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Frame Fit: Sit-Bones, KOPS, Saddle Setback, Stem Length
I put this up on rec.bicycles.tech yesterday, but in retrospect it seems more
MTB-specific because of the different style of riding. --------------------------------------------------------- The dilemma: A totally-competant framebuilder, using a CAD program, has come up with one saddle setback distance; I, using the dents made by my butt, have come up with another; and there's no way in hell I'm going to drop the kind of money a custom FS frame costs without thoroughly resolving that difference. "Resolve" could include making me understand where I've gone wrong. To wit: - I'm what somebody referred to as a "statistical outlier": 6'5" tall, 37" inseam....and so-forth - I've got two bikes that I consider TB almost dialed-in and dialed-in respectively. - Both of those bikes have seemingly-ludricous saddle setbacks: huge, and humongous. If you drop a plumb line through the center of each saddle clip, they're 11.25 (the FS) and 13.25 (the HT) inches behind the center of the BB spindle. On the hardtail, it's actually even more ridiculous in that the ThudBuster post I'm using gives about 7/8" more setback when compressed by the rider's weight. - Both bikes work in that my butt bones are no longer riding on the rear rail of the saddle. On the FS, they're sort of up against the rear rail, and on the HT, they're not touching it any more. Same saddle, adjusted 100% back both cases. - The FS with a straight post (9.75" setback) did not work for the first 1,000-or-so miles. During that time I went through a series of seemingly-too-hard saddles until it finally dawned on me that I was sitting on the rear rivets of the saddle. Then I went to a setback post and things got better with the setback increased to 11.25". - I'm in the process of getting a custom frame to replace the FS and, I thought, make my FS more like the hardtail that seems to fit so well. The framebuilder, however, is at odds with my numbers. He says 10" is more like it considering my body dimensions. He definately knows what he's talking about and I definately don't - except that I *do* know where my butt bones wind up. Some things that occur to me: - Apples and Oranges: My measurements and the builders are taken/computed differently. Previous measurements by Yours Truly have already been revised by a quarter inch in one case. - With my preferred setup, I'm 'way behind KOPS - as much as 2-3"...but that's a position my body found all by itself. - Stem length is a player here and my previous problems - that drove me to move the saddle backwards - were really a reflection of handlebars not being far enough forward. - My size 15 feet aren't in the builder's equation. - There's something missing from the builder's algorithm - like ratio of thigh to shin length. I hope to test the builder's proposal by setting up my current FS to match his setback/cockpit length/handlebar height and then riding it a hundred or so miles. My problem is that I need to resolve the (very large?) discrepancy between the framebuilder's number (10") with my number (13.25"). Again, "Resolve" could include making me understand where I've gone wrong. -- PeteCresswell |
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#2
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Frame Fit: Sit-Bones, KOPS, Saddle Setback, Stem Length
"(Pete Cresswell)" wrote in message ... I put this up on rec.bicycles.tech yesterday, but in retrospect it seems more MTB-specific because of the different style of riding. --------------------------------------------------------- The dilemma: A totally-competant framebuilder, using a CAD program, has come up with one saddle setback distance; I, using the dents made by my butt, have come up with another; and there's no way in hell I'm going to drop the kind of money a custom FS frame costs without thoroughly resolving that difference. "Resolve" could include making me understand where I've gone wrong. snip Again, "Resolve" could include making me understand where I've gone wrong. -- PeteCresswell Pete, I'd get a builder who did a better job and didn't leave me more confused than when I started. That's probably harsh because bike fit can be really tough. That said, why is the guy not working for you to resolve the difference instead of the other way around? Why isn't he explaining to you how he arrived at his numbers and whether or not your size 15 shoe has been taken into consideration? My suggestion is to contact another frame builder and get a second opinion. What you are saying, if I can condense it down to the lowest common denominator, is that you are not comfortable with the product this builder has provided you. Leave it at that and educate yourself further by discussing it with others in the profession. When you find the right situation you'll know and be happy with it. I've heard good things about Strong Frames and Zinn apparantly specializes in frames for tall people so he should be familiar with the issues you have. Talk to them both and tell one of them no. Good Luck. |
#3
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Frame Fit: Sit-Bones, KOPS, Saddle Setback, Stem Length
In article ,
(Pete Cresswell) wrote: I put this up on rec.bicycles.tech yesterday, but in retrospect it seems more MTB-specific because of the different style of riding. --------------------------------------------------------- The dilemma: A totally-competant framebuilder, using a CAD program, has come up with one saddle setback distance; I, using the dents made by my butt, have come up with another; and there's no way in hell I'm going to drop the kind of money a custom FS frame costs without thoroughly resolving that difference. "Resolve" could include making me understand where I've gone wrong. I think that personal preference plus being a "statistical outlier" plus mtb vs road riding could account for the difference. I like having my saddle slightly farther back for mtb riding, since there's more low-rpm seated climbing and less high rpm spinning than road riding. CAD programs or "fit kit" are no more (and no less) accurate than anything else; the data they work off is the same body of experience that an experienced "by eye" fitter would use. Measuring saddle position with different saddles is tricky, since the place where you sit is not always X% back from the nose or Y% on front of the tail on one saddle vs another. I try to estimate where the sitbones will actually reside for 'normal' riding, and then measure from there. You should never adjust your saddle position to compensate for stem length. But having found your position, you probably will want to stick close to it. Trying out the builder's position on your current bike is a very good idea. He might be right after all. I'd suggest trying it with a longer stem to keep the reach the same. Eric |
#4
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Frame Fit: Sit-Bones, KOPS, Saddle Setback, Stem Length
(Pete Cresswell) wrote:
I put this up on rec.bicycles.tech yesterday, but in retrospect it seems more MTB-specific because of the different style of riding. --------------------------------------------------------- I went through some of your problematics recently when I got me a new FS. We thought we had frame and cockpit all dialed in but I wasn't comfortable and I didn't feel balanced. I felt like I was falling forward onto the handlebars all the time. I got myself a setback seatpost from Thomson and a new stem that was 15 mm shorter, the same as the setback on the seatpost. This made a whole world of difference. All of a sudden my butt landed right on the saddle and I still had the same distance to the handlebars. My balance felt much better and my wrists weren't sore anymore. It actually felt so good that I started adjusting my roadbike towards the same geometry with disastrous results. The roadbike is back where I had it and I don't have the same geometry on them at all. But, I feel comfortable and balanced on both. Conclusion: I tried to mentally imagine the triangle that is in contact with the bike. feet hands and butt. If I move the two upper points back or forward simultaneously I shift my balance until it is too far back, forward or just right. -- Perre You have to be smarter than a robot to reply. |
#5
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Frame Fit: Sit-Bones, KOPS, Saddle Setback, Stem Length
On Wed, 21 Apr 2004 00:55:19 +0000, (Pete Cresswell) wrote:
My problem is that I need to resolve the (very large?) discrepancy between the framebuilder's number (10") with my number (13.25"). The one thing that seems immediately obvious, is that you do not seem to differentiate between saddle setback and cockpit size. I'm assuming, as an "outlier", you've been taking frames that are on the small side for you and using the layback of the seat to give you extra cockpit room. Your figures might be better if you compared headtube to saddle distance, rather than BB to saddle distance. A competent frame builder should be able to build in extra cockpit room by fitting a longer TT, thus giving you a more 'ideal' lower body position, but maintaining your current upper body posture. -- a.m-b FAQ: http://www.j-harris.net/bike/ambfaq.htm a.bmx FAQ: http://www.t-online.de/~jharris/bmx_faq.htm |
#6
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Frame Fit: Sit-Bones, KOPS, Saddle Setback, Stem Length
CAD programs or "fit kit" are no more (and no less) accurate than
anything else; the data they work off is the same body of experience that an experienced "by eye" fitter would use. One thing that's occurred to me: if their CAD program includes a stick figure of me on the bike (which I'm pretty sure it does...) they're actually a little shy of having the info that the "by eye" fitter has. Reason: that stick figure's dimensions may or may not be accurate and, whatever algorithms they use to represent the articulation of it's joints may or may not be accurate... |
#7
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Frame Fit: Sit-Bones, KOPS, Saddle Setback, Stem Length
RE/
I got myself a setback seatpost from Thomson and a new stem that was 15 mm shorter, the same as the setback on the seatpost. This made a whole world of difference. All of a sudden my butt landed right on the saddle and I still had the same distance to the handlebars. I suspect that's where I've been with my FS. Problem is that in doing that, I've defeated the design of the bike. I can live with the handling, but Tony Ellsworth would probably be unhappy if he thought anybody was going to judge his design based on the bike as I've got it set up. Seems to me like if my additional setback turns out to be valid, that the custom frame should be designed in such a way that I'm properly balanced with the extra setback - as opposed to the front wheel being really light. Otherwise, why bother with a custom frame? My modification of the Ellsworth weight distribution isn't a total crime against nature though. Today I took my first ride on the bike set up more-or-less per the custom maker's spec. Totally different ride. Definately properly balanced. No more front end popping on the climbs. The downside was that it took me a couple of dings on my knee to discover that the newly-weighted front wheel tends to follow ruts more strongly and the newly-lightened rear wheel no longer climbs obstacles at such an oblique angle as it used to. -) -- PeteCresswell |
#8
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Frame Fit: Sit-Bones, KOPS, Saddle Setback, Stem Length
RE/
The one thing that seems immediately obvious, is that you do not seem to differentiate between saddle setback and cockpit size. I'm assuming, as an "outlier", you've been taking frames that are on the small side for you and using the layback of the seat to give you extra cockpit room. Your figures might be better if you compared headtube to saddle distance, rather than BB to saddle distance. I've been concious of that - but maybe not 100% competant. I tend to focus on the distance between either the center of the saddle clip (more precise but harder to measure) or the back of the saddle (easier to measure) and a point on top of the bars on one side about the middle of where I grip them. That way, the difference in cockpit length due to slight rotations of risers can be accounted for. -- PeteCresswell |
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