I'm looking at a 2001 Jamis Aurora frame with a 58 cm seattube (just my size), but it has a very short top tube of 56 cm. Because I'm relatively new to cycling and have always ridden ill-fitted bikes, I'm not exactly sure what size top tube is workable for me. I know there are all types of variables involved, but I wonder if some might give opinions on advantages/disadvantages of short top tube lengths.
According to the fit calculator at competivecyclist.com, for the "French Fit" (touring) I should ride a frame with ~57.5 cm top tube. I've ridden frames with ~56cm top tubes, and I never felt particularly cramped, but then I never rode them more than 20 miles or so. I have had problems slipping down onto the nose of my saddle, which I'm quite sure wasn't a setback or saddle angling problem. Would a short top tube cause me to scoot forward in the saddle?
Conversely, might a short top tube afford a more upright position that, over the long haul, will be more comfortable? Jamis must have been thinking something when they designed the geometry this way...or perhaps it was simply poor engineering.
Thanks in advance!
(锕侊箒)~~~~----------------------------------------------------------------------
It's a comon approach to frame design. That doesn't make it the right one for you. You hear different explanations, that go back to earlier dimension in butted materials, to the most agile wheel base dimensions, to the presumed body of the average cyclist.
Simply though, if you look at the average touring frame grid, it will give something like different lengths for various tubes in various sizes. What probably doesn't vary though is the wheel base, the chainstay length, the forks or the wheel size. They may vary, but not so often. Sometimes the seat tube and head tube angles vary, or they don't. So if you maintain chainstay length, and wheelbase, and basically everything except the main tube dimensions, the top tube length has to be shorter on large frames and longer on shorter frames to maintian target wheelbase length.
That said, the frame itself is not necesarilly going to actually match those numbers. I have a 58 frame, and it's wheel base is almost 1" longer than claimed, so what the numbers market, and what the frame actually turns out at is another mater. I wouldn't be surprised if your frame was also longer than stated, but I have no specific info.
1.5 CM is not a ton of difference that can be corrected with a stem of the right length. It depends on what your true frame size should be. While the various fitting programs are good, a better measure can be previous bikes you have owned.
Shorter top tubes will create a more upright positioning, but that isn't necesarilly more comfortable if you are correctly fitted in the longer frame, as your fitting advice suggests. I don't want lower bars, I draw the comfort line there, but a well stretched out torso is a better back, lung, and aero position. I find it perfectly comfortable. Remember, we have ancestry of a parallel back to the ground, and a concave lower spine position.
(锕侊箒)~~~~----------------------------------------------------------------------
Traditionally, Bike's from the 70's & 80's are almost always measured c to c,seat post and top tube. In most cases, these frame's had what's called a "true" measurement and again in most case's, the top tube was shorter by 1 or 2 cm than the seat tube. Not to disagree with the previous poster, but wheelbase is also effected by head tube angle and as mentioned, chain stay length and of course frame dimensions. A longer wheelbase contributes to more comfortable touring IMO. It's considered by many, including myself, that a more upright position also contributes to comfort for the many miles a touring cyclist may cover. If the 58cm bike your looking at has the dimensional ability for you to have correct pedal stroke,(10 to 15 degree knee bend at the bottom of the stroke),proper fore & aft seat position,(ball of foot at the 90 degree,plumb from knee to approximate pedal axle bisection. If the bike meets these criteria,then make sure you can get the bars at least equal height to the top of the saddle! Cockpit dimensions are then,just stem length choices.
Richard (锕侊箒)~~~~----------------------------------------------------------------------
OP GOOD question, I had the same concern,I'll get a 55 or a 57 2005,6 or 7. The fit calcs.,first of all relate to "road race geometry",I know about the Merx and French fit aspects,use them as a sort of comparison. The bike you want is a road but not race bike, "traditional" race geo. need not apply. 56 is quite enough for me and I do kind of like to ride low and "racey". I've used 130mm stems,you could too.I'd guess you won't need to. Besides ,any bike in the realm of cyclecross / touring will have a not so long top tube. The Trek 520 has a bit longer tube than some,most have top tubes that don't get much longer as the frame increases in size.I thought I'd put my 2cents in. The members before me submitted great data. Jamis is a really fine bike!
