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Crank size for 7 year old  RSS Feed

#1 2018-02-13 12:08pm

blammo585
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Registered: 2013-02-17
Posts: 1066

Crank size for 7 year old

My wife's nephew lives near a track, and I was thinking of taking him there to ride when it gets warmer. He's 7 years old. I have a DK Banshee that is pretty short so I was thinking of putting some parts on it for him to ride. I don't want to go crazy with something he may not ride much but I want to try to make it so it will "fit" him. It will have 20 x 1.75 wheels on it so what size cranks should I put on it and what gearing? He's 7, and he seems short but I guess he may be average height for his age.

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#2 2018-02-13 1:39pm

Thrash
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Re: Crank size for 7 year old

whats his inseam?


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#3 2018-02-13 1:42pm

bnd
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From: Utah
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Re: Crank size for 7 year old

My newly turned 7 novie daughter is on the short side, her race pants are 22" I think. I just bumped her up to 135mm. It looks right & she says they feel right.

b.


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#4 2018-02-13 1:47pm

Kurt_Rad
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From: Cochrane, AB Canada
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Re: Crank size for 7 year old

Read this the other day

some guy on the internet wrote:

This is a regular discussion I have with other parents/riders and as this is an area of expertise for me, I thought I would share what the research suggests, rather than gaining the info from others based on anecdotal information.

First, realize than crank length is very personal and depends on a variety of different issues, including % of Type I & Type II muscles fibres, optimal cadence (can change with training), tracks, etc.  As the crank length gets longer, peak torque will go up (faster out of the gate), but crank length does affect pedal speed - longer cranks require a faster pedal speed to maintain the same cadence (more difficult to maintain a higher cadence coming out of corners for example).

For optimal crank length (not what other people are using), go to the article by Martin & Spirudo from 2001 (Eur J Appl Physiol, poages 413-418) - Determinants of maximal cycling power: crank length, pedaling rate and pedal speed. Their recommendation (page 415) is that the optimal crank length is 20% of leg length or 41% of tibia length. Also, need to look at your child's pedalling tendencies as optimal pedal rate decreased with increasing crank length, whereas optimal pedal speed increased with increasing crank length. This takes in to account optimal muscle lengths. This is a pretty technical article, but based on my research, the average optimal crank length for power output in children is around 160 mm for children and adolescents aged 10-19 with an average height of 163 cm (5'4"). Don't go too long as it may affect how fast they can pedal AND affect their ability to produce power!

By the way, inseam is not leg length in the above article. Leg length was defined as the difference between standing height & seated height and tibia length defined as the distance from the lateral malleolus to the cleft of the knee.


"Give a man a fire, and you warm him for a day. But set a man on fire, and you warm him for the rest of his life."

[img]http://uploads.bmxmuseum.com/user-images/56126/img_053859d1cd019a.png[/img]

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#5 2018-02-13 2:12pm

Garcia_58
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From: South FLA
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Re: Crank size for 7 year old

Kurt_Rad wrote:

Read this the other day

some guy on the internet wrote:

This is a regular discussion I have with other parents/riders and as this is an area of expertise for me, I thought I would share what the research suggests, rather than gaining the info from others based on anecdotal information.

First, realize than crank length is very personal and depends on a variety of different issues, including % of Type I & Type II muscles fibres, optimal cadence (can change with training), tracks, etc.  As the crank length gets longer, peak torque will go up (faster out of the gate), but crank length does affect pedal speed - longer cranks require a faster pedal speed to maintain the same cadence (more difficult to maintain a higher cadence coming out of corners for example).

For optimal crank length (not what other people are using), go to the article by Martin & Spirudo from 2001 (Eur J Appl Physiol, poages 413-418) - Determinants of maximal cycling power: crank length, pedaling rate and pedal speed. Their recommendation (page 415) is that the optimal crank length is 20% of leg length or 41% of tibia length. Also, need to look at your child's pedalling tendencies as optimal pedal rate decreased with increasing crank length, whereas optimal pedal speed increased with increasing crank length. This takes in to account optimal muscle lengths. This is a pretty technical article, but based on my research, the average optimal crank length for power output in children is around 160 mm for children and adolescents aged 10-19 with an average height of 163 cm (5'4"). Don't go too long as it may affect how fast they can pedal AND affect their ability to produce power!

By the way, inseam is not leg length in the above article. Leg length was defined as the difference between standing height & seated height and tibia length defined as the distance from the lateral malleolus to the cleft of the knee.

Great info KRad


I am not of this world ----- 1978 GLJ 1979 DG Racer

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#6 2018-02-13 2:30pm

blammo585
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Registered: 2013-02-17
Posts: 1066

Re: Crank size for 7 year old

Thrash wrote:

whats his inseam?

I'm not sure. I guess I'll have to ask but I just want to fix the bike up a little without putting too much in it since who knows how much he would actually ride after I take him his first time. I was just going to get some 1-piece cranks but finding short ones are difficult. On ebay I found 140mm and 165mm.

I know that for my 8th birthday I got a department store BMX bike, and I found out years later when I started changing stuff out that it had 165mm cranks and had 44-18 gearing. I rode that bike OK for a long time. I was probably a little taller when I was 8 than he is.

He rides an 18" X Games bike that a neighbor kid had outgrown and gave to him. He fits very well on it so maybe I'll just let him ride that, but I wanted something to do with the Banshee.

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#7 2018-02-13 2:50pm

Kurt_Rad
Pro Member“The Hammer”
From: Cochrane, AB Canada
Registered: 2011-01-10
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Re: Crank size for 7 year old

Garcia_58 wrote:

Kurt_Rad wrote:

Read this the other day

some guy on the internet wrote:

This is a regular discussion I have with other parents/riders and as this is an area of expertise for me, I thought I would share what the research suggests, rather than gaining the info from others based on anecdotal information.

First, realize than crank length is very personal and depends on a variety of different issues, including % of Type I & Type II muscles fibres, optimal cadence (can change with training), tracks, etc.  As the crank length gets longer, peak torque will go up (faster out of the gate), but crank length does affect pedal speed - longer cranks require a faster pedal speed to maintain the same cadence (more difficult to maintain a higher cadence coming out of corners for example).

For optimal crank length (not what other people are using), go to the article by Martin & Spirudo from 2001 (Eur J Appl Physiol, poages 413-418) - Determinants of maximal cycling power: crank length, pedaling rate and pedal speed. Their recommendation (page 415) is that the optimal crank length is 20% of leg length or 41% of tibia length. Also, need to look at your child's pedalling tendencies as optimal pedal rate decreased with increasing crank length, whereas optimal pedal speed increased with increasing crank length. This takes in to account optimal muscle lengths. This is a pretty technical article, but based on my research, the average optimal crank length for power output in children is around 160 mm for children and adolescents aged 10-19 with an average height of 163 cm (5'4"). Don't go too long as it may affect how fast they can pedal AND affect their ability to produce power!

By the way, inseam is not leg length in the above article. Leg length was defined as the difference between standing height & seated height and tibia length defined as the distance from the lateral malleolus to the cleft of the knee.

Great info KRad

My old College Swim coach has a PHD in kinesiology and biomechanics. He wrote it...his kids race BMX


"Give a man a fire, and you warm him for a day. But set a man on fire, and you warm him for the rest of his life."

[img]http://uploads.bmxmuseum.com/user-images/56126/img_053859d1cd019a.png[/img]

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#8 2018-02-13 8:57pm

Garcia_58
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From: South FLA
Registered: 2013-05-14
Posts: 8502
Bikes: 35
For Sale: 29

Re: Crank size for 7 year old

Kurt_Rad wrote:

Garcia_58 wrote:

Kurt_Rad wrote:

Read this the other day


Great info KRad

My old College Swim coach has a PHD in kinesiology and biomechanics. He wrote it...his kids race BMX

Very cool


I am not of this world ----- 1978 GLJ 1979 DG Racer

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#9 2018-02-13 10:24pm

happyshark
Pro MemberWelcome To The ThunderDome!
From: sa,tx
Registered: 2017-09-06
Posts: 515
Bikes: 9

Re: Crank size for 7 year old

My 7 rides expert frame what year banshee look like my banshee the crupi is custom built for 7 -9 yr crank is a 145 I have a 155 on standby for next yr  http://uploads.bmxmuseum.com/user-images/246333/img_01635a83d62121.jpg
http://uploads.bmxmuseum.com/user-images/246333/img_02465a83d62014.jpg
http://uploads.bmxmuseum.com/user-images/246333/img_02525a83d62048.jpg


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#10 2018-02-13 10:27pm

happyshark
Pro MemberWelcome To The ThunderDome!
From: sa,tx
Registered: 2017-09-06
Posts: 515
Bikes: 9

Re: Crank size for 7 year old

44 t w145 crank arms. The 155 cranks has 43 t it all about gearing when there little I would just look on craigslist or offer up and buy a mini or an expert of any bike DK redline etc. etc. it be cheaper than trying to outfit that banshee   for a seven-year-old

Last edited by happyshark (2018-02-13 10:38pm)


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#11 2018-02-14 5:36am

jimmymac
Pro Member
Registered: 2012-11-13
Posts: 388
Bikes: 2

Re: Crank size for 7 year old

This tool might help you get in the ballpark - worked for me, but came out a bit too long for my kid -

http://bmxultra.com/bmx-sizing-charts/

Have been checking this method also -
https://www.facebook.com/HeliumUSA/phot … 95/?type=3

How to measure crank length. Copyright © 2017 Damon Sprague
This is the method I prescribe (The Sprague Method). I worked this out myself so there is no-one to blame but me if you hate it, can't use it or think it's stupid.
So I have child stand close to but not touching wall, shoes on shoulder width apart. Use a clipboard or similar, place flat against wall with long edge in crotch high as possible before protest. Measure from floor to top edge of clipboard including fractions with tape measure. Convert fractions of an inch to decimals for equation, top number divided by bottom i.e 3/8 = .375. Example, 20.25 for a 20 1/4) inseam. Then divide total number by .173 for crank length in mm. So 20.375 divided by .173 = 117mm. Now if your child is a stomper, better to go with the longer size 120mm as you are unlikely to find a 117mm crank. If your child is a spinner, the next size down would be better such as 115mm. As a rule too long is worse than to short of a crank. Also note young beginners are not typically going to be "spinners". Don't count on feedback from young riders, careful observance to determine proper fit of all the components will result in safer, faster, better riding and more fun so scrutinize the fit. *notes: If using clipless (clip, clip in) pedals the shorter length will be better than long. Also try this on yourself to see if it is close to what you may be using on your adult bike.  You can also find a decent chart at BMX ultra though it will render long sizes for little riders. DO NOT use Dans- way way outdated. Other calculators use the ambiguous term "inseam" which means what exactly? My method tells you exactly how to get the measurement to plug into the formula. My measurement method works well with my formula and I stand by it. If you try and mix my measurement method with other calculators you will not get solid results- those are not designed for my measurement. They are designed to use what ever your interpretation of "inseam" is since it is not clearly defined.

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#12 2018-02-14 5:39am

jimmymac
Pro Member
Registered: 2012-11-13
Posts: 388
Bikes: 2

Re: Crank size for 7 year old

Kurt_Rad wrote:

Read this the other day

some guy on the internet wrote:

This is a regular discussion I have with other parents/riders and as this is an area of expertise for me, I thought I would share what the research suggests, rather than gaining the info from others based on anecdotal information.

First, realize than crank length is very personal and depends on a variety of different issues, including % of Type I & Type II muscles fibres, optimal cadence (can change with training), tracks, etc.  As the crank length gets longer, peak torque will go up (faster out of the gate), but crank length does affect pedal speed - longer cranks require a faster pedal speed to maintain the same cadence (more difficult to maintain a higher cadence coming out of corners for example).

For optimal crank length (not what other people are using), go to the article by Martin & Spirudo from 2001 (Eur J Appl Physiol, poages 413-418) - Determinants of maximal cycling power: crank length, pedaling rate and pedal speed. Their recommendation (page 415) is that the optimal crank length is 20% of leg length or 41% of tibia length. Also, need to look at your child's pedalling tendencies as optimal pedal rate decreased with increasing crank length, whereas optimal pedal speed increased with increasing crank length. This takes in to account optimal muscle lengths. This is a pretty technical article, but based on my research, the average optimal crank length for power output in children is around 160 mm for children and adolescents aged 10-19 with an average height of 163 cm (5'4"). Don't go too long as it may affect how fast they can pedal AND affect their ability to produce power!

By the way, inseam is not leg length in the above article. Leg length was defined as the difference between standing height & seated height and tibia length defined as the distance from the lateral malleolus to the cleft of the knee.

Interesting! Just ran the 2 methods I posted up and came up with 160 mm for my almost 11 year old - 5' 3" with 27" inseam. He had been running stock 170 mm cranks until they busted (yay redline!), and was having trouble after getting out of the gate usually ahead of his competitors.

Last edited by jimmymac (2018-02-14 5:40am)

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