   #2
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Chainstay length on horizontal dropouts is the distance from the center of the bottom bracket shell to the intersection of the centerlines of the seat and chainstays, or to the center of the dropout, and is measured parallel to the ground. Moving the wheel increases the distance from BB to wheel axle (which is sometimes called effective chainstay length) and overall wheelbase.

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Old 04-06-13, 10:54 AM   #3
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As explained above, chainstay length used to be measured to the end of the stay itself back in the days of horizontal dropouts. With verticals which position the rear axle at a fixed distance from the BB, most people measure chainstay length on centers (horizontal) BB to axle, which can also be called the effective chainstay length when the wheel is movable.

You have the latitude to experiment with (effective) chainstay with your long dropouts, but depending on how your RD is mounted, this will also move the sprockets with respect to the RD affecting shift performance.
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Old 04-06-13, 11:19 AM   #4
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OP: Play around with the location of the axle in the dropout, I bet you'll find a more noticeable change in shift performance and noise than you will with the handling of the bike.
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Old 04-06-13, 11:42 AM   #5
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I had wondered about mechanical function and how it would be affected. I would definatley prefer the best mechanical function over +/- 2cm chainstay length experimentation. It will be interesting to see what my LBS does.
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Old 04-06-13, 11:43 AM   #6
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Yes, feel free to experiment. After all it is your bike. And anyway, they wouldn't make long dropouts if they expected everybody to put the wheel in the same place (that's for verticals).

Besides the possible changes in shift performance, there may be changes in rim height at the brakes. So after moving the wheel, take a moment to make sure the shoes are still on the brake track, and not overhanging, especially not on the tire side, where they can slice through a tire very quickly.
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Old 04-06-13, 11:43 AM   #7
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One thing to know, part of the mutual benefit of pairing, a vertical dropout and Indexed rear shifting,
is when you put the wheel back in, its always in the same relationship to the derailleur, location.
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Old 04-06-13, 12:05 PM   #8
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Quote:
Originally Posted by corwin1968 View Post
I had wondered about mechanical function and how it would be affected. I would definatley prefer the best mechanical function over +/- 2cm chainstay length experimentation. It will be interesting to see what my LBS does.
Whatever the LBS does, you have the option of providing for faster wheel positioning in exactly the same place. I haven't seen these lately, but horizontal dropout bikes used to come with sliding axle stops which were positioned in the slot behind the axle and locked into place. These served the same purpose as the micro-adjust screws that Campagnolo and other high end dropouts included.

If you hunt around you might find one or a pair. If not you can improvise something with a flathead screw and nut and a pair of washers. You need the flathead to the inside so your system is as flush as possible to the inside lest it interfere with the outer sprocket of your cassette.
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Old 04-06-13, 03:16 PM   #9
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The frameset came with a couple of hex-head bolts that screw into the dropout from the rear but they don't feel all that sturdy.

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Whatever the LBS does, you have the option of providing for faster wheel positioning in exactly the same place. I haven't seen these lately, but horizontal dropout bikes used to come with sliding axle stops which were positioned in the slot behind the axle and locked into place. These served the same purpose as the micro-adjust screws that Campagnolo and other high end dropouts included.

If you hunt around you might find one or a pair. If not you can improvise something with a flathead screw and nut and a pair of washers. You need the flathead to the inside so your system is as flush as possible to the inside lest it interfere with the outer sprocket of your cassette.
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Old 04-06-13, 04:06 PM   #10
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I haven't seen these lately, but horizontal dropout bikes used to come with sliding axle stops which were positioned in the slot behind the axle and locked into place.
+1 - Yeah, they're around still - in fact, they seem to have made something of a comeback with all the fixie hipsters using them on their (semi)horizontal track(alike) frames.
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Old 04-06-13, 04:06 PM   #11
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indeed, most better classic horizontal dropouts had those skinny stop screws. the campy ones had knurled knobs on them. they are there so you can set where the wheel installs and not have to fidget around when replacing a wheel, they only need to stop the axle long enough for you to set the QR tight, the screws don't have to take any load.
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Old 04-06-13, 04:10 PM   #12
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...they only need to stop the axle long enough for you to set the QR tight, the screws don't have to take any load.
I've always been leery of the concept of a QR skewer being able to provide enough clamping force to hold back the inertia of a wheel braking from speed from moving in the dropout. It just seems more secure, the idea of the axle having the end of the dropout to butt against, rather than just thin air, and the clamping force of the skewer.
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Old 04-06-13, 04:14 PM   #13
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well, A) there's not much braking force on the back wheel of a skinny-tire bike. B) if your axle and its clamping ends can't hold itself in place, it has no place being on a bicycle.
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Old 04-06-13, 04:14 PM   #14
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Quote:
Originally Posted by corwin1968 View Post
The frameset came with a couple of hex-head bolts that screw into the dropout from the rear but they don't feel all that sturdy.
They're fine. They don't have to b sturdy since they're simply guiding stops for putting the wheel back. Once the wheel is tight they don't take any load at all.
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Old 04-06-13, 11:59 PM   #15
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The biggest steady load the QR clamp/dropout interface has to handle is due to the chain tension. 150# rider standing on the pedal is about 75 lb-ft of torque in the crankset. With a chainring radius about 0.25 feet, the chain tension is about 300 lb. If you work through the sprocket and wheel diameter, the driving force is only 25# at the rim of the wheel. The braking force is at worst of about the same magnitude, so the chain tension is the main point.
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Old 04-07-13, 06:41 AM   #16
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The biggest steady load the QR clamp/dropout interface has to handle is...
Wow - that's most informative. Thanks.
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