Sunday, December 18, 2011

Seatpost Setback and Related Matters

Origin8 Seatpost, Zero Setback
A couple of days ago I replaced the seatpost on my Rivendell Sam Hillborne with one that has zero setback, and the change has been interesting. Before I go any further, I will warn you that this is a continuation of the long top tube post. So if that one gave you a headache, please stop reading now and save your sanity! Or, continue at your own risk.

For those new to the concept of setback, seatposts come with different amounts of it. One of the things the setback does is move the saddle clamp back, thus altering a bike's effective seat tube angle. Say your bicycle frame has a 74° seat tube, and you buy a seatpost with 2cm of setback. Unless you counteract the setback by moving the saddle forward along the rails, your bicycle's effective seat tube angle will be 2° slacker, making it more like 72°. And you can make it slacker still by pushing the saddle further backward. By contrast, a seatpost that goes straight up with no setback leaves your frame's natural seat tube angle unaltered. Seat tubes today tend to be steep, so it is rare that anybody wants to make them steeper still. But with a zero-setback seatpost, it is possible to make the effective angle a bit steeper by pushing the saddle forward on the rails.

2 Year Riv SH Frame-a-versary
The other factor influenced by a seatpost's setback is the reach from saddle to handlebars. The more setback a seatpost has, the further the saddle moves away from the handlebars. Here it is worth noting that bicycle fit experts typically warn against messing with seatpost setback and saddle positioning in order to alter reach. Instead it is advised that one's saddle position preference should be fixed in relation to the bottom bracket. At least that is my understanding.

Getting back to my bike, it has a 52cm seat tube and a 57.5cm top tube - the latter being unusually long given the former. Additionally, it has a 71.5° seat tube angle, which is atypically slack. In previous posts I explained that when I ride this bicycle, I feel as if my body is not sufficiently forward. The long top tube will not allow me to fit the bike with a stem longer than 7cm, and the slack seat tube puts me further back still.

2 Year Riv SH Frame-a-versary
Originally the bike was built up with a seatpost with generous setback, making the effective seat tube angle even slacker than its natural 71.5°. Eventually I replaced it with a seatpost that had only minimal setback, but even that did not feel as if I were sufficiently forward. I was reluctant to go with a zero-setback seatpost, because everyone I spoke to acted horrified by the idea. "Zero setback? What are you trying to do, turn it into a racing bike?" However, after the "long top tube" post I came to the conclusion that a zero setback seatpost is the most obvious solution. Far from making the bike "racy," it would simply continue the frame's already slack seat tube angle without slackening it further. Or, I could move the saddle a tiny bit forward and make the effective seat tube angle a rather normal 73° (as it is on my other two bicycles with drop bars). So, that is exactly what I did.

The welcome side-effect of the new saddle position is that the long top tube problem seems to be resolved. My reach has been reduced considerably and I can get a longer stem if I want. But even with the current stem I already feel myself positioned significantly more forward on the bike than before. The subjective sensation of this is greater than I would have predicted: I feel more in control over the steering, and I feel that the bicycle is distinctly faster to accelerate and to start from a stop. Although visually the saddle comes across as being too far forward now, its relationship to the bottom bracket is actually quite normal for a roadbike (off-the-shelf road frames in my size typically have 74-75° seat tube angles). I need to take the bicycle on a longer ride before I can say more, but I think this setup may be just the thing.

It's been exactly two years since I received the Sam Hillborne frame as a holiday gift, and this bicycle has given me over 2,000 happy miles. I've changed a lot as a cyclist over this time and the Sam's frame is quirkier than I initially realised. But I am going to try and make it work for me - hopefully learning a thing or two in the process.

46 comments:

  1. Of course, "back in the day" (I am thinking of much older English bikes in particular), seat angles were really slack but the seat posts had no setback, or even had "setforward," or the saddles were pushed forward on the rails. This had the benefit of getting the saddle bag away from the thighs of the rider. Meanwhile, riders still got into an appropriate position, vis-a-vis the bottom bracket (as you have done now.) I'm all about riding "way back" by modern standards, but there's no reason whatsoever to combine a 71 degree seat angle with a seat post with setback, let alone a lot of it. Kudos to you for making this oddly proportioned bicycle work for you! (And it's still not in the least bit racy.)

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  2. That's such a great looking and versatile bike, I hope you can get it to work for you. It would be great if this simple change would make it significantly more ride-able. I'm trying to get the saddle position dialed in on a bike right now, but it's really something that could go either way for me.

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  3. cyclotourist - I am hoping that it will work as well; I am very much attached to the bike and in a way I wish I could magically change my preferences to where they were a year ago when "sitting too far back" and being slow uphill did not bother me. If it doesn't start snowing I am planning to take the bike on a nice long hilly ride over the holidays and put it through the ultimate test!

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  4. XO - Yes, the glorious "set forward" seatposts on turn of the (20th) century bikes! I included this reference initially, but removed it because I thought the post was already too convoluted, but also because I realised I did not know what the actual purpose of those seatposts was - to steepen the angle or to decrease reach. Since the old bikes, as I understand, were infamous for having the same TT length for different size frames, I thought it was more likely the latter. The saddlebag thing you mentioned had not even occurred to me!

    Anon - It's an Origin8

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  5. Velouria - Hope this works. Methinks it could be a prudent step, Petersen would probably appreciate it, it could well be in the conception of the bike. Why? Because there is more to this than setback. Think of your position on a bike as a triangle: feet-hands-bum or rather their contact with pedals-grips and saddle. This triangle is very much defined by the hip angle. Tilt the triangle back and you have the upright position of the leisurely rider or the city rider, tilt it forward - unchanged - and you have the position of a rider who wants and is able to apply a bucket of coal. (Though an untrained rider would probably want to have a more open hip triangle, everything else unchanged). A fourth element is gravitation, how much weight you have on all points which determine what saddle and grips you want, esp. how much weight your hands can stand. The slimmer your waist, the stronger your back, the less weight on your hands, the more forward tilted position you will want, also because your legs are probably stronger as well and you will want to pedal harder which in turn takes weight away from your hands.

    You are stronger than two years ago, your body has changed and wants a different position to be comfortable and the bike will most certainly allow that with the measures you have taken. Now, that Petersen is a man who knows what he does. If you look at the Riv geometry charts you can see that there are bikes intended for enthusiasts who probaly have more than one bike, like Rodeo or SimpleOne with a more forward tilted position and bikes like Sam H or Hunqapillar witch Grant stresses are intended as first or only bikes for people who nevertheless have the ambition to ride a good bike a lot. These bikes have a longer top tube - esp. in your size. Why? I think he might see a customer who wants a good bike and wants to ride, but maybe is not that welltrained and a little intimidated and therefore prone to choose a bike a little on the small side. This customer gets a bike that lets them ride in a more backtilted position and be comfortable, but has the ability to change with its owner as she or he gets stronger and more experienced.

    My guess at least. Good luck!

    Sipelgas

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  6. Can you explain about stem length? I don't understand why you'd go the trouble of getting further forward to then consider a longer stem. Or why not go for a shorter stem in the first place? What effect does changing the stem length have?

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  7. Just as you realized that Pashley did not work perfectly (though it is a great bike), it seems like it is time to set this Sam free (though it is a great bike).
    You have learned a lot in the last few years, and one thing your great blog has shown is that not every great bike is a great fit, literally or figuratively, for everyone.

    As Alan of Ecovelo has sold some of his bikes, I am sure you will be able to sell this easily, for a fair price.

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  8. Golly, me thinks that frame is not the right one for you. Nothing wrong with a zero setback seat post, but the saddle position and super short stem suggest to me that you need a different frame, not just a bunch of band-aid modifications used in the hope that they make the frame work. I think people fall in love with the idea of a particular frame, (this seems especially true of GP's offerings,) then go to absurd lengths to make them work. I doubt any good fitter would put you on a frame that required the current seat position and stem length and claim they were ideal.

    Steve

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  9. Am happy that the position feels better for you. Am curious what effect this might have on your loaded touring setup in regards to the back wheel kicking out when braking. Hoping to hear from experienced touring riders about that and how to avoid it.

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  10. Lots of words to explain this simple issue but I understand your concern.
    My other bike has a standard "square" geometry (57cm ST, ~57cm TT) and with minimal setback and long 100mm stem I felt being too stretched between the saddle and the bars. Recently, I installed a shorter 80mm stem and it makes my ride much more pleasant.

    This makes me think that those Rivendells are built in some weird way. I am not sure how I could comfortably ride a bike with drop bars and such a long top tube. How long is the top tube for the 56cm frame? Like 62cm?

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  11. I've found that all of those stict rules about rider positioning on the bike are ultimately secondary to what feels right to the rider, and that that can vary greatly.

    I've also found that there seems to be a great deal more variance for any particular rider in what can work as an efficient riding position re fore/aft position, reach, etc. Again, in the end, it's best to go with what feels right, rather than some abstract formula.

    In this sense, it's interesting to compare the extreme differences on these variables between road racing and triathalon bikes--the rider position on the latter is usually extremely far forward.

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  12. The setback dimension that allows us to compare one bike to another is measured by running a vertical line through the axis of the bottom bracket and measuring back to the nose of the saddle. That's called saddle setback. At risk of confusing things hopelessly, most all saddles are alike enough that this measurement means something. The joker in the deck is the saddle presently on your Moser.

    From the pictures published here all looks normal and fine. We can only look, you could measure.

    If the saddle is too far forward the rider is more liable to pain at the front of the knee. If the saddle is too far back there is stress and strain to the tendons behind the knee. In between you ride a bike. It's a fairly broad range. Almost no one goes back too far. If you attempt performance rides on a Dutch bike you can feel the stress behind the knee.

    UCI rules demand 5cm of saddle setback. Otherwise you have a BSO and not a regulation bicycle. Lots of commentary makes that rule out to be there because the old Italian men are fuddy-duddys. The rule is there so the bikes steer somewhat alike in close quarters, to keep the bikes from steering too fast, and to reduce the chance of flying over the handlebars.

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  13. What, you mean my earlier zero setback post suggestion awhile back was negated? I truly don't understand taking this kind of statement to heart when it lacks all basis in comprehension of what a racing bike is, let alone what proper setback is:
    " I was reluctant to go with a zero-setback seatpost, because everyone I spoke to acted horrified by the idea. "Zero setback? What are you trying to do, turn it into a racing bike?"

    "I feel more in control over the steering, and I feel that the bicycle is distinctly faster to accelerate and to start from a stop."

    Yeah, changing it from a Dutch bike to this set up will teach you both about muscle recruitment and putting more weight on the front wheel for more balanced handling. This was also something I pointed out when you were looking for a road bike.

    This kind of long tt with proper reach set up is one way to set up a bike to avoid the otb apocalypse of the other post.

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  14. Dweendaddy - It's tempting to draw the Pashley comparison, but I am not ready to throw the towel in yet. This bike has some unique capabilities that my other bikes (and most other bikes) lack and I have a feeling that the positioning issue is ultimately fixable. Also, I've looked into what other options are available were I to sell this bike and get another in the same category but "better," and it isn't realistic.

    GR Jim - The problem is that I am not confident in my understanding of bike stuff and there is so much conflicting advice flying around. I don't mean in the comments here either, but in person from experienced bike people. I've been told that a zero setback seatpost will ruin the bike, ruin my knees, ruin the fit, defeat the purpose of the bike, etc. But finally my own conclusion is that anyone believing this (1)is not really processing how slack the ST of this particular frame is, and (2)cannot, for whatever reason, get rid of the visual/stylistic association of zero setback with racing bikes.

    Anyhow, I wanted to write this post as a followup to the previous one, because otherwise this would get lost among the comments. I know there are people out there who bought this same bike around when it first came out and are now having similar issues to mine, so hopefully documenting this saga will be helpful.

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  15. "If you attempt performance rides on a Dutch bike you can feel the stress behind the knee. "

    Yes, it was interesting to notice this a some point : )

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  16. I get that. There are way too many experienced bike people out there who think they know everything. I'm one of them, but at least I get my info from the source and look at numbers and intended use.

    At some point you may want to attend a free fit seminar put on by a top notch racy shop. Forget every bike in the shop is carbon. You'll find a lot confirmed and learn a lot too. I learn stuff on the bike still every day.

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  17. Whatever feels right to you is what you should do. There's nothing magic about KOPS. If your bike has a super long top tube, it's probably better to get a zero setback post than go with an overly short stem, which might throw off the way the bike . I did the same when I had SOMA double cross with a long 59cm virtual top tube. You go, girl!

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  18. Something tells me that the slack seat angles on Rivendell frames are there to help accommodate the Brooks saddles. The Brooks seems to scoot you forward a cm or two compared to a modern saddle.

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  19. "Can you explain about stem length?... What effect does changing the stem length have?"

    A longer stem puts the handlebars further forward, if you can picture that, which in turn places more of the rider's weight forward. Moving the saddle forward transfers more weight forward as well. Doing the two things together is akin to transfering the rider from the back of the bike more toward the front of the bike.

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  20. I really appreciate the detailed description of what you've done to dial in on a comfortable riding position. I can relate to the endless tweaking to find what works best. I hope the new seat post does the trick!

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  21. Saddle setback - the measurement from vertical plane through BB to nose of saddle - cumulates and includes seat tube angle, seat post setback, saddle adjustment, all of it. The bike clearly still has setback so it doesn't matter how you got there. It does photo a bit funny from some angles if you care about that.

    UCI says 5cm setback or more or it's not a bike. The old Cinelli manual suggests 1cm for sprinters of small stature up to 6cm for long legged rouleurs. If you tried to abide by both rules you'd have 10 millimeters to play with and no more. I ride with 6 to 10cm setback and don't expect anyone to do what I do. And am happy my bikes are not all the same.

    If you want a bike no one will ever dare to criticize Hilary Stone is offering a 20-1/2" Hetchins Vade Mecum Tourer for less than what your SH would sell for. Or go to velorama.fr for a very fine Meral Randonneuse, 53x53. No one will kick sand in your face.

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  22. How are zero setback seatposts associated with racing bikes? Most road bikes ridden by elite pros have setback seatposts with the saddles pushed all the way back (just check out bikeradar). I usually associated zero setback seatposts with mountain bikes, where they are quite common.

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  23. I don't know Duy, maybe it's the Thomson thing. I personally associate straight seatposts with vintage bikes.

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  24. I was fine with your explanation of the problem,my head hurts listening to all the arguments pro and con. I've got a funny body,long torso and short legs. Every bike I have had has needed "tweeking," primarily to adjust the effective top tube length. After my meager 40 years on the street only one question matters,
    How does it feel the day after a full day in the saddle?

    Marc

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  25. This sounds like an excerpt from my bicycle history.

    This is purely anecdotal, but I have gathered from looking at photographs of very old bicycles with extremely slack seat tubes that it was very common for saddles to have "negative" set back (although it probably was not thought of in those terms). Have you seen photos of the old 7-style seatposts that paralleled the shape of the stem? As seat tubes angles grew steeper, seat posts with set back became the norm.

    The old straight-post-and-clamp used for several decades to mount the saddle (and still available on inexpensive bikes today) can be set up with negative set back by flipping the clamp around. I have often done this. Indeed, sometimes I have replaced a modern single-piece piece with the old clamp style so that I could have negative set back. I like a steep seat tube angle, I suppose.

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  26. Nick, I've done that too. In fact, some Pashley Roadster models are assembled with the clamp forward at the factory (the seat tube is quite slack).

    I have three bikes with classic seat posts (1 modern, 2 vintage) and they all have the clamp forward. I like a steep seat tube angle, too. In my size frames, the saddle ends up roughly where you would expect it on a fast road bike.

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  27. For what it's worth, I got a custom touring bike that was made with a 54cm TT instead of my usual 52cm, because it's too dangerous to have toe overlap on a loaded touring bike. This is how it arrived: http://antbikemike.files.wordpress.com/2008/07/marianna-f-007.jpg

    With a setback seatpost and a 9cm stem. On my initial ride around the block, I almost tipped over because I felt like superman stretching for the bars. I immediately put a no-setback seatpost on, with the saddle pushed all the way BACK because I felt too close to the bars. I put on an 8cm stem and was like "oh, way better, nothing else needs to be changed!" And then I rode it like 10 miles and my arms and shoulders hurt like hell. Then I cried a lot because it was an expensive bike built FOR ME that didn't fit. Eventually I measured the distance from saddle nose to BB and compared it to my commuter, and discovered the saddle needed to come all the way FORWARD. Now it fits and I love it and it looks like this: http://www.flickr.com/photos/tinyhonkshus/6534800177/

    So basically: 1) I think your solution could very well work for you, 2) It happened on my custom bike, so it might not just be a "bad frame for you" as some people are suggesting.

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  28. Marianna - that's a great looking ANT! Glad the fit worked out for you.

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  29. We are all different shapes. I am nearly 4 inches taller than my father, yet we have the exact same length torso. The result of this is we can ride the same size frame comfortably (Most would say his frame is "too big" for him), I just need more seat post and sometimes higher bars. I also find that the standard 100-120mm or so stem length is generally too long for me and prefer around 80 or 90mm. If it works and doesn't cause any pain or discomfort who cares what anyone says - they ain't riding it.

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  30. I hate to complicate things more but isn't seat setback just as personal as leg length/torso ratio? I remember one of the good cycling books of the late 1980s that included ways of taking anatomical measurements. I found I had a long thigh to leg length ratio requiring a more setback seat position, either from a slacker seat tube angle and/or more setback seatpost. It was stated that this proportion was characteristic of women just as the long leg to torso ratio is. Ultimately, the issue is where your knee is relative to the cranks and this is determined by both thigh/leg proportion and a little bit by foot length.
    Question: did you raise your seat when you moved it forward? Moving it forward without raising it shortens the seat-to-cranks/pedal distance.

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  31. I was going to mention anon 5:28's question, also. It always seemed to me that one would want to start with the seat height, then move the saddle fore or aft to get into the best position of knee in relation to pedal, then adjust stem length and height to place hands and arms in the most efficient and comfortable place, then re-adjust seat height. Of course, as other posters have mentioned, you're the one riding this and it's your comfort which is most important. As long as you are capable of sustained, pain free, pedaling for hours -- which is what a touring bike should allow -- then all this is just talk. Hope the bike works as great as it looks!

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  32. It's almost an axiom in bike fitting that you DO NOT change the saddle setback to correct a reach problem. This link describes a good check for saddle position.

    http://www.velocipedesalon.com/forum/f7/riding-tip-1-a-550.html#post7428

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  33. Anon - Right (and the post says as much). The setback in itself did not feel right to me, and that is why I changed it. The correction of the reach problem was just a (welcome) side-effect.

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  34. Seatposts have setback? Zero setback seatposts? Aahh! My lht has a long top tube and have struggled with the bike since day one. I have no idea if the seatpost has setback or not. How would you know?

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  35. Another vote here for it all working out for the best. And maybe even turning out better than what you started with!

    I do feel that frames are inscribed with intention from their maker. You can't just throw things together and expect to get the best out of a frame. But that doesn't mean that their intention is completely closed to manipulation. GP is one of the most open bike builders there is (nearly every Riv bike is an All-Rounder of some sort as they used to be called). And what you're aiming for doesn't seem radically conflictual with the bike's original set-up.

    I've got pretty much the same set-up on my Mercian: a Thomson elite "zero" setback with the saddle a little bit forward and an 8.5cm stem. Mine is a 73 deg ST to begin with (so by your deductions I guess I'm pushing a 74 deg tube!). I came to this set-up after changing numerous different aspects and now it's by far the most comfortable and responsive bike I've ever ridden.

    On your bike, the one thing I would wonder about is how the shortened stem reacts with the larger tires?

    E in Ireland.

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  36. I have a 7cm stem and Nitto Noodles on my Sam with 650 Pari Moto tires, so 38s, and haven't noticed any issues with steering the bike. It tracks pretty effortlessly. If I changed the bars to any of the other ones I have laying around (basically stock bars I accumulated off bikes through the years) I probably would go to a longer stem too. The noodles seem to have much more reach built into them than the other ones I have.

    When I first got my Sam, I set up my Brooks saddle with what seemed like a good position; it turned out that it was too far back and I developed some rear knee pain. Finally getting the help of my friend to eyeball it, I found out my knee was too far back during the pedal rotation and sliding the saddle up a good 1.5cm cured the issue.

    I do feel a little stretched out when I ride on the hoods but I find that a blessing. It helps my back and I feel like I am actually riding in the bike instead of on top of it.

    One thing is for sure, getting a bike set up to fit your own riding style takes a little time and generally component swaps. It certainly helps to have different saddles, stems and bars laying around to get any bike to be utterly comfortable. I certainly enjoy tinkering with them.

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  37. I've put zero-setback seatposts on several bikes-- previously, a 90's road touring frame and my current bike, a 1985 ATB with drops. Both frames feature slack angles, and were improved by the change, in addition to sliding my Brooks saddle forward. I'm glad to see you are finding your niche on that bike; a performance hybrid as I call it-- with fat tires and drop bars-- is a revelation in comfort and performance, simultaneously. Don't let these old guys bully you with their wisdom.

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  38. I have a similar issue with my Sam - the reach is a bit too long with Noodles and a 10 cm stem. I use a brooks B17 with a VO setback post, which I like, so I'm planning to change the stem for an 8, which arrived today. Crossing fingers.

    Jay

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  39. I ride two bikes: A 60cm Sam H and a 58cm Trek CX bike (which was my first bike). When climbing or accelerating (seated) on the Sam I always feel like I can't quite get myself "on top" of the pedals--a feeling I've never had on the Trek.

    I'll try some adjustments this winter (I give the Sam winters off here in Boston) and will be sure to slide the seat forward to see if it alleviates this feeling.

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  40. Aaron - Yes, that is the feeling exactly. I'll be interested to know whether the change in saddle position resolves this for you.

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  41. To make matters worse, there is the confusing verbiage on their website. If you read the Rivendell website, they go to great lengths to explain why you should probably be riding a larger frame size than normal. I too was interested in Hilbourne or perhaps a San Marcos, but when I measured my PBH (they refused to consider inseam, even though one can derive almost the same information from that measure), it put me at the LOWER range of a 52, when I normally ride a 54. I looked at the sizing of at least two of their other bikes and found the same thing happened.

    When I called and inquired about this, they said that those comments and instructions on riding larger sizes than normal don't apply to frames with sloping top tubes because those are "expanded." Well, their website doesn't say this, anywhere. In fact, it says that one will be "invariably" riding a larger frame size than what one is used to if they buy a Rivendell frame. Their words not mine.

    Apparently, the stuff they wrote about frame sizing is the exact opposite of what the real world facts are.

    This is compounded by the fact that their seat tube measurements don't measure the whole thing. Something like center of bb to some other point, which leaves a good inch or so off the total. That is what they told me.

    So, why then, do they go to such great lengths to say that on a Rivendell, the frame size number will be larger than what you are used to, when they measure the frames in such a way that their numbers are designed to understate that number? ARe they just trying to be confusing and obscure things? I probably have about 3 hours of research into what should be a simple matter of "does it fit in this size or not." I fortunately am interested in this frame, but am put off enough by their obfuscation that I might not buy it just because it ticks me off a little.

    I normally ride 54s. I am 5'10" with an inseam of 30". I have a relatively long torso. I can get a 52 Hilborne and it might actually be a good fit for me, due to the peculiarities of my body geometry.

    But if I read their guide to sizing and did what they suggested, ordered a 56, I wouldn't be able to reach the ground.

    Adding to the confusion is that in the blogosphere there are people who claim to have purchased a larger size Hilborne frame than the size they normally ride on a road bike. What are these people doing, riding in elevator shoes?

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    1. That is called center to center measuring. They measure from the bottom bracket to the intersection of the center of the seat tube, to the center of the top tube. Yes, this is very misleading. Also worthless on frames that have sloping top tubes. Just have to know that a 54 will ride more like a 56 or 57 when it comes to "exposed seatpost".

      Also, KOPS is a good starting point and work from there. You can't just slap a seat with any old seatpost on any bike. Then again it depends how much you ride. 2 hours a day on an ill fitted bike won't be fun. Compared to a 30 minute weekend ride a few times a month where it doesn't matter how the bike fits.

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  42. Hi,

    Wondering if your seatpost tweaking managed to make your ride more comfortable. Thanks!

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  43. Velouria, did you get it all to work? Just stumbled across this thread and am curious how it worked out.
    I am finding, after numerous builds, that I need to fit the seat height and setback first, then dial in the reach and drop to suit the type of riding I'm planning for a particular bike.
    Here's what I've learned. If it fits you, you will feel it right away. It's like trying on a pair of shoes that are comfortable right out of the box. If things feel awkward at first, your body may indeed adjust but, if you're like me, something will always seem off.
    Anyway, best of luck. I really enjoy reading your stuff.

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  44. Weird that you describe the setback as increasing the seat tube angle. It's a static addition of length.

    The seat tube angle determines the amount of increase of length over a certain distance. Moving the triangle back 5cm doesn't change the angles.

    So while increasing/decreasing setback will affect your reach, it won't have any affect at all on your seat tube angle.

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