V20 finding that fit

[ratz]

Ok here's an audience participation exercise. I would like to see how other people current setups compare to the following calculations

This assumes the straight Boom/Slider for the boom length calculation.
This assumes the Stock seat pan for the boom position calculation.

Boom length has alway been easy to calculate:

Calculate Lower Leg Length (LLL) as:

LLL = The height of the top of you thigh off the floor when sitting in a chair with leg bent at 90 degrees​

Then:

Boom Length = LLL + Crank Length + 8 cm​

I've been wondering if we can quantify the best Boom forward Aft starting position once the boom length is set.
Working from standard bike fit numbers that are used to fit road bikes I get this as a possible optimal starting numbers:

BB distance from front of Seat Pan = Inseam - ((CrankLength /2) + X))

where X = thickness of the seat cushion after it is compressed by the rider's weight.
Ventisit Comfort 3cm Pad is ~2.25cm under load as it compresses about 0.25cm.
Ventisit Classic 2cm Pad is ~ 1.75cm under load as it compresses about 0.25cm.
​

I'd like to compare that to what people are actually riding to see if there is something like a standard correlation.

So checking these these for myself:

Step (1) Measure your inseam using a spirit level. Shoes off measure from top of level to floor.


picture borrowed from the interwebs.


Step (2) Measure the distance from the center of your Bottom Bracket (aka Crank Axle) to the front edge of your seat pan

So for me with inseam of 31in and 155mm cranks

BB-2-SP = 78.74cm - ((15.5cm /2)+ 2.25cm)= 68.74cm = 27.06inches

The bike fit numbers would seem to indicate that I have my boom is forward about 1" too far at 28inches. I have been tending to ride with my toes pointing forward which I feel might be robing power. So based on this exercise and my previous thoughts about bringing the feet closer; I am going to move the boom back that full 1" (but keeping the same length) to see if that gives me a little more power delivery and a flatter foot position relative to the power stroke.

I'll be paying attention to the knees with this adjustment and watching for pain at the front of my knee

Problem: pain at the front of the knee.
Solution: adjust the boom forward moving feet forward

Problem: pain at the back of the knee.
Solution: adjust the boom backward bring feet back.

Don't know if this will give us a common formula; but it's interesting.

Edit: Corrected math; Larry spotted that I generated the formula incorrectly when I copied out of my spreadsheet. That's the problem with doing multiple atomic steps in multiple cells; easy to get the parenthesis wrong when combining.

 

[LarryOz]

Step (1) Measure your inseam using a spirit level. Shoes off measure from top of level to floor.

picture borrowed from the interwebs.

Note: Make sure the level is Vendetta yellow and has been adequately chilled in garage at <40 degree F before using.

 

[ratz]

Results. Much more power on the ride; better anchor into the seat for power.
Same results for Pluckblond but her legs are NSFW so you get mine instead with the math verification for the magic 150degrees for fit.

 

[MrSteve]

Since my right leg, crushed by a car, is shorter, weaker and more crooked,
my formula for proper fit is the one true spot that mostly works well for both legs, most of the time.
Even so, my effective X-seam measurement will vary under both cadence and load, whether I'm sitting upright
or not and will be shortest when I'm slouched down, out of the slipstream.

This makes the mental gymnastics you're having fun with worthless for people
like me who're outside the norm.

 

[RAR]

I too have one leg shorter than the other, I put a quarter inch spacer in my shoe too make them equal.

 

[LarryOz]

math verification for the magic 150degrees for fit.

S0 that is what we are shooting for: 150 degrees?
You did not fell like you "other" knee was bent too much since you brought everything in an inch or so?
Next - you can consider changing your cleat to mount to the center of you shoe - I know some strong bent riders that swear by that method. (read up the some DF riders are also switching to this)
I am going to work on some kind of adapter that I can try out at a variety of mid-shoe positions.
...
my stats: 150mm cranks, 3cm ventist pad, 30.25" inseam
76.835cm - ((15.0cm /2) + 2.25cm) = 67.085cm = 26.4"
So, right now I am riding with a 28" measurement from center of BB to front of seat pan. Which is is 1.6" longer than the formula suggests. Wow!
I will try and get some photo's with angles of before and after a change and see how it feels on my endurance ride tomorrow.

 

[Rick Youngblood]

One thing to mention, as I've aobserved researching boom/seat/crank position, is how wold the formula be affected angle of the anckle. For example Larry rides with toes pointed almost up vertical. My toes are pointed more forward.

 

[Rick Youngblood]

Next - you can consider changing your cleat to mount to the center of you shoe - I know some strong bent riders that

I haven't read about this, and may try it this spring. I have mine adjusted all the way back.

 

[cranky cyclist]

Ok here's an audience participation exercise. I would like to see how other people current setups compare to the following calculations

This assumes the straight Boom/Slider for the boom length calculation.
This assumes the Stock seat pan for the boom position calculation.

Boom length has alway been easy to calculate:

Calculate Lower Leg Length (LLL) as:

LLL = The height of the top of you thigh off the floor when sitting in a chair with leg bent at 90 degrees​

Then:

Boom Length = LLL + Crank Length + 10cm​

I've been wondering if we can quantify the best Boom forward Aft starting position once the boom length is set.
Working from standard bike fit numbers that are used to fit road bikes I get this as a possible optimal starting numbers:

BB distance from front of Seat Pan = Inseam - ((CrankLength + X)/2)

where X = thickness of the seat cushion after it is compressed by the rider's weight.
Ventisit Comfort 3cm Pad is ~2.5cm under load as it compresses about 0.5cm.
Ventisit Classic 2cm Pad is ~ 1.75cm under load as it compresses about .25cm.
​

I'd like to compare that to what people are actually riding to see if there is something like a standard correlation.

So checking these these for myself:

Step (1) Measure your inseam using a spirit level. Shoes off measure from top of level to floor.

View attachment 4045
picture borrowed from the interwebs.


Step (2) Measure the distance from the center of your Bottom Bracket (aka Crank Axle) to the front edge of your seat pan

So for me with inseam of 31in and 155mm cranks

BB-2-SP = 78.74cm - ((15.5cm + 2.5cm)/2) = 68.74cm = 27.06inches

The bike fit numbers would seem to indicate that I have my boom is forward about 1" too far at 28inches. I have been tending to ride with my toes pointing forward which I feel might be robing power. So based on this exercise and my previous thoughts about bringing the feet closer; I am going to move the boom back that full 1" (but keeping the same length) to see if that gives me a little more power delivery and a flatter foot position relative to the power stroke.

I'll be paying attention to the knees with this adjustment and watching for pain at the front of my knee

Problem: pain at the front of the knee.
Solution: adjust the boom forward moving feet forward

Problem: pain at the back of the knee.
Solution: adjust the boom backward bring feet back.

Don't know if this will give us a common formula; but it's interesting.

Check out Pedaling Innovations.com. I am not advocating that you buy their product but there is information on pedal technique. There is a myth that you can get more power by using your hamstrings in the upstroke of the pedal. The old idea was that you had to pedal toe down. Actually you should pedal with ankle down. It is an interesting read.

 

[ratz]

S0 that is what we are shooting for: 150 degrees?

Yes that's considered the optimal for the power stroke; it leaves enough play for ankling under different loads; and for "pumping" the bike under the rider left and right which translates well to the pumping of the MBB that Jim Promotes.

You did not fell like you "other" knee was bent too much since you brought everything in an inch or so?

Nope felt natural but that's one ride; I plan to give it a week. The primary change was that I was much more FIRMLY planted in the seat which let me get a firmer push under heavy load. I don't feel I lost the full inch; I was probably a little loose on the bike before.

I am going to work on some kind of adapter that I can try out at a variety of mid-shoe positions.

the would be handy for testing I agree that a mid foot position might have some significant advantages.

 

[LarryOz]

One thing to mention, as I've aobserved researching boom/seat/crank position, is how wold the formula be affected angle of the anckle. For example Larry rides with toes pointed almost up vertical. My toes are pointed more forward.

fwiw: I think current thinking is you can get more power with your feet pointed straight up, which is also why some cyclist are moving toward center of the shoe cleat attachment.

 

[ratz]

Check out Pedaling Innovations.com. I am not advocating that you buy their product but there is information on pedal technique. There is a myth that you can get more power by using your hamstrings in the upstroke of the pedal. The old idea was that you had to pedal toe down. Actually you should pedal with ankle down. It is an interesting read.

Yep we've reviewed that before; height of BB relative to the rider effects what is "perceived" as point toys. See this thread for the pictures.

http://cruzbike.com/forum/threads/a...crank-and-chainstay-length.11420/#post-128950

 

[LarryOz]

Since my right leg, crushed by a car, is shorter, weaker and more crooked,
my formula for proper fit is the one true spot that mostly works well for both legs, most of the time.
Even so, my effective X-seam measurement will vary under both cadence and load, whether I'm sitting upright
or not and will be shortest when I'm slouched down, out of the slipstream.

Wow - that is a bummer - ;(
You could machine a crank down to match the "shortness" in you one leg.
Then you might be able to get a better fit for yourself and maximize everything!

 

[ratz]

fwiw: I think current thinking is you can get more power with your feet pointed straight up, which is also why some cyclist are moving toward center of the shoe cleat attachment.

Mostly......

The height of rider will make that "vertical position" be relative. The shorter you are compared to someone else riding the same length chain-stay the higher your BB will be; and thus the more vertical you foot will appear. What you are really saying is foot perpendicular to the primary axis of the pedaling stroke motion.

http://cruzbike.com/forum/threads/a...crank-and-chainstay-length.11420/#post-128950

 

[ratz]

Since my right leg, crushed by a car, is shorter, weaker and more crooked,
my formula for proper fit is the one true spot that mostly works well for both legs, most of the time.
Even so, my effective X-seam measurement will vary under both cadence and load, whether I'm sitting upright
or not and will be shortest when I'm slouched down, out of the slipstream.

Ouch, a true enigma wrapped in his Vendetta

 

[Mathew Fy]

Ok here's an audience participation exercise. I would like to see how other people current setups compare to the following calculations

Here's my Vendetta set-up:
Measured inseam 81.25
Measured Boom Length 88.9
Measured BB to Seat Pan 72
Measured Lower Leg Length 55.9
Measured knee angles 77*, 150*
Calculated Boom Length 81.4
Calculated BB to Seat Pan 71.25
My boom length may be shorter than most because I use a mountain bike handle bar with Ergon GP5 Grips.

 

[ratz]

Calculated BB to Seat Pan 71.25

Measured BB to Seat Pan 72

Measured knee angles 77*, 150*

Ok so that puts you within the range of accuracy <8mm off and roughly 150 degree would match with being just a hair long

What length cranks?

 

[Charles.Plager]

Wow - that is a bummer - ;(
You could machine a crank down to match the "shortness" in you one leg.
Then you might be able to get a better fit for yourself and maximize everything!

Assuming it is from the knee to the foot that is shorter, I don't think that would help as much as getting shims under the cleats to make the short leg longer (you want both legs to have the same motion when pedaling and shorter crank on one side would muck that up).

If the leg is shorter between hip and knee (too), then this might work.

But I'm not that kind of doctor, so this opinion isn't worth too much.

 

[MrSteve]

Wow - that is a bummer - ;(
You could machine a crank down to match the "shortness" in you one leg.
Then you might be able to get a better fit for yourself and maximize everything!

The fitting solution I pursued is peculiar to both my skillset and me.

Rather than shorten the crank for my original-equipment side to match my artificially shortened
side, which, if you mock it up, you'll see for yourself that this approach does not work....
I shimmed up the pedal to meet my artificially shortened leg and match the original equipment side.

Interesting and unexpected positive side effect of adding platforms to pedals:
pedaling on platforms mimics the forces on footpads when walking.
No hot-foot.

We now return you to the Original Poster's project,
formulating a "Standard Base Fitment Formula for Normal People."

 

[Mathew Fy]

Ok so that puts you within the range of accuracy <8mm off and roughly 150 degree would match with being just a hair long

What length cranks?

155 CobbCycling cranks