trail_monkey

When a manufacturer steepens the seat tube angle as the frame size of a the same model gets smaller, do they compensate for the CG balance point by shifting the BB shell further forward or is the rider of the smaller size frame literally that much more over the BB than the larger size with a relaxed STA? I recently warrantied my older size 58 frame for the same model (newer design) and went with a size 56 as it was a more appropriate size for me and I feel as if I could use a little more setback. My new 56 has a 73.5 STA and my 58 had a 73 degree. Through the years on this model the 56's have always had .5 more of a degree steeper angle than the 58's. Why do manufacturers do this as I have read it is common? Wouldn't the smaller riders want the same CG balance point of the larger riders?

Carbonfiberboy 

On smaller frames, like 50-52, it's foot overlap. IDK for the frame sizes you fit.

Iride01 

I always thought that it was because of the average femur lengths for various height persons. I don't think that as we move from a short person to a tall person that femur lengths and other body parts proportionally get bigger in a linear way.

Also, if you kept the steeper angles, the CG with rider will be shifting further back on the rear wheel which might produce some stability and steering issues. Some bikes did change wheel base as they grew but other didn't.

At any rate, that's what I've imagined and gleaned from sources I've read over the years but don't even know if they were credible. So TIFWIW. Probably less than 2˘

trail_monkey

I'm sort of understanding what you're saying. You're implying that they are factoring in the average change in femur lengths for different height people and therefore on average justify the difference in the seat tube angle? I feel pretty good on my new frame although I'm still tweaking things like stem length etc. I'm still not convinced I wouldn't feel a little better with another centimeter of setback but that would put my seat at the extreme end of the rails and I don't want to bend them. I'm already running a 20 mm offset post.

trail_monkey

Actually I have a little toe overlap on this 56 that I never had on the 58. but during actual riding this isn't anything that ever is noticed.

79pmooney

Part of the equation is wheelbase and weight balance between the wheels. Smaller frames often get shorter in front to reduce the reach. If the front wheel comes back, then if you want to keep a proper weight balance between the wheels, yo u need to bring the rear wheel forward, This means reducing the clearance between the tire and the seattube or FD clamp. Solution? Steepen the seattube or bend it. Not a lot of fans of curved seattubes and the bike's gotta sell.

And the seatpost/setback issues? Changing seatposts to a larger setback post is easy. (Well on any bike that doesn't use a proprietary seatpost and tube or mast.)

I have my customs built with steep seattubes (74 and 75) for weight balance and run custom 60mm setback posts with the clamps centered on the rails. Bikes handle great and the ti railed seats love being centered, (Slamming them means breakage at over $100 each time and a lousy ride home. I also make sure the clamp has a "soft" edge, especially at the lower back. Use a file on it if it doesn't)

Ben

trail_monkey

Thanks for the answer. Now I understand

Road Fan

Good response, Ben!

I'm curious about your "soft" seat bolt clamp edge. Can you say something about the shape you file it to? I would think not a lot of material needs to be removed, but a fine-tooth file (don't cut it too fast) with a round section is needed. There's a fine line between just shortening the length of seat rail that is supported, and reducing the stress riser that exists where the rail exits the clamp. A non-gradual chamfer would just shorten the supported length and it could still have a significant stress riser.

79pmooney

I don't get scientific. I just make sure it isn't exactly square, that it has a little round to it. Not that different from good machining practice for any cut metal part that is going to see stress. Now I'm not worried at all about the seatpost. What I don't want is a sharp edge causing a highly concentrated load on the seat rail, basically acting as a knife edge. (When you hit a bump with your seat pushed back, yor sitbones are putting much of your weight, multiplied by the bump, behind the seat clamp. Seat rails are acting as levers supporting you. The see the upward force they provide as a bending moment. Bending moments require opposing moments. The is provided by the front of the clamp. But forces have to total out to zero or you go flying off to somewhere else. (The cursed Newtonian physics thing.) So the seat rail sees around twice what you put on it at the rear clamp edge. A hard, sharp edge? Ti really doesn't like that.)

So I just check that edge (and the front and the corresponding top edges and hit them once or twice with a fine round file. (Fine sandpaper around a small dowel would do the same thing.)

Ben

Road Fan

Ok, so you just soften the edge a little with a little radiusing, I would just rotate the file while I stroke it lightly, just a stroke or three!

I agree, we are worried here about the saddle rail, not the seatpost. I also use a lot of setback, probably for the same reason, to keep the weight off my hands when in a tuck. Somehow my pedaling has adapted to this oddball position, and despite the efforts of the new bike industry my body is not changing it's mind.

I'm still trying to be able to pedal well enough to fly off to somewhere else!

I have a Selle AnAtomica carbon saddle still waiting to be tried out - I'm sure this will also help out its carbon rails!

Road Fan

For the fore-aft balance thing, I think there might be two things, but I think only one is significant. First is the weight on the front and rear wheels. Going back to the mid-1970s, "Bicycle Frames" by Joe Kossack says (p. 24) that British designers of the day generally tried to get 45% of the bike+rider weight on the front wheel and ... well, the rest on the rear. This is termed as being for good handling. I've never seen anything that says what goes wrong if you don't achieve this.

More significant for me is the location of my body center of gravity (CG) to the BB. When I ride no-handed I need either be planted on the saddle or to have my CG above the BB. If I don't I'll fall forward or backward. When I'm in the drops or deep into the hoods, I don't want too much pressure on my hands and especially if i have to remove my hands to negotiate a bump, scratch an itch, take a drink, or make clothing or anatomical adjustments, I don't want too much sensation of weight pulling me forward. I also like to lift my body, sometimes including my hands, off of the contact points to negotiate bumps (no, I do not know how to bunny hop). That body weight distribution is managed by moving the saddle back relative to the BB, either by sliding the saddle along the rails or getting a seatpost with more setback. Obviously, KOPS is not a significant part of this beyond being a rough philosophy. I near always end up with my knee significantly (1 to 2 cm) behind the pedal spindle.

Carbonfiberboy 

Bunny hopping is totally worth learning how to do. The downside of it is that it takes time and therefore distance to set up, Timing is also critical. You don't want your rear wheel to hit the object at the end of the bounce, the worst possible outcome. That said, it's fun to do and a worthwhile skill to develop. Nothing to do with weight balance of course. On my 52cm road bike, the seat tube has been made more steep, which I counteract by using a special large setback post, just as you say. IOW, the seatpost angle gained me nothing. Stupid. The only possible gain is that my seatstays are slightly longer than they would be otherwise, which may give me a slightly smoother ride in the back. I am grateful that the BB is where it is, no toe overlap.

79pmooney

I raced in the '70s. I was/still am long and skinny with very little power and a lot of wind resistance, I got right away that I needed keep a horizontal back nearly always. My first season I raced a fairly standard bike fit-wise. Next season I bought the last year's Fuji Pro in the basement of the shop I worked at. 59 cm, steep 74 ST, longish TT', 75 HT. Quick, quick steering! Despite the much shorted wheelbase, it had toeclip clearance. I never touched. But if I put my fingers between the rear tire and seatstay, I'd lose 'em., Basically the bike I was riding before except with the BB shoved back as far as you could get it. This did several things. Seat was much further forward relative to the BB. Much more weight on my hands. But the huge change was that my abdomen was opened up, allowing much deeper breathing. Overnight, I was radically faster, stronger and had more endurance.

I flunk the weight centering thing. I flunk the weightless hands. I can still go all day upwind with a low back. (And I take seat position and tilt as super critical adjustments. Always nose down or it stats to close up my abdomen. And I will always be close to fanatic about handlebar shape, position, tilt, brake hood shape and position. I need really good landing places for my hands, especially as I get older.

Oh, and the tire balance issue. My Fuji put real weight on the rear tire despite my far froward position because the BB was so far back and the chainstays so short.. It handled like a dream (once I was used to how quick it was. That took weeks every spring.) I trusted it completely on unseen corners at race speeds I didn't get to choose. Every bike I've had since until my two recent customs put far less weight on the tire. Rear end gets light when I position for intense high speed corners. Bumpy and off camber corners get scary. On those bikes I have to remember to push back to weight that rear tire. The new customs go back to the steep seat tube and short chainstays. Great cornering without having to remember.

So my bikes flunk the metrics. But those extreme three (the new customs and old race bike) were all bikes I could put very long days on and suffer nothing more that the consequences of doing that much work. 175 miles twice on the race bike. Both customs have taken me 130+ in my 60s.

Bunny hopping - good skill to have.

Ben

Road Fan

I can see your point about teh seat stays, in theory. But when I got my old Woodrup I looked and said, why do these look so heavy? the ODs were 18 mm, where my older and Columbus frames were 16 mm, and the Mondonico was biconical, even, I think it is 14/16/14. The widest part is at the brake bridge, which makes sense.

So it's not clear that you get a little more cushiness if all that is done is to lengthen the SS's. I think you'll have to get your seat tube re-angled, after all, LOL!

What deep-setback seatposts are your faves? I like the Nitto S-84 and the old CLB single bolt. I don't have any use at the moment for zero-setback posts.

79pmooney

I have the Nitto S-84 on the Mooney and TiCcyles custom posts on my two customs. (60mm setback, Thompson clamp hardware, ti post.) All of those posts are two-bolt and joys to set up and adjust. Other two bikes are standard setback. A two-bolt Suntour and one-bolt Campy Chorus. (Nicest single bolt post I've every used. No clicks. I can set the tilt anywhere exactly. But I have it on my winter fix gear that I never touch for position.)

Road Fan

Nice, 79! My CLB also has no clicks.

60 mm setback with an S-84? I measure about 35.

79pmooney

Sorry, Road Fan. I meant the customs. Nitto says 40mm on their website. You might be closer. I haven't measured mine.

Carbonfiberboy 

I only have the one on that 52cm. It's the Velo Orange Grand Cru. It's infinitely adjustable for angle, but I don't like it because changing the angle also changes the setback due to the design. Total PITA because one has to get the saddle angle and position on the rails correct at the same time using just the one bolt. 30mm setback if the stars are properly aligned.

AnthonyG

You have put your finger on it, and you have asked the right question.
The answer?

It's a fudge.
Simple as that. The name of the game in mass manufacturing bikes is to put as many people as possible on the smallest variety of sizes with the smallest variety of part sizes possible.
Small people don't want to or benefit from sitting further forwards in relation to the BB than larger people do but there is only so far that the front/centre distance can be reduced for a given wheel size.
So what do they do?
Rather than using a smaller size wheel, which is a different part and therefore more costly, they move the saddle forwards instead. Not in the riders interest, just in the manufacturers interest.

It gets more complicated (WAY more complicated) than this but that's it in a nut shell.
I'm a short guy who rides custom made small bikes and also a bit of a crank on this subject.

What's the RIGHT answer for short people. Well it starts with smaller wheels, shorter cranks, low bottom brackets and RELAXED seat tube angles. NOT steeper seat tube angles.
This whole subject is a rabbit hole but slowly things are getting better. Children's and juvenile bikes are getting MUCH better than they were 5-10 years ago as they slowly evolve back to shorter cranks, smaller wheels and relaxed seat tube angles.
End Rant/

Road Fan

I've looked at those on-line. I've never really warmed up to single-bolt 'posts for the same reasons. The CLB is one, but it seems to stay stale, so I'm only juggling one factor at a time. I really prefer two-bole designs, high-setback or not. Campy Record, Thomson, some of the Nittos, and a few others, are what I've done well with.

Road Fan

For some of their stuff Nitto is very clear about the measurement standard, and for others not. It's an issue for stems, as well.

Holding the seatpost at the 70-ish angle from the vertical, I tend to measure from the centerline of the shaft that goes inside the seat tube, to the center point of the seat rail clamp.

I don't care, really, if my measurement matches Nitto's. I measure so that I can compare a given post with another regardless of manufacturer, to figure out if I should or could substitute one for the other. I don't care about "whether Nitto is wrong."

So where did you get custom seatposts?

79pmooney

TiCycles. They make stems, seatposts and a lot of other one-off stuff on request. Also a wide range of repairs on steel and ti. Very good; not cheap. I like them because I can go to them with a very specific idea and they will build it, not say "we only do it this way" or "you want this". (You will get questioned "why" and may well have it pointed out that if you do "that", "this will happen and you might want to take this other approach..)

woodcraft

Another way to look at it is that a smaller frame has a shorter top tube,

but the front wheel can only be so close to the BB.