Overlay Fun, Sizing, and Sloping Top Tubes

Bryant-Hillborne Overlay
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Here’s a new overlay showing a 58cm Civia Bryant superimposed over a 60cm Rivendell Sam Hillborne. The Civia has a “downsloping” top tube, meaning the size is measured along the seat tube from the bottom bracket to a theoretical top tube above the actual top tube. The Rivendell, on the other hand, has an “upsloping” top tube, meaning the size is a true measurement to the top tube, with the top tube sloping uphill from there. That difference in measuring methods explains the greater than 2cm difference in size between these frames even though they list as 2cm apart on a spec sheet. It’s also interesting to note that because of the difference in handlebar set-ups, the smaller bike has a slightly longer reach to the bars. The moral of the story? There’s the numbers, then there’s the actual bikes and how they fit; don’t confuse the two.

22 Responses to “Overlay Fun, Sizing, and Sloping Top Tubes”

  • Velouria says:

    “There are numbers, then there are the actual bikes and how they fit; don’t confuse the two.”

    Couldn’t agree more. So much confusion and so many misguided purchases seem to happen when we go just by the numbers without taking individual bike geometry into consideration, not to mention which method of measurement is being used!

  • bongobike says:

    That’s an excellent illustration that will help newbies understand the complexities of bike sizing.

  • Dalton says:

    The numbers comment is great and I think its a great point to make. Alan, I know they are two totally different bikes, but when it comes to the sizing, which do you think works better for you? Do you now wish you had gotten a 60 in the Civia or maybe a 58 in the Rivendell? Just curious.

  • John Lascurettes says:

    I still find bike sizing (and fitting) to be the most intimidating factor in selecting and purchasing a bike.

  • Alan says:

    @Dalton

    58cm is my target size. I’ve found more bikes in that size that enable me to get the bars where I want them while also providing the standover clearance I prefer.

    Regarding these bikes specifically, the Civia is spot on. The Hillborne is only available in a 56 or a 60. I wanted less seat post showing on the Riv, and I also wanted Moustache bars at slightly above saddle height, so the 60 was the better choice. If a 58 had been available, I would have went for that size.

    Alan

  • Andy E says:

    I still think “upsloping” is just the retro-grouch justification for building a “down-sloping” top tube bike.

    interesting that they both have a very similar wheel base.

  • Dalton says:

    Thanks Alan! Like John, I too find the sizing to be the most intimidating factor of buying a bike. I am in the market this summer possibly sooner than later and this is the one thing I keep going back and forth on. Being right at that 6 foot mark it seems that I can go 58 or 60 and both would fit, but I just don’t know which I would prefer to ride.

    Its also a problem for me that I don’t have any local dealers for the two bikes I am considering. Doh! I am realizing that I need to find some more major brands with close to the same geometry as the two I am considering. I suppose its a good thing that I have to do some homework when I am considering spending over $1000 on a bike.

    Great blog Alan, thank you for all that you do!

  • Don says:

    I didn’t know about that aspect of measuring a compact frame. I’m still trying to understand the whole concept of trail and how that translates to “feel.”

    It looks like the Bryant pivots you ever so slightly over the bb in an aggressive direction. Is that accurate? Is that how it feels?

  • Alan says:

    @Andy E

    It would certainly be easier on everyone if all of the manufacturers agreed to use the same method of measurement. My preference would be for “center of bottom bracket to center of theoretical horizontal top tube, measured along the seat tube”.

    Alan

  • Aaron C says:

    I’m with Andy. It’s purely a matter of perspective and setup. There is nothing different between the two frames functionally with regards to the size of the intended rider. The Riv has a less severe slope than the Civia, which results in a much longer seat tube. In addition, it’s a little bit taller.

    If you were to yank the seatpost out of the rivendell another about four inches and put a level stem on it, you’ve got a “downsloping” bike for a much taller person who likes low bars. To think of it another way, if that Sam were a size smaller, getting your contact points in the same spots would result in a very standard looking “lotsa seatpost” sloping top tube bike.

  • Alan says:

    @Don

    Wikipedia has a pretty good article on trail and wheel flop:

    http://en.wikipedia.org/wiki/Bicycle_and_motorcycle_geometry#Wheel_Flop

    Jan Heine’s quote at the end of the section is instructive: “A bike with too little wheel flop will be sluggish in its reactions to handlebar inputs. A bike with too much wheel flop will tend to veer off its line at low and moderate speeds.”

    All other things being equal, the greater the trail, the greater the wheel flop (and vice versa). Head angle also affects wheel flop.

    You’re correct, the Bryant has a steeper seat tube angle (73 versus the Riv’s 71.5) which shifts the rider’s weight forward over the BB. Some of that is offset by the fact that the S-A saddle mounted on the Civia has longer rails than the Brooks and I have it set further back in relation to the seat post.

    Alan

  • Alan says:

    @Aaron C

    Agreed. It’s really a matter of semantics and how the frame designer thinks of it and the marketers want to sell it. Throw inch measurements and “sm-med-l-xl” size designations into the mix and it’s no wonder so many folks are confused.

    Alan

  • Daniel says:

    Bike geometry, fitting is always a fascinating subject. Alan I wonder if you have any thoughts on getting a good fit on a Brompton. It’s a bike that come in one size and has only 3 ways to adjust for size, seat post height, moving the seat forward or back and tilting the handle bars. How different sized people make these different adjustments seems interesting.

  • John L. says:

    Alan, I find these overlays fascinating and very instructive. One request: before you sell your LHT, how about a Civia-LHT overlay?

  • Garrett says:

    Very interesting idea! I would like to see the same image with bicycles aligned at the bottom bracket – the logical “origin point”. That way we could also discuss the varying seat tube angles in relation to saddle set back, the unimportance of wheelbase, among other things.

  • Alan says:

    Daniel,

    I believe the Brompton’s limited adjustment range is at least partially behind the widly varying opinions on how they can be used. Those who can get a good fit should have little trouble riding a Brompton longer distances, whereas those who are unable to get a good fit will become uncomfortable fairly quickly.

    The main decision is which handlebar to go with. The “S” is the lowest, followed by the “M” and the “P”. Anyone over around 6’2″ that doesn’t like being hunched over should definitely be on the P-type. I’m 6’0″ and I’m comfortable on the M-type. I owned an S-type a couple of years ago and found the bars too low. Those with PBHs over approximately 85cm should also use either the extended or telescopic seat post. Beyond those two decisions, there’s not much more in the way of adjustments on these bikes.

    Alan

  • Alan says:

    Hi Garrett,

    I almost aligned this one at the BB, but I like to see the fork rake/head tube comparison, so I went with the front axle again. I’ll align the BB on the next one.

    Thanks,
    Alan

  • Alan says:

    @John L.

    Will do! :-)

  • Aaron C says:

    Alan, I would suggest also propping up the rear wheel to match the height of the front so that they are level.

  • Garrett says:

    Alan, makes sense. It’s interesting to see the slight saddle angle difference resulting from the higher set handle bar.

  • Alan says:

    @Garrett

    “It’s interesting to see the slight saddle angle difference resulting from the higher set handle bar.”

    Good eye… :-)

  • Don Stevenson says:

    Thanks for the primer, Alan. It’s the kind of thing that is hard to articulate but is instantly obvious once you get on the bike!

 
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