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quantifying design criteria



 
 
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  #1  
Old September 15th 05, 04:38 PM
buckyllama
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Default quantifying design criteria

So I'm beginning an experiment with designing a recumbent. I'm also a
complete bent newbie but I do have some metalworking/composites
experience and a lot of upright wrenching experience so I figure it's
worth giving it a go.

Having said that I'm trying to pin down what the bent community means
when they say a bike is "fast" or "slow". I mean it's obviously
related to seat angle and BB height. a Tour easy is a differnt beast
than a bacchetta aero. But how about the different between something
like the aforementioned aero and a street machine. BROL describes the
aero as fast and the street machine as being kind of slow. But the
riding positions are similar. (particurlarly on the 26/20 version of
the aero) A quick analysis of the side pics of both bikes suggests
that with a similar seat angle the frontal area would be quite similar.
Is weight the major player in this comparision then? Or is it a more
amorphus "this bike feels fast" kind of judgement.

My goal is a high-racer/fast touring style of bike. Challenge serian
or similar. I haven't yet sold myself on teh 26 front wheel tho. I've
never ridden a bike witha seat that high and I was wondering how it
might fare in stop and go traffic. The extra visibility would be nice,
but I'm worried about getting a foot down. I have a 35 mi rt commute
of which approx 5 mi each way is in traffic with a lot of stoplights.
Thoughts?

Cheers and thanks for helping a newbie out.

-Tim

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  #2  
Old September 15th 05, 07:42 PM
Jon Meinecke
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Default quantifying design criteria

"buckyllama" wrote

So I'm beginning an experiment with designing a recumbent.
[...]
My goal is a high-racer/fast touring style of bike. Challenge serian
or similar. I haven't yet sold myself on teh 26 front wheel tho.


I have a Volae Sport (650/650) that probably meets
that description. You can find high-racers used for perhaps
not a lot more than you could build one yourself, -- though
rolling your own may have certain rewards. I'd still suggest
that you ride as many types as you can before you finalize
your design and begin to build it.

I've never ridden a bike witha seat that high and I was
wondering how it might fare in stop and go traffic.


I've only been riding my Volae for about 5 months. Getting feet
down to stop hasn't been a problem. Getting them back up
to start is only rarely a problem. %^P

The extra visibility would be nice,


Between my two more upright seating recumbents
(Tour Easy and BikeE) and the Volae, I don't know
that I feel much extra visibility. Head height is not really
that different.

I will say it's easier for me to look behind on the Tour Easy
or BikeE than on the Volae because of head/neck angle.
I have mirrors on all my bikes, but still like to glance back.
The more horizontal the neck angle the less you can easily
see by looking over my shoulder. Because of this, I feel
that in traffic, the more upright seating is slight advantage.
Bottom bracket height is not really an issue.

The Volae is definitely faster/easier, but with narrow
(650x23) tires it is not much fun on bumpy/rough surfaces.
If I were commuting on it I would probably get another
wheelset and beefier tires. But that's why I have and
ride three different style recumbents...

I broke a spoke on the Volae last weekend, luckily only
about three blocks from home. I rolled the bike back
home, took off the wheel, strapped it behind the seat
on my Tour Easy and rode to the local bike shop to leave
it for repair. A person I know was shopping at the LBS
when I got there and I related my story. When I said
I'd "gone home and changed bikes", she commented
that I'd said that like someone else might say they had
"gone home and changed clothes".

I told he she was right. %^) Perhaps you may want
to design and build several different bikes!

Good luck,

Jon Meinecke


  #3  
Old September 16th 05, 01:32 AM
Sunset Lowracer [TM] Fanatic
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Default quantifying design criteria


buckyllama wrote:
...
Having said that I'm trying to pin down what the bent community means
when they say a bike is "fast" or "slow". I mean it's obviously
related to seat angle and BB height. a Tour easy is a differnt beast
than a bacchetta aero. But how about the different between something
like the aforementioned aero and a street machine. BROL describes the
aero as fast and the street machine as being kind of slow. But the
riding positions are similar. (particurlarly on the 26/20 version of
the aero) A quick analysis of the side pics of both bikes suggests
that with a similar seat angle the frontal area would be quite similar.
Is weight the major player in this comparision then? Or is it a more
amorphus "this bike feels fast" kind of judgement....


While frontal area is an important factor as it largely determines
aerodynamic drag, there are other issues in seating position. Combining
a reclined seat with a relatively low bottom bracket (BB) will reduce
the amount of power the rider can generate for acceleration and
climbing. A seat with variable recline [1] could be used, but would add
weight and mechanical complexity.

Another issue is rider comfort. A rider will generally be faster
overall if he/she is comfortable on the bicycle, even if there is an
objective performance tradeoff to gain that comfort. Leave riding fast
in pain to the UCI professionals.

To state the obvious, rolling resistance and drivetrain friction will
also affect performance, but to a lesser extent than aerodynamics
except for low speed climbing.

[1] Here is one of the very few bicycles with variable seat recline
http://www.fortebikes.com/images/gallery/SEAN2.JPG.
--
Tom Sherman - Fox River Valley
"Abnormal would be a mountain lion speaking English." - M.V.

  #4  
Old September 16th 05, 02:32 PM
buckyllama
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Default quantifying design criteria

I'm not sure what you mean by variable seat recline. I'm planning on
making the seat adjustable within a reasonable range (approx 30 +/-5
deg abovr horiz for the hip-shoulder section of the seat, upcurved top
to limit neck/upper shoulder strain) using a similar system to a
t-bone. My current drawing puts the BB center at about the same level
as my hip joint when seated (a few inches above the seat base).

Speed at all costs is not my primary goal but the thing needs to be
fairly fast and also climb reasonably well. This is more fast tourer
than racer. My primary design consideration is to optimize this bike
for my commute. It need to handle traffic adequately when needed. It
needs to be fast on the flats and it needs to climb reasonably well.
My morning commute consists of a long gradual downhill which I usually
maintain about 25-30 on on the road bike. followed by a streatch
through town which is all stop and go. Then a ~10 mile streatch of
flat to rolling hills which I maintain about 20 on the road bike, then
a 3 mile climb (2 - 10%) , a 2 mile downhill and a 1/2 mile climb (4 -
15%). Avg speed on the computer is usually a hair over 16mph avg. at
the end. If I can match that rolling avg on the bent with a bit of
practice, I'll be happy. I'd rather gain a bit of speed on the flats
in exchange for a little climbing ability since the flat roads are
heavily travelled and the faster I am, the safer I am.

-Tim

  #5  
Old September 16th 05, 02:49 PM
Edward Dolan
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Default quantifying design criteria


"Sunset Lowracer [TM] Fanatic" wrote in message
oups.com...

buckyllama wrote:
...
Having said that I'm trying to pin down what the bent community means
when they say a bike is "fast" or "slow". I mean it's obviously
related to seat angle and BB height. a Tour easy is a differnt beast
than a bacchetta aero. But how about the different between something
like the aforementioned aero and a street machine. BROL describes the
aero as fast and the street machine as being kind of slow. But the
riding positions are similar. (particurlarly on the 26/20 version of
the aero) A quick analysis of the side pics of both bikes suggests
that with a similar seat angle the frontal area would be quite similar.
Is weight the major player in this comparision then? Or is it a more
amorphus "this bike feels fast" kind of judgement....


While frontal area is an important factor as it largely determines
aerodynamic drag, there are other issues in seating position. Combining
a reclined seat with a relatively low bottom bracket (BB) will reduce
the amount of power the rider can generate for acceleration and
climbing. A seat with variable recline [1] could be used, but would add
weight and mechanical complexity.

Another issue is rider comfort. A rider will generally be faster
overall if he/she is comfortable on the bicycle, even if there is an
objective performance tradeoff to gain that comfort. Leave riding fast
in pain to the UCI professionals.

To state the obvious, rolling resistance and drivetrain friction will
also affect performance, but to a lesser extent than aerodynamics
except for low speed climbing.


The only thing I would add to Tom's excellent message on recumbent bike
design is to be aware of high bottom brackets. Very many of us will get numb
feet if the BB gets much above the seat. A high BB will also affect handling
to some extent. Hence, the reason why so many recumbents have a low BB. It
is the classic design and for good reason.

I have always thought that the Tour Easy could have a slightly higher BB in
order to ensure more rider comfort. But the BB cannot be higher than the
seat or I will get numb feet. I think this may be more of a factor as you
age, but I am not sure about that. I have also known a few young riders who
cannot take a high BB.

--
Ed Dolan the Great - Minnesota
aka
Saint Edward the Great - Order of the Perpetual Sorrows - Minnesota


  #6  
Old September 17th 05, 02:35 AM
Sunset Lowracer [TM] Fanatic
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Default quantifying design criteria

buckyllama wrote:
I'm not sure what you mean by variable seat recline. I'm planning on
making the seat adjustable within a reasonable range (approx 30 +/-5
deg abovr horiz for the hip-shoulder section of the seat, upcurved top
to limit neck/upper shoulder strain) using a similar system to a
t-bone. My current drawing puts the BB center at about the same level
as my hip joint when seated (a few inches above the seat base)....


By variable seat recline I mean a seat where the angle of the seat back
can be changed while riding. This is complicated by needing to have the
seat base move forward to keep the rider's leg extension constant while
pedaling. The Forte lowracer I posted a link too is one example of such
a bike. [1] The variable seat recline allows for an upright seat back
for more sprinting and climbing power, and a reclined seat back angle
for minimizing frontal area on flat and downhill sections.

Further testing is needed, but variable seat recline could potentially
improve overall climbing ability of recumbent bicycles by allowing
changes in position. [2]

[1] The only other I am aware of is the one ridden (and built?) by John
"Rocketman" Williams which reportedly has a seat recline angle
adjustment from 10 to 40 degrees from the horizontal. [3]
[2] Studies have shown the fastest way to climb longer grades on an
upright bicycle is to alternate between sitting and standing.
[3] By convention, recumbent seat back angles are measured from the
horizontal.
--
Tom Sherman - Fox River Valley
"Abnormal would be a mountain lion speaking English." - M.V.

  #7  
Old September 17th 05, 03:51 AM
buckyllama
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Posts: n/a
Default quantifying design criteria

ah, the variable seat position is a very neat idea. A bit out of the
scope for this project but maybe someday.... hmmm. I can envision a
3 part seat so that the base, back, and shoulder sections move relative
to one another so that each position is comfortable and effective.

  #8  
Old September 17th 05, 05:01 AM
Sunset Lowracer [TM] Fanatic
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Default quantifying design criteria


Edward Dolan was on topic when he wrote:
...
The only thing I would add to Tom's excellent message on recumbent bike
design is to be aware of high bottom brackets. Very many of us will get numb
feet if the BB gets much above the seat....


Back in the ancient past when I owned a 20/20 Wishbone RT [1] with a
13-inch/33 cm seat height and 26-inch/66 cm bottom bracket height, I
would suffer from foot pain unless I wore shoes with proper fit and
very stiff soles (i.e. SiDi Dominator II Mega). The SiDi shoes also
offered the advantage of being black, as a rider's feet will be in
his/her vision on a bike with a high bottom bracket. Brightly colored
shoes would be an annoying distraction.

[1] See http://www.ihpva.org/incoming/2001/wbone2.jpg.
--
Tom Sherman - Fox River Valley

"We are discussing whether humans as prey are 'natural'. Clearly,
they are, or that mountain lion wouldn't have been trying to eat a
human." - M.V.

 




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