stem length for rando bikes
Michael Arciero
Igor Belopolsky
Greg Walton
Optimal stem length as a part of fit seems to be discussed among road cyclists, but not so much among the rando/handlebar bag crowd. For rando bikes a longer stem with shorter top tube will put the bag further out from the steering axis, increasing wheel flop (well not technically but will feel more floppy, and handling is affected). A longer tt with shorter stem mitigates this, but the longer tt will change the handling also, and longer thinwall tubes may be more prone to shimmy. All the BQ rando test bikes are pretty much 90-100. The old French bikes seem to be 80-90 or even less. I currently have an 80 with 58 tt, though I dont have a typical bar bag setup, nor fork trail. What are peoples experiences? Is there a blog post, BQ article, or thread I've missed where this was discussed?
--
You received this message because you are subscribed to the Google Groups "650b" group.
To unsubscribe from this group and stop receiving emails from it, send an email to 650b+uns...@googlegroups.com.
To post to this group, send email to 65...@googlegroups.com.
Visit this group at https://groups.google.com/group/650b.
For more options, visit https://groups.google.com/d/optout.
mitch....@gmail.com
On Tuesday, December 20, 2016 at 12:10:56 PM UTC-7, zybariver wrote:
Stem length is a relative issue, with the total stack height (head tube length + quill or steerer) the other major contributing factor. Because the head tube slopes, as total stack increases, the stem needs lengthen for the bars to be in the same vertical plane.I haven't done the trigonometry, but here is a link to a simplistic sketch that illustrates the idea, three theoretical stem lengths to accomplish the same vertical position.
Michael Arciero
Greg Walton
--
You received this message because you are subscribed to the Google Groups "650b" group.
To unsubscribe from this group and stop receiving emails from it, send an email to 650b+unsubscribe@googlegroups.com.
Steve Palincsar
Mitch, it works either way. I have modified the sketch here to show two different frame sizes, a & c, with the same head tube angle and down tube position. https://flic.kr/p/PaXS7g
mitch....@gmail.com
On Tuesday, December 20, 2016 at 2:30:05 PM UTC-7, zybariver wrote:
Mitch, it works either way. I have modified the sketch here to show two different frame sizes, a & c, with the same head tube angle and down tube position. https://flic.kr/p/PaXS7g
Igor Belopolsky
erick
On Tuesday, December 20, 2016 at 2:51:03 PM UTC-3, Michael Arciero wrote:
........ All the BQ rando test bikes are pretty much 90-100. The old French bikes seem to be 80-90 or even less.......
Max
On Tuesday, December 20, 2016 at 12:51:03 PM UTC-5, Michael Arciero wrote:
Igor Belopolsky
Eric Keller
riding long distances over fairly short periods of time, 200k, 300k,
400k, etc randonnees?
On Tue, Dec 20, 2016 at 2:10 PM, Greg Walton <gwa...@gmail.com> wrote:
> Stem length is a relative issue, with the total stack height (head tube
> length + quill or steerer) the other major contributing factor. Because the
> head tube slopes, as total stack increases, the stem needs lengthen for the
> bars to be in the same vertical plane.
position. I know from experience that I'm not careful about
transferring my position from one bike to another. This is really
frame/stem independent, the stem and seat setback need to change to
make up for any differences in frame geometry. I think most
randonneurs use a little more upright position on their rando bikes.
I think it's a mistake to go too far away from that. To me, much more
than a 1/2" raise in stem is pretty noticeable in how I feel about the
bike. I also feel cramped if I reduce my stem length much. I wouldn't
mess with seat position relative to the pedals at all.
I certainly wouldn't change stem lengths to acommodate a bag, that's a
design parameter in the frame.
Eric Keller
Boalsburg, Pennsylvania.
Michael Arciero
I guess maybe I was not clear with my original post. I am not talking about bike fit or position on the bike. I am assuming the rider contact points have been established, and are fixed. Also assume as fixed the type of brakes and handlebar one is using (so the bar reach is constant). Also assume that head tube and seat tube lengths and angles, fork rake are fixed; chain stays and seat stays lengths and angles also fixed. Now assume top tube and stem are horizontal (i.e., non-sloping) but lengths can vary. One can design a frame with those fixed specs, to fit those given, fixed rider contact points, by varying stem and tt lengths, by making the stem longer and tt shorter, or vice-versa. One can leave everything else on the bike constant, aside from what is directly affected by tt length, such as wheel base and down tube length and angle. Varying top tube and stem will have an effect on weight distribution and handling. A shorter top tube (with a longer stem-again to keep the contact points fixed) places the rider more forward in relation to the front wheel, so more weight on the front wheel. (The steering action is also different since the hands move through an arc of greater radius when turning since they are farther from the steering axis-this may be negligible). There has been a lot of discussion about this sort of thing with respect to racing and road frames. Most builders of road/racing frames will tell you what he/she/they feels is optimal weight distribution, and will design a frame around that. You don’t see criterium bikes with 80mm stems, mostly due to the fact that that does not result in the weight distribution that gives the handling characteristics optimal for criteriums.
On a rando bike, the situation is complicated because of the effect of front load on handling. Typically the bag sits about even with or just in front of the bars. If you have a bar bag that is even with the bars in height, the bag must be spaced forward enough to give your hands room on the tops. A long stem puts the bars tops far from the steering axis, and will necessitate positioning the bag even further forward and away from the steering axis than would a shorter stem. A load far from the steering axis has a large "moment" about the steering axis. This is what creates the heavy steering feeling when your bag is loaded down heavily. Much of the point of low trail is to mitigate this effect. A shorter stem also mitigates this effect by allow the bag to be positioned closer to the steering axis. Having the bag closer to the stem will result in handling being less sensitive to light vs a heavy load in the bag. But short stem necessitates longer top tube (remember-all contact points must remain constant) so affects weight distribution, steering, and handling in other ways, and may be more prone to shimmy
Again, the location of front load relative to the steering axis seems to have been largely absent from the discussion about fork rake, trail, the optimal geo for front loads, etc. (In one recent blog post Jan Heine did remark that a very large prototype bag had resulted in unfavorable handling, and somewhere else he remarked that slightly higher trail was better for porteur bikes.) As far as I know, Jan has mostly noted only that lower load is better. But in fact, the height of the bag along the incline of the head tube affects only center of gravity, and not the moment about the steering axis. I would bet that the former plays a much lesser role than does the latter.
satanas
And I can see keeping the combined top tube plus stem length constant. However, I'm not convinced that one would want to keep the head angle and fork offset constant if doing this. I'd be more inclined to keep the *trail* constant, to keep the front centre so the wheel was where it needed to be, and to vary the head angle and offset so that was what happened, otherwise the front centre and weight distribution will change, which may or may not be a good thing. It's always best to minimise the number of things changed if trying to test something out.
My *%#$! Soma GR has a 120mm (nominal) 3T stem, which equates to a 110mm Nitto. With a Berthoud bag on a Nitto front rack, the bag is closer to the bars than ideal, but the bag won't really move fore/aft relative to the head tube unless one uses a different rack, or unless it's attached directly to the bars a la Ortlieb. This could be changed with a custom rack with less "reach," combined with a longer top tube and shorter stem, and in fact that's what I plan to do eventually, partly to increase hand space on the tops, and also hopefully to reduce the effect the bag's mass has on steering. It would be nice to have room for top mount secondary brake levers too.
Since I've already reduced the fork offset and thus increased tco somewhat, chopping 20mm off the stem and adding it to the top tube and front centre would put the front wheel more or less back where it started. IIRC Fred Blasdel on the 650b list believes that fore/aft bag position has little or no impact on handling, but even if it falls into "marginal gains" I don't see how it can hurt, and TUEs don't really apply in randonnees!
Later,
Stephen
Alex Wetmore
When I switched from mid and high trail bikes to low trail bikes I found that I preferred a longer top tube with a shorter stem. My average bike switched from having a 56-57cm top tube with a 10-11cm stem to having a 58-59cm top tube and 8-9cm stem. The shorter stem allows me to bring the handlebar bag rack closer to the head tube while still allowing me to open the bag and get access to the top. The bike's steering is light enough that I don't need a longer stem to get leverage (especially with a handlebar bag and front panniers... I used to tour on high trail bikes).
My homemade racks tend to have the tombstone (back of the rack) about 2-3cm closer to the head tube than production racks. Here is a fairly extreme example, the pockets on the bag on this rack come back to the head tube:
https://www.flickr.com/photos/88336152@N00/16752501761/in/datetaken-public/
https://www.flickr.com/photos/88336152@N00/16131583734/in/datetaken-public/
I can't run a tall handlebar bag on that bike because it will hit the stem. However the flap doesn't really hit the stem when opening, so it works well with that smaller bag.
alex
Sent: Friday, December 23, 2016 4:21:38 AM
To: 650b
Subject: Re: [650B] stem length for rando bikes
I guess maybe I was not clear with my original post. I am not talking about bike fit or position on the bike. I am assuming the rider contact points have been established, and are fixed. Also assume as fixed the type of brakes and handlebar one is using (so the bar reach is constant). Also assume that head tube and seat tube lengths and angles, fork rake are fixed; chain stays and seat stays lengths and angles also fixed. Now assume top tube and stem are horizontal (i.e., non-sloping) but lengths can vary. One can design a frame with those fixed specs, to fit those given, fixed rider contact points, by varying stem and tt lengths, by making the stem longer and tt shorter, or vice-versa. One can leave everything else on the bike constant, aside from what is directly affected by tt length, such as wheel base and down tube length and angle. Varying top tube and stem will have an effect on weight distribution and handling. A shorter top tube (with a longer stem-again to keep the contact points fixed) places the rider more forward in relation to the front wheel, so more weight on the front wheel. (The steering action is also different since the hands move through an arc of greater radius when turning since they are farther from the steering axis-this may be negligible). There has been a lot of discussion about this sort of thing with respect to racing and road frames. Most builders of road/racing frames will tell you what he/she/they feels is optimal weight distribution, and will design a frame around that. You don’t see criterium bikes with 80mm stems, mostly due to the fact that that does not result in the weight distribution that gives the handling characteristics optimal for criteriums.
On a rando bike, the situation is complicated because of the effect of front load on handling. Typically the bag sits about even with or just in front of the bars. If you have a bar bag that is even with the bars in height, the bag must be spaced forward enough to give your hands room on the tops. A long stem puts the bars tops far from the steering axis, and will necessitate positioning the bag even further forward and away from the steering axis than would a shorter stem. A load far from the steering axis has a large "moment" about the steering axis. This is what creates the heavy steering feeling when your bag is loaded down heavily. Much of the point of low trail is to mitigate this effect. A shorter stem also mitigates this effect by allow the bag to be positioned closer to the steering axis. Having the bag closer to the stem will result in handling being less sensitive to light vs a heavy load in the bag. But short stem necessitates longer top tube (remember-all contact points must remain constant) so affects weight distribution, steering, and handling in other ways, and may be more prone to shimmy
Again, the location of front load relative to the steering axis seems to have been largely absent from the discussion about fork rake, trail, the optimal geo for front loads, etc. (In one recent blog post Jan Heine did remark that a very large prototype bag had resulted in unfavorable handling, and somewhere else he remarked that slightly higher trail was better for porteur bikes.) As far as I know, Jan has mostly noted only that lower load is better. But in fact, the height of the bag along the incline of the head tube affects only center of gravity, and not the moment about the steering axis. I would bet that the former plays a much lesser role than does the latter.
Michael Arciero
Stephen Poole
@Alex: Thanks for that - your words and pics give me hope!
@Michael: I tried attached-to-the-bars bags on road bikes with relatively long stems many years ago and really hated the leverage they exerted. If you're going to use that sort of bag (ie Ortlieb) then I'm inclined to think it's good to keep the stem short, and to use a small and light bag; the Ortlieb "compact" bar bags are small enough they can't easily be made heavy, and will hold a small camera, brevet card, wallet, etc.
Also, one of the things on display at Semaine Federale last year was a Nitto-like front rack with an Ortlieb-style mount integrated into the tombstone, so a standard Ortlieb bag fitted lieer down and further back, and was supported from below as well. Such a rack could also have integrated light and fender mounts, etc.
There are lots of interesting ideas coming out of the bikepacking scene too, many of which are worth borrowing or adapting.
Later,
Stephen
Alex Wetmore
Stephen: Michael is talking about my way of using Ortlieb pannier hooks to secure the bag from the bottom. Look at the photos that I posted and nearby ones in the set. I give credit to Rory Cameron for coming up with this great idea.
I agree with you on Ortlieb handlebar bags, but that isn't what I'm showing here.
alex
Sent: Friday, December 23, 2016 12:30:11 PM
Cc: 65...@googlegroups.com
Subject: Re: [650B] stem length for rando bikes
@Alex: Thanks for that - your words and pics give me hope!
Stephen Poole
Thanks Alex - I'd seen the Ortlieb hook solution before but couldn't remember where. It might be a good option for me, and the Ortlieb parts are easy to get; I'll need to make a rack in any case.
By the way, do you think keeping the bag closer to the head tube improves handling???
Thanks again,
Stephen
Michael Arciero
Alex-in addition to the ortlieb pannier bracket attchment on the bottom of the rack, i did see a photo of a steer tube mounted decaleur that looked like it was designed to accept an ortlieb-type pannier bracket attachment, or perhaps something similar.
They are apparently working on something like that at Ocean air as well. You can see prototype in the pictures for the Doceana. It appears to be pretty clunky/cumbersome, but is just a prototype using Nitto lamp holder parts.
Mike
Hahn Rossman
--
You received this message because you are subscribed to the Google Groups "650b" group.
To unsubscribe from this group and stop receiving emails from it, send an email to 650b+unsubscribe@googlegroups.com.
