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Bike geometry query -- anyone with an opinion, step up . . .(17 posts)

Bike geometry query -- anyone with an opinion, step up . . .bill
Dec 17, 2003 1:11 PM
Pegoretti built me a bike using only anatomic measurements. He came up with a 51.5 seat tube, 53.5 top tube, and 73 degree seat tube and head tube angles. I've always thought that the bike was trifle small, but it certainly worked. The drop between saddle and bar was about 9 cm or so, which required 2 cm of spacers (Pegoretti also extends the head tube). The saddle was about 20 cm over the top tube. I'm 5'8" tall, 160 lbs, 81 cm (31.5") inseam, in all other respects I think very average proportions.
Bike was trashed in a crash, and I'm looking to replace it with another Pegoretti. I built the first bike to race, and I'll race the replacement.
I've been talking to the importer about how I might want to change the dimensions of the bike. No direct contact with Pegoretti -- it's all through the importer (who does seem to have a fairly good and regular relationship with Pegoretti).
The "off the rack" 52 cm frame is very similar, with a 52 c-c seat tube, a 53.5 top tube, and 74 STA and 72.5 HTA. I don't know why Pegoretti customized my geometry, and I'm trying to find out.
I liked the old bike. I have bikes that handle quicker (my current Fondriest and my old Litespeed handled quicker), and slower (my 52 cm Douglas, which has a 73.5 STA and a 71.5 HTA, is probably a little slower). The bike was a great tweener, honestly, although I'll probably use its replacement mostly in crits.
I don't know how or whether to change the customized geometry of the former Pegoretti. Any observations? Advice? Etc.?
If you have bikes and feel the need to replace the Pegdzrider
Dec 17, 2003 1:36 PM
I'd replace it with as close a replica as possible. A Fondriest and a Litespeed would keep lots of people happy. I think the question is more about making you feel good than about needing a bike to race crits where quicker handling is desirable. You could probably reach the same riding position by sliding the saddle back and getting a slightly shorter stem. Would it feel the same? I don't know. How much more does custom cost?
custom doesn't cost any more, nor does it take any less time tobill
Dec 17, 2003 1:52 PM
get. So, it's no matter on those counts.
The Fondriest is actually a great race bike, period. Stiff and quick. The only problem is that it's CF, and it won't ding, it'll break. I have raced it in road races, but I'm squeamish about crits.
The Litespeed, I sold. I probably should have kept it just for crits, because it was indestructible, but something about it I started just not to like. It was actually too stiff for me, which I attribute almost entirely to the fork (a Wound-Up with a steel steerer). I replaced it with a Douglas scandium alu, which actually turns out to be a not very stiff bike and not really great for racing -- the perils of Internet buying and believing what you read. I haven't decided what to do with the Douglas, as we speak, but I'm happy to have the backup now.
maybe too small...C-40
Dec 17, 2003 1:52 PM
I would say that the bike was vertically too small, at least in the head tube length. A properly sized head tube should not require any steering tube spacers if a stem like a Ritchey WCS with an 84 degree angle is used.

The saddle should also not be more than 18cm above the top tube.

What does not quite add up is your 81cm inseam and your saddle height. I would recheck your inseam by blocking up the wheels on your bike until you get saddle-like crotch contact when standing over the bike in bare feet. Then measure from the floor to the top of the top tube and see if it's not closer to 83cm. Understating cycling inseam is a common mistake which leads to a too-small frame. If your inseam is really only 81cm, then you may have the saddle too high.

As an example, I have an 83cm inseam and a 71-71.5cm saddle height.
The distance from pedal spindle to saddle (although I'mbill
Dec 17, 2003 2:03 PM
never confident of where exactly I'm supposed to take that measurement on the saddle, because varying the place can give a significant variance in distance) is like 72 cm. How are you figuring the proper saddle height?
I'm pretty confident of the inseam dimension. I checked it a number of times using the book in crotch method.
saddle height....C-40
Dec 17, 2003 2:09 PM
is measured from the center of the crank (not the pedal) to the top of the saddle, along the center of the seat tube. 72cm sounds pretty close.

If your inseam is really only 81cm, then you could not ride anything larger than a 52cm (c-c) since the standover height of the frame would be about 79cm.
oh yeah, bb, not pedal spindle. But . . .bill
Dec 17, 2003 2:31 PM
if I measure along the seat tube, and one bike has a 73 STA and the other has a 74, and I have adjusted my fore-aft position to compensate, then I'm hitting the saddle at a different place, aren't I? Where the measurement could vary significantly.
BTW, I'm becoming more convinced of the sensitivity to fore-aft adjustment -- I don't think it has anything to do with power as such, and I don't believe in any magic to KOPS as anything other than an approximation of the spot where you're balanced. I think, however, that there is something to the balance thing -- where you need to balance your weight over the rear with the weight on your arms.
yes, but...C-40
Dec 17, 2003 3:16 PM
1 degree changes the meassuring point by only 1.2cm. Saddle height has to be eyeballed, so it't not real accurate anyway. If you actually use the same saddle on two bikes with diffrent STA, then it would be easier to decide on where the measuring spot was.

If you have different saddles, they can sit differently, so an identical saddle height measurement may not actually place your butt at the same height. The error generally shouldn't be over a few millimeters.
About that saddle height ...HouseMoney
Dec 17, 2003 4:15 PM
"The saddle should also not be more than 18cm above the top tube."

I just measured mine & it's 18½cm vertically from the tt to the front tip of the saddle, a little more if you measure to the top of the saddle (placing a ruler across the top).

I'm 6' tall in bare feet, maybe slightly taller. My inseam is 88.2cm. Using the I x .65 formula = 57.33; subtracting 28cm from I = 60cm. Although I just bought that Large Giant OCR2 yesterday that I asked about, I'm already looking to possibly replace my "main" ride (58cm c-t CAAD5). If so, should I consider going up to the next comparable size, depending on the bike company/sizing method?

For example, say I was looking at a Colnago. Three sizes (600x577, 610x582, 620x585) look to be in the ballpark. Before I plunked down that kind of $$, I'd want to be more than reasonably sure that I was getting the correct size. In fact, the largest of the 3, 620x585, has the same tt (585) & seat tube angle (73°) as the Giant I just bought!

Your comments would be appreciated.
Anything wrong with the CAAD5 fit????C-40
Dec 17, 2003 5:08 PM
If there's something you don't like about the fit of the CAAD5 now's the time to fix it, if possible. From your description, it soound like it's not bad. You saddle height above the top tube is about right for a relatively tall rider.

The C'dale has an effective TT length of about 58.2cm compared to the Colnagos. All of the Colnagos (60-62) are larger than your C'dale. Note the Pcc dimension (c-c) are all larger than your C'dale which would be about 56.5cm c-c.

If you want a Colnago yo should probably resign yourself to using a longer stem. Large Colnagos have shorter than average TT lengths. I've said it many times before, DON'T BUY A FRAME BASED SOLELY ON TT LENGTH. As long as a 130mm will be long enough, there's no problem.

I would compare the head tube length of the Colnagos to your C'dale, keeping IN mind that the total HT length with headset will be 25-30mm longer than the values in the Colnago geometry chart.
Why not just replace itLeroy
Dec 18, 2003 6:38 AM
You said you liked the bike. Why not get a new one just like the old one?
Because I'm the kind of guy who should not be allowed to buybill
Dec 18, 2003 8:06 AM
custom bicycles, because I don't know enough about what works or what I want. I always thought that the Pegoretti was a little small, although it was more of an aesthetic issue than a true fit issue -- it always rode well, but it had a very generous amount of seatpost showing, with an extended headtube and 2 cm of spacers. If I just tell the guy I want a bigger bike, he's likely to change the dimensions and geometry in other ways, too, because that's what happens when bikes get bigger. I'm trying to avoid the rule of unintended consequences. 'Course, we're talking pretty small changes in the scheme of things.
There are guys in my club who have been racing for fifteen, twenty years and know exactly what works and what they want; I don't. I'm looking for advice.
Don't be a slave to numbers.dzrider
Dec 18, 2003 8:35 AM
You had a bike and knew you liked it better than other, quality bikes you owned. The numbers are designed to predict what will work; you already know what worked. You may not need a custom bike because the variances are pretty small, but you had something you liked and, imho, trying to duplicate its angles seems like a no-brainer.
Bill, Bill, why you no come to your uncle Dan on this one?djg
Dec 18, 2003 1:22 PM
You weren't merely satisfied with that bike, you loved it. And it did not look strange at all (although I wasn't wild about the paint). A world-class builder took your measurements and made you a bike that you thought felt great. It was great. If you want another Peg, I say get the same darn thing. Yes, his stock frame will work just fine for you--heck, without the tape measure you may not notice a difference at all. But don't start fiddling with the angles based on guesses about what differences might be nice.

About racing: so, those super thin-walled tubes folded like cheap oragami in a crash that broke zero bones. Weird stuff happens (and weird, distinctive, force vectors get generated) in particular crashes, but I'd think twice about waiting another six months for another custom peg and throwing it into a bunch of crits next summer. OTOH, it's your dime. I know the Douglas isn't as stiff as you might like, but do you really sprint slower on it? Does it make a difference? If so, why not replace just the frame with something cheap and board-like? Just get the same basic color--C won't notice. You know, I wouldn't even jump to the conclusion that the Fondriest is less crash-worthy than the Peg was, although I make no warranties.
Yeah, I kind of did go walking around saying how, every time Ibill
Dec 18, 2003 3:12 PM
got on the bike I liked it a little more, didn't I? Was that just for the cameras? Maybe not.
I've sent the relevant excerpts of your post to the importer.
Thanks, Dan.
So is he cutting you a break?djg
Dec 18, 2003 3:20 PM
I figure I may not be able to ask in person when I see you next, what with the sort of heightened security we're seeing around DC these days.

The walls have ears and all.
He claims yes, although we haven't seen any numbers yet. Thebill
Dec 18, 2003 3:45 PM
other deal is that, while we're dicking around, the dollar slowly fades into the toilet.