Trim for speed

[Duncanleon]

I've been riding for season and a half now and once I was thankfully able to get passed the "learnnng and break-in" period, I was able to actually start paying attention to speed and power output. I have follwed the formula for establishng a proper fit and have been experimenting with the position of the MBB relative to the ground. I'm curious about the aerodynamics of how the apparent wind at roughly 20 mph reacts to the body leaning back using a 20 degree seat incline.

I recently read a reposting of some of the the results from the V20 wind tunnel testing that occured a couple of years ago and I've followed some of the offhand comments and pictures from other V20 riders. Im seeing the results are very mixed and most have to do with the comfort of the fit for riding.

What are other's opinions regarding where maximum aerodynamic efficiency might be if the adjustment is made to the height of the BB and pedals relative to the height of the handlebars, the seat pan and the riding position of the body. Is it more efficient to keep the BB slightly lower allowing the body to be laying relatively straight at an angled plane thereby driving the wind along the full length of the body? Is it better to raise the pedalling feet higher to create a smaller "hole"? Would that apparent hole really be any smaller or streamlined considering the impact of the rider's face to elbows to flailing feet? If it does make a difference then at what point would the BB be considered too high which would start diminishing efficiency? Where is the happy medium?

 

[Mathew Fy]

I've been riding for season and a half now and once I was thankfully able to get passed the "learnnng and break-in" period, I was able to actually start paying attention to speed and power output. I have follwed the formula for establishng a proper fit and have been experimenting with the position of the MBB relative to the ground. I'm curious about the aerodynamics of how the apparent wind at roughly 20 mph reacts to the body leaning back using a 20 degree seat incline.

I recently read a reposting of some of the the results from the V20 wind tunnel testing that occured a couple of years ago and I've followed some of the offhand comments and pictures from other V20 riders. Im seeing the results are very mixed and most have to do with the comfort of the fit for riding.

What are other's opinions regarding where maximum aerodynamic efficiency might be if the adjustment is made to the height of the BB and pedals relative to the height of the handlebars, the seat pan and the riding position of the body. Is it more efficient to keep the BB slightly lower allowing the body to be laying relatively straight at an angled plane thereby driving the wind along the full length of the body? Is it better to raise the pedalling feet higher to create a smaller "hole"? Would that apparent hole really be any smaller or streamlined considering the impact of the rider's face to elbows to flailing feet? If it does make a difference then at what point would the BB be considered too high which would start diminishing efficiency? Where is the happy medium?

I don't race, but I do love chasing KOMs, so I'd also like to know which is faster. I think personal comfort, power output, and general preference are more important than the aerodynamics resulting from chainstay lengths. As they found out from their 2017 wind tunnel tests a higher BB can bring the "knees into the clean airstream above (the) shoulders" while a lower BB can put the "heels into the clean airstream below the seat." I suspect if you choose an elevated head position with a Thor Seat, Adjustable Carbon Fiber Seat, or Foam Wedge Backrest Insert you may benefit more from a longer chainstay. Only by testing in a wind tunnel or using the Chung method can you really know. I've been looking at mywindsock and it seems that on windy days my estimated CdA is higher, so I don't know if that's helpful or not.

 

[chicorider]

Anecdotal experience from a guy who rides purely by feel and doesn't even know what CdA stands for: I rode my V20 with a short 16-inch chain stay and curved slider for six years. Now I'm riding a V20c with the stock medium chain stay and straight slider (they don't offer a short stay in carbon). This put my feet about 2.5-inches higher, and, with everything else being about the same, Strava says I'm riding about 1 mph faster on average. An incremental improvement for an incremental change in position.

 

[Opik]

What are other's opinions regarding where maximum aerodynamic efficiency might be if the adjustment is made to the height of the BB and pedals relative to the height of the handlebars, the seat pan and the riding position of the body.

The position by Warren Beauchamp

The Recumbent Bicycle and Human Powered Vehicle Information Center

Recumbent and Human Powered Vehicle Information Center

recumbents.com

The specific recumbent make and model doesn't matter that much, but if you want to go fast it needs to be able to put you into "the position". This recumbent position provides low aerodynamic drag and enough ergonomics that you can see over your knees. Basically the position is a seat angle between 15 and 25 degrees from horizontal, and a bottom bracket that is several inches above the seat base.

 

[Duncanleon]

I don't race, but I do love chasing KOMs, so I'd also like to know which is faster. I think personal comfort, power output, and general preference are more important than the aerodynamics resulting from chainstay lengths. As they found out from their 2017 wind tunnel tests a higher BB can bring the "knees into the clean airstream above (the) shoulders" while a lower BB can put the "heels into the clean airstream below the seat." I suspect if you choose an elevated head position with a Thor Seat, Adjustable Carbon Fiber Seat, or Foam Wedge Backrest Insert you may benefit more from a longer chainstay. Only by testing in a wind tunnel or using the Chung method can you really know. I've been looking at mywindsock and it seems that on windy days my estimated CdA is higher, so I don't know if that's helpful or not.

"knees into the clean airstream above (the) shoulders" while a lower BB can put the "heels into the clean airstream below the seat."

Thanks guys,
Identification of that slipstream is the the key here as both your quotes from the wind tunnel article and the "configurative" comments from Craig (Chicorider) are indicating. In my case, without changing the current seat configuration (Ventisit with an adjustable headrest), I'm tall enough so that i can't commfortably lift the BB high enough for knees to really paricipate in this discussion, but the location of the heels relative to the bottom of the seat becomes a focal point. Ive also recently discovered that if the BB is up too high, the slipstream "hole" might be smaller but required power output and ability to sustain a climb of even a mild 2% incline requires a significantly greater effort to maintain speed and power without having to sit up further than before in the seat. Looks like there is a very small window to dial into to find the right mix.

I appreciate your input

 

[M.J]

I think you're over thinking the situation. You don't automatically lose power if you raise the bottom bracket, especially not in a "every inch of rise results in a drop of X percent" kind of way. That would assume that the stock position is optimized for everyone right out of the box.
My bike is set up for speed above all else. I have a non-stock seat fitted, but not a Thor. It provides proper support without putting my shoulders in the wind. My bars are 44cm wide bullhorns, fitted "upside down" and with the stock Rival brifters. This allows for low-ish, straight arms, which is probably the best compromise when you're dealing with MBB and can't safely run some super narrow bars.
I have the long (23", pretty sure) chainstay fitted. This puts my seat to BB rise at 10.5", which is pretty radical but works great in practice. I find the position to be powerfuI, efficient, and very aero. It also sets the boom parallel to the ground, not presenting itself to the wind.
The "knees in the airstream" thing has been taken as gospel since the wind tunnel test, but I think there are mitigating factors. Those results were for Maria Parker, who is a petite woman. Small frame, narrow shoulders, presumably small feet. For someone my size (a broad shouldered 5'11" with size 10 feet) a different set of factors would need to be taken into consideration. I'm pretty sure that my heels dragging through the otherwise clear air under the bike was way more of a problem than my knees being higher now.
I'll attach some pics of my current setup. I used this bike earlier this week to do a 10 mile loop course TT at 28.2mph on 271 watts.

 

[vosadrian]

I'm reading on with interest. One other consideration for BB height is crank length. Running shorter cranks means the knees will not go as high AND the heels won't go as low. In my experience, there are many benefits to short cranks, and I run 155mm on my V20, but would like to try shorter. With shorter cranks, the aero senstitivity of BB height is probably reduced quite a bit. I think I am quite aero. I only use MyWindSock for CdA calculation, and I get varying results depedning on the course I do, but typically ranging 0.13-0.18, so I am probably around the middle of that range.

@M.J, in the photos your heels look to be good, but knees going above shourlders. The front biew looks like knees as high as your eyes. So you are on the higher side. Seems to work well for you, but I wonder if shorter cranks would work even better?

 

[M.J]

It's the angle of the oncoming pic that makes it look like that. The side view is more accurate.
I'm ok with the cranks I have on. I wouldn't want to change my current fit by fitting really small cranks. I've done lots of experimenting with this and other bikes and am very fond of my current hip angle. Any more open and I have trouble maintaining power for race distances.

 

[vosadrian]

It's the angle of the oncoming pic that makes it look like that. The side view is more accurate.
I'm ok with the cranks I have on. I wouldn't want to change my current fit by fitting really small cranks. I've done lots of experimenting with this and other bikes and am very fond of my current hip angle. Any more open and I have trouble maintaining power for race distances.

Do short cranks change the hip angle if you adjust boom length accordingly? They certainly reduce the range of hip angle, but with other bike adjustments, I would expect you could get into your peak hip range of motion with lower range.

My case is slightly unique. I have hip issues. I have a condition where as my knees get close to my chest, my hip gets to a range where I get bone on bone impingement in the ball and socket. It has caused chronic hip issues and I have had one surgery to one side because of it. It is not that uncommon and several high profile sports people have suffered it (like Andy Murray). I first started having issues on my DF with normal 172.5 cranks trying to get low in the drops with my knees close to my chest. When I realised what was going on, I went to the shortest cranks I could find to open up the hip angle. In the process I found my pedal stroke get smoother (I had more control over the top of the stroke) and my power numbers went up (I could work more efficently in a smaller range of hip motion). This was quite immediate. The first ride I felt smoother pedal stroke immediately and went on to post some PRs on climbs within a week of the change.

The V20 has even more benefits than a DF. With the short cranks it has the same biomechanical benefits for me of reducing hip range of motion, but there are some other benefits also. Firstly it reduces the vertical height of your knees/heels so you reduce frontal area and improve aero. Secondly, it means your knees get less close to your handlebars which give you more space to play with getting the optimal hand position (previously with the longer cranks I could only move my bars as far forward as my knees hitting them, but now I have been able to move them further forward with shorter cranks. Previously I was brushing my thigh tops above the knees on my bars, but now I have good clearance there. Of course the same could be achieved with some unique shaped bars (more reach).

It does change your riding style a little, but you adapt very quickly (a few hours riding). You tend to run shorter gear at the same speed and spin higher cadence, so it effectively lengthens the gearing, as you can spin out your top speed gear to higher cadence..... but you will struggle a little more at low cadence in the shortest gear with a little less torque due to a shorter crank arm. Before going shorter cranks I was looking for a larger big ring than the 52 I run currently, but with the shorter cranks I can spin out the 52-11 gear to high enough speeds (60-65kph) that I am not looking for a larger big ring. On the other hand I might want something bigger than the 32 on the back now!

I am 155mm on my V20 (165 on my DF), but I would like to try something around 145 on the V20 which I think is close to what Larry runs.

 

[Frito Bandito]

Sorry you are having hip issues with the longer cranks Adrian. I sometimes feel that my 165mm cranks are too short. I have 172.5mm on my DF bike, and I had 175mm on my Specialized, both before I started with my V20, and maybe those lengths are what my legs feel comfortable in. I can't say I am willing to buy some longer cranks to test out, but quite a few times when I sat lower in my seat to increase the hip angle I just felt I could keep going forever. So, I might give 172.5mm or even 175mm cranks a try if I can.

 

[vosadrian]

That is surprising to hear @Frito Bandito. In my DF circles (I have many DF friends as I have raced DF for many years), I have convinced a number of high level riders to try short cranks and all have loved them and never gone back. The length of the crank does not set your average/base hip angle. It sets the range of motion of your hips and knees (from the base angle). The average hip and knee angle is set by other aspects of the bike setup (on a DF bike by saddle height and setback and bar position). So in the middle of the crank cycle in the neutral position your hip and knee angle will be the same, but they will move less far from that position at the extremes. It means your hips and knees will have less angles on the joints and this is considered better for joint health. You will be less likely to damage knees/hips with shorter cranks

If you google some papers on crank length, you will see that lots of testing has been done, and no negatives have been found of shorter cranks. Riders who have used long cranks for years with proper fit and biomechanics have not lost any power by going significantly shorter in the cranks, and many report less issues with injury etc. In theory many thought that you would lose power because a shorter leaver arm produces less torque with the same force.... but in reality the body automatically compensates with higher cadence and power = torque * RPM. The body does not feel cadence, but feels pedal speed. because of the smaller circle, the pedal speed is the same at higher RPM. The end result is most riders unadapted make the same power as they did with longer cranks... but more aero and healthier joints!

 

[Frito Bandito]

That is surprising to hear @Frito Bandito. In my DF circles (I have many DF friends as I have raced DF for many years), I have convinced a number of high level riders to try short cranks and all have loved them and never gone back. The length of the crank does not set your average/base hip angle. It sets the range of motion of your hips and knees (from the base angle). The average hip and knee angle is set by other aspects of the bike setup (on a DF bike by saddle height and setback and bar position). So in the middle of the crank cycle in the neutral position your hip and knee angle will be the same, but they will move less far from that position at the extremes. It means your hips and knees will have less angles on the joints and this is considered better for joint health. You will be less likely to damage knees/hips with shorter cranks

If you google some papers on crank length, you will see that lots of testing has been done, and no negatives have been found of shorter cranks. Riders who have used long cranks for years with proper fit and biomechanics have not lost any power by going significantly shorter in the cranks, and many report less issues with injury etc. In theory many thought that you would lose power because a shorter leaver arm produces less torque with the same force.... but in reality the body automatically compensates with higher cadence and power = torque * RPM. The body does not feel cadence, but feels pedal speed. because of the smaller circle, the pedal speed is the same at higher RPM. The end result is most riders unadapted make the same power as they did with longer cranks... but more aero and healthier joints!

I do watch my power closely while riding, and sometimes I notice that when in a taller gear with a lower cadence I can feel a bit sluggish when trying to increase power, but as soon as I drop to a lower gear there is a small jump in the power with that higher cadence. A sort of powerband like in an engine. So, that seems to support in essence what you wrote. I don't know if what I felt was due to a bit of play in the pedals in changing my foot position or maybe I was incorporating different upper leg muscles. Of course the cleat position was not any closer or further than at the beginning of the ride so it mustn't have been that. I can replicate it though so I'll pay a bit more attention to that next ride so see if I can tell what it was.

 

[vosadrian]

Power Meters do not measure power directly. They typically measure Force or torque depending on the PM type. In the case of a pedal based PM, they measure force. Then they convert to torque as you have to enter the crank length in the setup of the PM, and Torque is Force * Radius. Then once they have Torque, they still need Cadence to convert that to power as Power = Torque @ RPM. So you can make more power by staying at the same cadence and pushing more force on the pedals.... or you can keep the same force on the pedals but change to a shorter gear and go higher RPM.

In terms of body physiology, generating power by pushing harder on the pedals at low cadence will build up lactic acid and make your legs feel more sore, but your Heart rate will stay lower. High cadence, lower force tends to keep the legs feeling better (and it better for your joints also), but tends to get the HR running higher. On big efforts I tend to alternate between high and low cadence. On climbs on a DF I alternate between standing at low cadence and seated spinning at higher cadence (every 30-60 seconds). The muscle groups used for each option are different also, so it moves the load around.

 

[chicorider]

Seven years ago I started with 165mm cranks on my V20 because they were readily available. Then, with a little looking, I found 160mm cranks, liked them better, and rode those for a long time. Of course, I began to wonder if 155mm cranks would be even better, but didn't want to pay a lot of money to find out. When I found a set of used 155mm SRAM Apex cranks, for only $100.00, I figured here was my chance to experiment. I rode those for a while. I gave them a good run. But I ultimately went back to 160mm length. I liked the taller high-end gearing that came with the 155's; I picked up a couple new downhill KOMs with them. But I didn't like losing that little bit of torque/leverage on the low end for climbing--and I climb a fair amount. !60mm because my Goldilocks crank length.

None of that is to say that any particular crank length is THE right length. It will vary from rider to rider for many different reasons. What I am saying is that I was glad that I experimented a little bit--nothing like what Larry and a few others have done, but enough for me to feel confident that the cranks I am using are the right ones for me. It was worth the effort and small expense.

 

[vosadrian]

Interesting @chicorider. I found that shorter cranks lengthened my effective gearing and I am more effective at high speeds on downhills. Added about 5kph to the speed I can comfortably pedal at. Did you run the same chain rings with each crank length?

The theory says shorter cranks have lower foot speed at the same cadence. The body's limit on cadence is typically foot speed when you can no longer engage the muscles quick enough to apply useful power on the down stroke as the pedals are travelling so quickly. Shorter cranks reduce the foot speed so you can typically go a little higher on cadence until you hit your foot speed limit and this has the effect of lenthening the gearing. But the physics does not explain the biology and perhaps some people lose some power due to less torque at same cadence so have more trouble getting on top of the gear?

 

[Frito Bandito]

That is nice to know your crank length through experimentation Chico. I don't think engineers get it wrong, especially when (1) science says it and (2) others try it and it works out just as it was predicted. 165mm feels fine for me, but I still wanna try some long cranks though just to satisfy my curiosity. If I can get some 155s then I will try those too.

 

[chicorider]

@vosadrian, I did experience a bit of what you describe as a lengthening of gearing when going downhill, and I did enjoy that. But I lost a bit at the other end when going up hill, and my knees didn't enjoy that too much. I did run the same chainrings, yes. Part of my crank length preference might also be because I am a little bit of a masher; always have been. I did have to spin a little more with the shorter cranks, which slightly affected my aerobic capacity, which slowed my speed down a bit, which my brain didn't like, which would make me mash harder, which made my knees complain, which, eventually, sent me back to the slightly longer 160mm length, where none of that was happening.

I can explain exactly none of the physics of it all because I majored in English.