Seatpost setback, effective ST angle

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shimmeD
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Location: eNZed

by shimmeD

I didn't want to say that frame manufacturers are cheating shorties! Because I don't know whether I've 'normal' limb measurements. I sure know that 74 degrees doesn't seat me far back enough, and using a SMP saddle (which I'm a fan of) doesn't work either because the seat clamping mechanism is wider than the saddle at the front 1/3 or so of the rails. The protrusion wears out shorts where my inner thighs rub.
Less is more.

kulivontot
Posts: 1163
Joined: Sun May 16, 2010 7:28 pm

by kulivontot

I guess I don't quite get what you get out of a smaller frame with the same reach. With the prevalence of compact geometries with mega-short seat-tubes, you can just lower the saddle. So the only benefit at that point is a shorter head tube. Yes, you can always shorten the stem to some degree, but it just seems silly to have a 49cm frame that still needs a 70mm stem to get the correct amount of reach. I realize we've kind of digressed from op's original question, but the point I was trying to make is that the ST angle is really only one part of the problem.

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shimmeD
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Location: eNZed

by shimmeD

I agree with OP's fitter wrt start from where you sit (to maximise pedalling efficiency). If the other contact points are achievable, then the frame is in a sweet starting point.
Less is more.

FreaK
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by FreaK

wingguy wrote:You're thinking about it backwards. Smaller frames have steep STAs so that you can get a shorter saddle/bar reach for a given BB/HT reach. There's a limit to how far the headtube can come back while leaving clearance for the front wheel (unless you use a super slack HTA or 650c wheels) so to make the bike feel shorter the STA comes forwards. The trade off is that the saddle may end up too far forwards for weight distribution and pedalling efficiency.


As a guy who rides in the roughly 52cm range I find that this kind of sub-optimal geometry comes up far too frequently in this size range.
Sometimes the basic hack is to optimize the rear triangle but slacken the headtube angle so as to maintain the desired reach, which is nice in that it leaves your weight balanced over the cranks to develop power and ride off the rear wheel, but riding a road bike with a slack headangle and a just-workable front center isn't all that fun either. I'd rather toe overlap and balanced handling than extra space and a wonky ride.

This is one of the reasons I'm all in favour of the 650b trend for road, as opposed to mtb, as 650c was a little smaller than necessary for most people who's bikes were sub optimal with 700c.
it's actually possible to come to the conclusion even before realising it makes no sense at all
-
tymon_tm

wingguy
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by wingguy

kulivontot wrote:I guess I don't quite get what you get out of a smaller frame with the same reach.


Well, like I said again, if you really need it you can reduce the effective length of tiny bike without resorting to non-standard wheel sizes, unusable short front-centres or super slack HTAs... But you might be compromising saddle setback.

Ultimately, 700c wheels are too big to package (for example) a 5'0" person's bike around and keep everything as it should be. Something's gotta give and it's often STA. Not saying that's right or wrong, just saying it is what it is.

kulivontot
Posts: 1163
Joined: Sun May 16, 2010 7:28 pm

by kulivontot

I still don't follow the logic that a shorter person's torso is as long as a taller rider. I guess the idea is that if you need a shorter reach get a shorter stem. Which is fine if it were socially acceptable to ride 70mm stem without somebody suggesting you need a bike fit without actually seeing you on the bike. I still think there are far more bike fit issues related to reach rather than seat tube length/angle.

wingguy
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Joined: Thu Mar 08, 2012 11:43 pm

by wingguy

I still don't follow the logic that a shorter person's torso is as long as a taller rider.


That's not the logic. Why do you think that's the logic?

kulivontot
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Joined: Sun May 16, 2010 7:28 pm

by kulivontot

Because the reach doesn't change

wingguy
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Joined: Thu Mar 08, 2012 11:43 pm

by wingguy

But the STA is coming forwards. They're cheating the overall saddle/bar distance by bringing the back forwards not the front back. With the trade off, like I said before, of maybe a less than optimal saddle position. That is the logic bike designers are using when they're bringing STAs really steep on small sizes.

I don't know how much clearer I can make what I'm saying...

AJS914
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Joined: Tue Jan 28, 2014 6:52 pm

by AJS914

Additionally, they are not accommodating a shorter rider's reach. The shorter person uses a shorter stem.

kulivontot
Posts: 1163
Joined: Sun May 16, 2010 7:28 pm

by kulivontot

I just find it to be a poor trade off to sacrifice reach for saddle setback. Saddle setback on a straight seatpost is already adjustable by +/-30mm just by moving the clamp point on the rails. You can add another 30mm of adjustability by adding a setback seatpost. If you ship a 49cm bike with the same reach as a 54cm and slap on a 80mm stem stock, you're not really giving a whole lot of range for reach adjustment. Maybe this assessment is specific to my situation, and I am projecting at this point, but I still don't think it makes sense. If someone can suggest manufacturers that size the frame reach progressively, I'd appreciate it.

wingguy
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Joined: Thu Mar 08, 2012 11:43 pm

by wingguy

Again, I didn't say it was a good thing, I just said it was a thing.

Cervelo scale stack and reach on a straight line, as do Argon-18. Although Argon's smallest bikes are still relatively long.

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