Varying geometries of CV bikes
 

As I look at more and more classic bikes for sale in far away lands, I wonder how you all navigate bike fit for bikes you can't get your hands on.

I have a custom Dave Kirk, made for me, with a 55.5cm seat tube and 56.5 top tube. I have my saddle 74cm from the center of my crank and the tip of my stem 70cm from center of saddle. And it fits perfectly. So I use that 74cm / 70cm measurement when looking at bikes that I can't get my hands on. My 60s Cinelli is around 59 by 56 but with seatpost adjustment and stem length I can get my required measurement.

But at what point do you think this will lead me astray? Standover plays a part in there but sellers aren't always wise to bike measurement. How do you all do this?

I size my bikes by seat tube length only. When I worked in bike shops in the 1980s/1990s, top tube length was never even considered, unless you were getting a custom-built frame, I suppose. The saddle fore/aft adjustment and stem length took care of any issues presented by the top tube length.

standover + TT length

I find most useful measurements to transfer from bike to bike are frame stack and frame reach:
https://cimg1.ibsrv.net/gimg/bikefor...59dc5ae3c3.gif


And in terms of fit, I know that my ideal measurement from the center of the crank to the top of the saddle is 73cm, so that'll usually be the first thing I set up before I figure out stem length and angle.

What is your height and inseam length? Do you mean c-c?

The seat tube angle needs to be considered when presuming what top tube length is best for a particular rider, since each degree steeper moves the entire top tube and thus head tube forward by roughly 1cm (depending on frame size).

And the HT angle affects not only the front-center and thus toe clearance to the front tire, but also plays into what length of stem is likely to give the bike neutral steering characteristics while a given rider is in the saddle and while the same rider of off of the saddle.

The front-center dimension along with the chainstay length affect weight distribution and thus steering stability, with more weight up front helping to stabilize the steering.

The modern "reach" dimension is useful, but provides an accurate comparative function only when the two frames being compared have a similar "stack" dimension (since the HT isn't quite vertical).

How does top tube length have anything whatsoever to do with femur length/crank length--even considering your "AND AND AND?"

Most critical for me is the distance between my butt and hands (length and drop). Much of that can be controlled by the stem, bars and sliding the saddle back and forth if the top tube is close to "ideal". Where my feet land in relation has always been inconsequential. The relative BB position impacts me more if I ride out of the saddle, but then I don't ride out of the saddle much it that case.

I'm 6'1 and my inseam / pbh is 84cm (33 inches). That doesn't seem to be a typical combo.

The distance from center of crank to top of seatpost can only get you so far. The standover of a 59cm c-to-t Specialized Expedition is too high for me. But my Cinelli has the same 59cm c-to-t and it's fine.

I guess I'm answering my own question: I have to do research on the bike's geometry if I'm going to buy from afar. I just need to learn what to look for.

That doesn't sound right. Different wheel sizes? Something is up.

Logical?

My first thought was bottom bracket height, which is of course a function of the tire's rolling diameter.

Schwinn "electro-forged" road bikes had a really high bottom bracket and thus a high top tube, so I wondered if it was intentional (to keep the flat pedals from striking the ground) or if it might have been because these mass-produced frame's geometries had perhaps been originally designed around 26x1-3/8" tires(???).

This is basically what I do. I also happen to use 73cm from the center of the crank to the top of the saddle, though there is a bit of guess work involved in the angle of this measurement. I generally measure along the seat tube, but I try to adjust the angle a bit if the seat tube angle is not 73 degrees. After that, I use the frame reach to figure out what stem length I need and adjust the height to be a little below the saddle height. Then I set the saddle fore/aft position to get my preset distance from the center of the handlebars to the tip of the saddle. I try to use the same handlebars and saddle on all my builds so that this measurement is meaningful. If for some reason I'm using a different saddle or handlebar, I adjust a bit as needed to account for that.

To answer the OPs question about sizing of a bike I don't have in my hands, I generally judge it based on seat tube length and head tube length, with some consideration for top tube length if it's unusual. Using the system above, I find that I can usually make anything from a 55cm to 59cm seat tube work. Accounting for top tube length is a little tricky because the seat tube angle can make a top tube "fit" shorter or longer. I have a spreadsheet that I use to calculate frame stack and reach from geometry, but I usually have to guess at things like the seat tube angle.and bottom bracket drop. Of course, there's also the question of whether or not you can trust a seller's measurements. Generally, I find seller measurements to be unreliable, so I'm trying to teach myself to smoke test it based on pictures of the frame (mostly based on head tube length). I also often ask sellers for head tube length, because it's much easier to measure and provides a reasonable reality check for claims about seat tube length.

Stand-over height and seat tube length are quite important, IMO, because stems can only extend upwards a very limited amount.

If you buy a too short seat tube and use a long seat post to compensate, your torso will have an exaggerated downward lean to get to the bars. You can buy a long stem to correct this but you end up with a frame that's too small, a super long seat stem and a super long stem. You may also have a top tube that's too short. That bike doesn't fit. You've kludged it,... but why?

Get a frame that fits.


P.S. - Be careful with just the seat tube lengths, as bottom bracket height can vary.

If buying sight-unseen, get the stand over height.

The Wall thing is great when you have the bike.
Communicating with seller before buying or getting in the car to check it out? Make, model, year, and pictures tell most of the story. ST and TT measures can give you more confidence. Standover only matters if you're buying a beach cruiser and you will be stopping to pick up chicks. (Or your buying a race bike and you like to pedal through as many corners as possible.)

wrt this: But at what point do you think this will lead me astray? When you slide your seat forward/aft you change the center of gravity.

Don't forget n+2-1= twice as much fun for only n+1.

We’d use stand-over height to get a bike in the customer’s ballpark. The general rule of thumb for road bikes is you should be able to straddle the top tube and be able to lift the front wheel off the ground about an inch before hitting your junk. After that, a bike should be sized based on proper leg extension while pedaling.

Unless you get a custom frame, top tube length will be beyond your control. The only frame dimension you can control on a stock frame is seat tube length. Once this is determined, the proper positioning is then finalized using seat post, saddle and stem adjustments.

For non-custom bikes, I do my homework: read the manufacturer’s suppplied specs and know your proper size for that brand and model, which sometimes varies by year. After looking at a lot of pictures of bikes for sale, one also gets a pretty good ‘eye’ for what their size bike should look like. My ideal size is 57 CTC seat tube and 56 CTC top tube & I can usually spot one a mile away. I also know that I can make a 56 or 58 sized frame work if it’s something rare that I really desire. The rest is just detail fitting.

it sounds like gravelinmygears replicates the relative location of BB, saddle and bars using as a baseline a bike custom fit to him. That is the same thing I do. As long as the frame size is in the ballpark you should be fine.

I'm 5' 10" on a good day. If I have to eye-ball a frame that I am considering, I make sure the top tube is no longer than 57cm c-c,
The seat-tube length is much less of a concern. I can, and do, ride anything from a 54cm c-c to 58cm. I like 56cm x 56cm the best, I think.

On all my bikes, I have the same measurement for saddle height from the center of the BB to the top of the saddle and I drop a plumb line from the back of the saddle and keep the same measurement from that line to the center of the BB. I use the back of the saddle instead of the nose, because all my saddles are Brooks Pros and, with tensioning, the noses extend more on some than others. Finally, I get the measurement from the back of the saddle to the center of the bars the same by varying the stem length. Once again, I use the back of the saddle for the same reason. My stem lengths vary from 80mm to 95mm. Yes, I use short stems. I like to be in a more upright position. It's easier on my crash-damaged shoulders.

I used to only consider "seat-tube" length but have found over the past few years that I prefer frames with tight racing geometries. I feel more comfortable on them and they are more of a joy to ride, especially in the big city.

What does one have to do with the other? I ride a 58 cm whether I’m on my crit-angled Cinelli or a touring bike.

"Geometry"

that's the subject of this thread

You made some weird connections between seat tube length and frame geometry. If you can’t answer the question, no worries.

YOU made a "weird" connection.

go troll someone else


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