From: bleep@ux1.cso.uiuc.edu Date: Mon, 15 Mar 93 14:29:24 CST Subject: crankset height (call for discussion) I'm working on a recumbent tricycle design. I'd like to hear the current philosophy re crankset position. Say, if my seat angle is about 45 degrees, should the "bottom bracket" be at the seat level, above it, or below it? and why (scientific-like explanation with lots nifty statistics)? Is there literature out on this topic--if so, would someone send me a list. Thanx. Tho X. Bui. Mat. Sci. Dept., U of Illinois. From: Dave Garnett Date: Tue, 16 Mar 93 17:47:04 +0000 Subject: Re: crankset height (call for discussion) I think that you will find in practise that your options are limited by other things. 1) The crankset must be positioned so that the pedallers feet clear the road surface under ALL conditions. Personal experience of touching down at 30mph plus here ! 2) You want the c of g (ie pedaller) as low as possible, or corners become quite exciting. My experience is that you require the crankset as low as possible consistent with the above because :- a) If the user is sitting fairly upright, then you have to pedal with your legs at an acute angle to the body, which I certainly find very awkward. (Something to do with hip flexing, perhaps ?) b) If the user lies right back, then you have problems with things like locating the driver so that you can get a good push, not to mention things like seeing where you are going. You also find that wheel clearance may be a problem. Doubtless there are negative implications with this philosophy for things like drag, but what the hell, if you can't pedal comfortably, you won't go very fast anyway ! My (probably faulty) recollection of the medical story is that there are negative implications for having the crankset above the seat line, something to do with blood pressure/flow. Dave From: drela@henry.mit.edu Date: Tue, 16 Mar 93 12:58:22 EST Subject: Re: crankset height (call for discussion) On the Daedalus airplane we found that the seat angle was fairly critical to long-term comfort. We built a mock-up with an ergometer to play with the geometry. The angles we ended up with were: crank-seat line 10 deg above horizontal (crank above seat) seat-back line 50 deg above horizontal The seat angle was therefore 120 deg. This is a "very recumbent" (almost supine) position, which was done for streamlining reasons and to put the pilot's CG very close to the ground. It may not be practical for a utility HPV, and rotating the entire configuration more upright is probably better. Power doesn't seem to be affected. We used a setup similar to Daedalus on the MIT hydrofoil boat, except that it was rotated forward 5 degrees, so the crank-seat line was 5 degrees above horizontal. This worked well even during sprint efforts. Mark Drela MIT Aero-Astro From: "Nickolas E. Hein" Date: Tue, 16 Mar 93 10:25:54 PST Subject: Re: crankset height (call for discussion) Dr. (Tom?) Too gave a presentation on this subject at the Human Power Conference at Yreka last Summer. He showed test results from two series of tests conducted on various students. The seat back angle was laid back much less than 45 degrees. The first series showed the effect of raising the seat above the cranks showing variation of maximum power output with seat height for several different riders. The plotted results showed that the most power was output when the line from the bottom bracket to the seat base was 15 deg. above horizontal. The second series showed the effect of tilting the whole setup (with a constant seat angle) on maximum power. I didn't really memorize those results because I interpreted them as being the effect of hills. As such, power did decrease at a noticeable rate as the rider was rotated "uphill" and increased when the rider was tilted "downhill". The actual plots should be published in the proceedings, whenever they come out. Dr. Too's specialty is physiology, so he concluded that pumping blood "uphill" to the legs reduced the maximum power. My specialty is closer to physics and I would add to his explanation that the amount of force you can put into the seat back is an additional factor in the variation. I suspect there is an optimum position where you can redirect your weight into the pedals and the seatback to get the most force. Rotating the bike "uphill" reduces the max. force on the pedals in what may be a very predictable relation. Keep in mind that these tests were for maximum power, if your application is longer-term, endurance riding they may not be directly applicable. Hope this helps! Nick From: bleep@ux1.cso.uiuc.edu Date: Tue, 16 Mar 93 13:56:42 CST Subject: Re: crankset height (call for discussion) In Message Tue, 16 Mar 93 11:26:38 -0800, drela@henry.mit.edu writes: >On the Daedalus airplane we found that the seat angle was >fairly critical to long-term comfort... >The seat angle was therefore 120 deg. This actually seems much less than most LWB bikes (Tour easy, etc.) which were designed for comfort. Can you elaborate on the "long-term," and/or will some LWB owners please speak up on their experience? >We used a setup similar to Daedalus on the MIT hydrofoil >boat, except that it was rotated forward 5 degrees, >so the crank-seat line was 5 degrees above horizontal. >This worked well even during sprint efforts. Interesting. I don't know much about recumbent sprinting. Do you brace against the seat's back (lumbar region?) to get more effort? Tho X. Bui. Mat. Sci. Dept. U of Illinois. From: R.Stclair@EBay.Sun.COM (R D St.Clair) Date: Tue, 16 Mar 93 13:31:18 PST Subject: Re: crankset height (call for discussion) The rule of thumb I have used, and broken in both directions, is to keep the bottom bracket at the same level as the hip joint for near maximum power. Dropping the bottom bracket below this point pays a slight penalty in maximum power, but is often percieved as more comfortable, expecialy by inexperienced riders. (I think it mostly has to do with the impression that your feet are closer to the ground where they can steady you if you stop. This would not realy apply to a trike. My experience is with bikes.) Raising the bottom bracket much above the hip joint definitely results in a feeling of premature deadness in your legs. With conditioning this feeling is reduced, but who needs it if it can be avoided. My understanding of the phenomenon is that it has to do with the fact that the vesels in your legs (whole body actualy) have one way valves. The rythmic contraction of the leg muscles coupled with these valves provides significant pumping action and aids the heart. When the legs are low, as nature intended, gravity helps fill the legs, and muscle pumping works well. If the legs are high, the pressure creeated by the heart must push the blood up into the legs before muscle pumping can move it along from there. By the way, I agree with all of the other valid points raised in responce to your earlier request. My recollection of the information presented at the IHPVA conference was that the optimum hip angle was very similar to that experienced on a conventional diamond frame when riding on the drops. If you can keep this same relative body position, but just rotate the whole thing back untill you are recumbent you will be close to optimum. This results in a somewhat more folded up body position than you might at first chose for a recumbent. Nevertheless, this has shown to be optimum for maximum power output, even in a recumbent. However, maximum power output should probably not be your primary consideration. A more laid back seat back may be less than optimum for power but feel more comfortable to the less competitive rider, and also result in less wind resistance. The result in an un-faired vehicle, could actualy be faster. I would caution against going more than about 45 degrees seat back angle unless you provide upper sholder and head support. Many people will find that their neck gets tired constantly pulling their head forward. Naturaly conditioning can overcome this to some extent, but who needs the pain. There is some variation with individual build. If you have a more curved upper back and a natural body posture with your head more forward on your sholders (like me), even 55 degrees is no problem. Hope this helps. R.D. From: "Nickolas E. Hein" Date: Tue, 16 Mar 93 14:42:24 PST Subject: Re: crankset height (call for discussion) > Tho X. Bui. Mat. Sci. Dept. U of Illinois. writes: > >The seat angle was therefore 120 deg. > > This actually seems much less than most LWB bikes (Tour easy, etc.) which > were designed for comfort. Can you elaborate on the "long-term," and/or > will some LWB owners please speak up on their experience? > > Interesting. I don't know much about recumbent sprinting. Do you brace > against the seat's back (lumbar region?) to get more effort? My experience is that if you have a high seatback you use it for power-climbing on hills, sprinting and just changing positions on a long ride. Most 'bents I've seen don't have one, the ones I've used with a high seatback were homebuilts. Hope this helps. Nick From: R.Stclair@EBay.Sun.COM (R D St.Clair) Date: Tue, 16 Mar 93 16:03:38 PST Subject: Re: crankset height (call for discussion) It seems to me, if we want to provide useful information here (even in the form of personal opinion or personal experiences), we had better define some of our terms (just so we all have a common understanding of what we are talking about). How about the following as a starting point? Seat back angle: The seat back angle measured relative to horizontal. Typicaly in the range of 90 to 150 degrees. (my previous postings of 45 to 55 degrees were relative to vertical, so under this new definition it would properly be called 135 to 145 degrees). (You can take the approximate angle of the spine for bikes without a seat back. Ex. ~10 to ~90 degrees for a diamond frame) Seat pan angle: The seat pan (base) angle relative to horizontal. Typically in the range of +10 to -10 degrees with positive angles denoting a seat pan that slopes down towards the back of the seat. Negative angles denote sloping down towards the front of the seat. (Less meaningful, or at least has a different meaning for, a seat you stradle rather than sit on.) Seat back to crank angle: The angle between the seat back and a line drawn through the hip joint and the bottom bracket. Typically ranges from 90 to 150 degrees. If the bike has no seat back you could use the approximate angle of the spine instead. Hip joint to bottom bracket angle: The angle between a line drawn through the hip joint and bottom bracket relative to horizontal. Ranges from ~-90 degrees for a diamond frame, to ~-30 degrees for a Tour Easy, to ~0 degrees for a lightning to ~+10 degrees for some very low tricycles or recumbent boats. Anyone wish to clarify their previous statements relative to this system of measure, (or any other that you care to clearly define)? Thanks, R.D. From "Nickolas E. Hein" Date: Tue, 29 Sep 92 9:47:01 PDT Subject: Seat vs. Crank height At this years' HPV conference a presentation was given showing the results of maximum-power pedaling tests with variation in the angle from the seat base to the bottom bracket center. Output power was plotted against this angle (measured from vertical). The plot showed significant dropoff in power for off-optimum angle, the maximum power was achieved when this line was 105 deg. In other words, the crank center was on a lined 15 deg. below the seat base. From my own experience this is a very comfortable pos- ition, I can get it on the bikes I ride by pushing myself higher up the rigid seatback. It seems to provide the best combination of using the seatback to push against and using a component of gravity to provide some push against the pedals. I'd really like to see that earlier article you spoke of because when I tried the Lightning it didn't seem like a very comfortable or practical position, and I've heard of people getting numb feet after riding with their feet up like that after a while. The paper I spoke of earlier also studied the effect of rotating the entire test rig at various angles to simulate the effect of being at uphill and downhill angles. With increasing uphill angles the maximum power also dropped off pretty severely. This led to the suggestion that a "seat jack" might be installed to get back to original seat/crank angle. -- I take responsibility for everything I say, but I could be completely wrong. ============================================================================= Nickolas Hein | Voice: (206) 237-8935 Boeing Commercial Airplane Group | Boeing net: neh3568@aw2.fsl.ca.boeing.com P.O. Box 3707, M/S 96-04 | Seattle, WA 98124-2207 | ============================================================================= From Date: Tue, 29 Sep 92 12:08:43 -0700 Subject: Seat/cranks angle v. power output. I was talking with a person who'd done some research physiology. He says that power output is a function of both relative blood pressure and absolute blood pressure. Elevation of the body's center above the legs does not affect relative blood pressure but does affect absolute. He said that decreasing the absolute decreases power output. Some training can adapt for low absolute blood pressure, but he didn't know to what extent. Power output is obviously a function of many things, but according to the above, upright positions have an intrinsic power advantage, keeping the relative positions the same but chaning angle (hill climbing) can reduce power output, and training can compensate but only partly. The person I was talking to says that absolute pressure vs. power is in a number of physiology texts, but I haven't done any digging. ;-D on ( Train ore ) Pardo From mckay!dwight@ecn.purdue.edu Date: Tue, 29 Sep 1992 22:23:17 -0500 Subject: Re: ATP R-20 Experiences (really HPV seats) > ASIDE: How do the rest of you feel about seatback angle? > What is the angle of your seat? Do you feel that > angle is optimum? Have you tried other angles? > What was the result? Is adjustability important? > How does angle affect performance? Etc? The seat on my Trice is at about a 40 degree angle (I'm guessing) and is made of molded fiberglas so no adjustment is possible. The seat does have a headrest and is covered with a removable cloth covered foam cushion. The seat looks sort of like this: | | / / / / ----- The slope is less steep then the slash character, but you get the idea. I've not found the headrest too useful as I can't put my head comfortably against it with my helmet on. (Maybe I need to get a more modern helmet then my trusty old Tourlite!) I like the angle of the seat. Originally I felt it left me far to low to the ground as the seat angle combined with a seat hieght of about 8 inches off the ground leaves your head only about 36 inches off the ground. But, after a few rides I got used to it. It's a lot like riding in an MG or Miata (sp?) or other small sports car. I've not done any formal testing of the performance aspects of the Trice, but it subjectively feels faster then my Infinity. The speeds I've measured are about a mile or two faster on the Trice over the same route. The Infinity like the Ryan has a more upright seat so the difference might be due to aerodynamics, but I really can't tell. --Dwight D. McKay From Stuart E. Strand Date: Wed, 30 Sep 1992 08:43:26 PDT Subject: Seat vs. Crank height I have some of the old references to this topic. The data was collected in the 60's and early 70's as far I as I know (exercise physiology is not my field), so subjects trained to exert in the prone position may not have been available. //////////////////////////////////////////////////////////////////////// / Stuart Strand, Univ.Washington sstrand@u.washington.edu / / AR-10, Seattle WA 98195 206-543-5350 uunet: beaver!milton!sstrand / //////////////////////////////////////////////////////////////////////// ------- From "Nickolas E. Hein" Date: Wed, 30 Sep 92 9:55:44 PDT Subject: Seat vs. Crank height Actually the seatback angle in the presentation I mentioned was kept constant at 90 deg. It was the line from the seat base to the bottom bracket center that was 105 deg from vertical. (15 deg below horizontal). I suspect the reason for buyers to prefer having the bottom bracket lower than the seat is that they have good visibility forward. I find this to be very helpful when riding. Also, as I mentioned before, when your feet are above your hips you tend to get numbness when going long distances or climbing hills. In the vehicle I ride I found that this effect severely restricts the amount of power I am able to put out. -- I take responsibility for everything I say, but I could be completely wrong. ============================================================================= Nickolas Hein | Voice: (206) 237-8935 Boeing Commercial Airplane Group | Boeing net: neh3568@aw2.fsl.ca.boeing.com P.O. Box 3707, M/S 96-04 | Seattle, WA 98124-2207 | =============================================================================