Terry Butterfly vs Thomson seatpost

[laura*]

I'm assembling a brand new MTB and have purchased a (setback) Thomson seatpost and a Terry Butterfly saddle. What is the fore/aft adjustment range for this combination?

The Butterfly's rails are marked like this:
STOP> |....i....|....i....|....i....|....i....| <STOP

The ruler portion is 40mm long. Between the tips of the STOP arrows is 47mm. Unfortunately, the Thomson rail clamp is 41mm long.

Does this combination really have only 6mm of official adjustment range? :eek: Or are the Butterfly's markings meant to indicate the range for the center of a (much shorter) clamp?

Right now I tentatively have the saddle pushed back as far as possible with the front edge of the clamp right up to a bend in the rails. The middle of the clamp is still within the 40mm interval. However, I don't know if this will be back far enough. My rough measurements show that the saddle on my old MTB is even further behind the bottom bracket - but that saddle might never have been properly adjusted.

[Andrea]

You're correct to think that those marks are more important for seatposts that have less clamping area. The Thompson's setback is a little shorter than others that are out there, so you may end up needing something else in order to get your saddle in the right spot. A basic bike fit could be helpful.

[Miranda]

Yea... what Andrea said. Finding a good bike fitter (yes, the kind you $pay$ to measure you etc.) is an enlightening experience if you have never done so. I'm getting a new ride to resolve the mystery of why all my stuff hurt on my TOO big bike. The fit was worth every penny!

My point is that the fitter will be able to tell you what geometery will give you the best KOP (knee over pedal) position. Like ok... maybe the Thomson (which are great post for adjustability & in general) is not enough set back. That's a limiting factor about them (they offer one set type of setback post). Plus, how far back you need to go depends a lot on the STA (seat tube angle) of the bike.

If you are smaller, like me, one thing that happens in an effort to shorten the top tube in smaller bikes, the STA gets steeper. If you try riding dead in the center on the saddle (like the mid point of that Terry saddle), your knees are all scrunched up and hurt. When you start pushing back to compensate for too steep of a STA to get your legs stretched out right, your reach increases... and body/pelvis rolls forward to get to the bars. Part of my issue--*all* the saddles I tried hurt cuz of this.

Sooo... short of the long answer is that of how close the bike geo on STA is to what your legs need--and is current combo of post & saddle enough set back +that (geo). Look at the bike specs (if not with your booklet when bought, google manufacturer on line for a chart) to see how steep it is. If you got both of the post & saddle all the way back, and your KOP is still cramped up/not right--it's not enough then. PLUS--what's the geo/STA on your old mtb? Compare the two (old mtb & new mtb) geos. Did everything feel ok? If so, try to match that???

This is not the greatest link, but the header line says that the Thomson Elite set back is 16mm (1.6cm). If you go to Thomson's web site, they state some info about the longer rails. Like it's to offer more strength support structurally.

http://www.bike-x-perts.com/en/product_info.php/products_id/132908

Plus, here's a link to Thomson that talks about their rail length and design.
http://www.lhthomson.com/elite_features.htm

So I think 6cm sounds about right (5.6cm=4cm+1.6cm)... 4cm in the Terry saddle fore/after rail, plus 1.6cm w/the lay back of the Thomson seat post angle.

[laura*]

(reply part 1)


If you are smaller, like me, one thing that happens in an effort to shorten the top tube in smaller bikes, the STA gets steeper.

I'm taller. The old stand over test might have a salesperson rolling out their biggest bike - a bike where I wouldn't be able to reach the handle bars. Back in 1990 I bought a new MTB which made my shoulders hurt. At the time I didn't know enough to know that it was too big. In 1994 that bike was stolen. The replacement was a tiny bit smaller and fit a lot better.


Look at the bike specs (if not with your booklet when bought, google manufacturer on line for a chart) to see how steep it is.

I actually didn't get any literature at all with the frame! But that's OK. I'm assembling this new MTB from a bare frame, choosing every component. Thus, I have data downloaded for every component. One reason I'm building this bike from scratch is to get a bike that's tall enough yet short enough.


PLUS--what's the geo/STA on your old mtb? Compare the two (old mtb & new mtb) geos. Did everything feel ok? If so, try to match that???

Hey, I have the tool available for measuring angles. So why hadn't I? ...scurries out to the garage and back... The old (non-suspenion) MTB measured at 72.5 STA. The new one (hardtail) is speced at 73, but with a suspension fork of course that's true only at the designed for fork length.

I was trying to compare the old and the new but with suspension in the mix, it was hard to compare seat fore/aft position. A change in assumed fork length would cause seat position to measure different. Plus of course 15 years of evolution in bike fit theories.

One comparison I can be fairly certain of is BB to saddle distance. I have an even older Schwinn road bike that I last road 20 years ago. Its saddle was only 8mm taller than my old MTB's saddle. And come to think of it, every spring I've had to raise the MTB's saddle because it seems to sink during the winter. Thus the two might agree "exactly".


So I think 6cm sounds about right (5.6cm=4cm+1.6cm)... 4cm in the Terry saddle fore/after rail, plus 1.6cm w/the lay back of the Thomson seat post angle.

Not 6cm, but 6mm. Thats 47mm - 41mm to keep the Thomson clamp entirely within the STOP> <STOP range. The back of that range really does seem like a "don't cross this line" limit. The saddle rail starts curving there.

[laura*]

(reply part 2)

Today, I actually got to ride the new bike. I coasted downhill and walked the bike to a local bike co-op where I installed the chain. That let me pedal back home. :) My impressions:

The saddle needs to be raised slightly. This will of course also push it back which will give some adjustment range.

Knee-over-pedal seems OK. I need to measure more precisely instead of just eyeballing against a door frame or street sign.

The bike seems to be short enough to fit me!