Tuesday, September 12, 2006

Sloping Top Tube Madness: Version 2.0

I posted a rant of sorts about the recent trend in design of 29"er hardtails that makes the seat tubes on these bikes unusually short. The thread got some pretty lively discussion, so you might want to check out the comments. At any rate, it was readily apparent to me that folks were not totally understanding where I was coming from. So, I offer version 2.0 for your approval.

I'll cut right to the chase here: the dang seat tubes are getting too short! There are reasons why designers and manufacturers do this, but I think that perhaps we need to break out of the "being fashionable/traditional mode" and rethink hardtail 29"er designs a bit.

The current philosophy seems to be this: The head tube has to be where it is to clear the tire, (duh- Mr. Obvious here!) and allow for one to mount a suspension fork with a specified amount of travel. Okay, head tubes are where they are, check! Riders want standover, so the top tube, in it's journey from the head tube, has to angle, dip, curve, or whatever, to clear the bits of any rider that is expected to fit any particular frame size. So, top tube clearance, check! The final piece of this puzzle comes in the form of a seat tube, which is connected to the top tube at some point along it's length. Obviously, if you connect the two tubes, and leave more than an inch or two of seat tube sticking above the top tube/ seat tube junction unsupported, you are asking for frame failure or at least, some problems. So, seat tube length above the seat tube/ top tube junction must be kept to a minimum, check!

All right. I hope you are following me here so far. Here's where it all comes together. Some of the current crop of steel hardtails out there have copius amounts of top tube clearance. Examples would be two of the three 29"ers that I own: the Inbred 29"er and the Raleigh XXIX. This means that the top tube follows a severely sloping angle from the head tube to the seat tube. In order to keep a traditional double triangle frame layout, the seat tubes then have to be short to avoid having an unsupported section of seat tubing above the top tube/ seat tube junction.

This results in an inordinately short seat tube in comparison to the effective top tube length. For instance: The Raleigh XXIX in a medium size has an effective top tube length of 23.6" and a seat tube length of 16.5"! ( measured center of BB to the top of the seat tube) The Inbred 29"er, dubbed an 18" frame size, has an effective top tube length of 23.75" and a seat tube, measured center of BB to the top of the top tube where it meets the seat tube of 16"!

It is generally accepted that in order to be safe, one must insert a seat post to pass just beyond the junction of the seat tube and top tube of any frame. So, in order to do that and get your leg extension correct, you are going to need a very long seat post. Raleigh's spec of an Easton 350mm post won't cut it for alot of riders. I had to order 410mm Salsa posts for both my Inbred and now for my Raleigh. (By the way, my inseam is 33.5" and I am 6'1" tall, for the record)That's a hassle and costs extra. Not to mention, longer seat posts flex more, which you will either love, ( bump compliance) or hate( seated climbing flex).

On the subject of standover, I checked my Inbred 29"er last night. FIVE inches under the tires when I lifted the bike off the ground while straddling it. That's way more clearance than I need. I can do with three inches, thank you very much! Three inches of clearance means that the top tube could have been designed to meet the seat tube at a higher point. That means that I would have been able to use a traditional mountain post length of 350mm. (Some certain Englishman thinks 350mm posts are for compact roadie frames. Nice try, but 350mm has been a standard post length fot mtb's like.......since the eighties!)

I would like to see a couple of things happen if the designs stay with these "super sloping" top tubes. One: use a larger seat post diameter, like 31.6mm, which would keep the flex in check for us bigger guys and would also be stronger for such a long length of exposed post. Second: supply these longer, (at least 400mm long) posts with your bikes. That way a shorter rider can cut off the excess post, but a longer legged rider can accomodate his leg extension without buying another seat post or going a size bigger in the frame and compromise his top tube length.

Whatever happens, this sloping top tube madness has got to end!

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