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Old 02-15-09, 01:37 AM   #1
DerekVT
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BB height on a touring bike?

I've been building up an early 90's trek 520 to be my touring bike. it's been a winter project and coming along slowly, but i've finally got it mostly together. i've noticed that the BB height seems kind of ridiculously high. seat tube measures c-c something like 48 and top tube is 56. stand over is about perfect for me (i usually ride a 52), and length wise it is fine as well. i've got my seat post jacked up pretty high though and am rocking a nitto technomic stem. seems to fit fine, i'm just wondering why a touring bike would have a high BB?

i checked the trek website and all it gives is a 27.2cm bb to ground measurement, which doesn't mean a whole lot to me. i checked the bb drop on the surly long haul trucker and rivindelle atlantis and some others, and they seem to be a bit lower than comparable road/cross bikes.

i've heard a bit of arguing over center of gravity and how raising it can help with maneuverability, though i think loaded up, the CG would still be pretty low on this thing. i guess i'm just wondering if this is normal for a touring bike?
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Old 02-15-09, 09:34 AM   #2
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How much BB drop does the frame have?
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Old 02-15-09, 09:57 AM   #3
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that's what i don't know. i just know it's high. it's higher than my cross check's is.
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Old 02-15-09, 10:00 AM   #4
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The current production stock geometry Waterford 1900 Adventure Cycle Touring frames have an 80mm BB drop.

The 1992 Trek catalog shows the 520 geometry on page 47 (page 3 of the pdf), and describes the 520 on page 50 as equipped with 700 x 30c Matrix Cross Country K Kevlar belted tires.

The radius of 700c (622mm bead seat diameter) rims with those tires is 311mm + 30mm, or 341mm, so a line through the centers of the front and rear dropouts on a frame with those tires will be 341mm above the ground.

The 1992 Trek catalog (link above) shows dimension F (center of BB shell to ground) as 29.3cm, or 293mm. Subtracting 293 from 341, the BB drop will be 48mm. To me, that's quite high for a touring bike and is close to the BB drop of modern track bikes.

If you use the 27.2cm (272mm) BB center to ground dimension you say you saw on the Trek website instead of the 29.3cm shown in the catalog, 341-272= 69mm, which is close to the BB drop of modern road sport bikes.

If you have the frame and want to measure the BB drop instead of relying on the Trek website or the 1992 catalog dimension, stretch a string between the centers of the rear dropout and the front fork dropout and measure the distance from the string to the BB shell center.
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Old 02-15-09, 10:28 AM   #5
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cool, thanks a lot. i guess that confirms that it is actually higher than normal, still not really sure what the logic would have been though, i'm not going to be cornering too sharply or riding this thing in the woods
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Old 02-15-09, 10:32 AM   #6
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this one in the 92 catalog looks like my exact frame. same colors and same tubing...
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Old 02-15-09, 05:58 PM   #7
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Quote:
Originally Posted by Scooper View Post
If you have the frame and want to measure the BB drop instead of relying on the Trek website or the 1992 catalog dimension, stretch a string between the centers of the rear dropout and the front fork dropout and measure the distance from the string to the BB shell center.

Do as Scooper says here. To measure is to know. It's silly to speculate on what the geometry is when you can measure it.
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Old 02-18-09, 12:22 AM   #8
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I go by BB height, sure it varies with tires, but it is what I design for not the BB drop. I start with the wheels and their actual centers. It is easy to measure the height if you have the wheels on, it should be in the 10.5 to 12 range. I know of currently existing bikes that run that range. I just got off one that was around 11.5, and it felt high off the ground, but it was not hard to handle or otherwise ill manered on the several tours I took on it. Seems like the cooler bikes these days shoot for 10 5/8"ish. People seem to like the Trek, so it must be OK.
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Old 02-21-09, 10:53 AM   #9
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If 48 mm is the actual BB drop, and I think Scooper's method can be done accurately, then it is real high. But that's a common issue with small frames. If you have a short small (48 cm remember) frame, as the TT is made shorter the front wheel moves closer to the BB. If the head tube anlgle is not changed, the tire to downtube clearance can get too small for fenders. One part of mitigating this is to raise the BB and hence move the downtube. Another aspect of course is to lay back the head tube, and decrease rake to restore desired trail.

It's hard to directly compare small frames (<51 or 52 c-c, I think) to more standard sizes, like 56 cm. Unfortunately, Trek did not publish their geometry for all frame sizes. The usually only talked about the 22.5 inch, 56 cm sizes.

My 52 cm Woodrup, built originally for 27 inch wheels and fenders, has a 6.5 mm BB drop. It always feels a little teetery to ride.

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