RUBY THE ROOKIE U/15 JUNIOR

For all things Velodrome
RR-M
Posts: 25
Joined: 27 Aug 2012, 11:20

Postby RR-M » 30 Aug 2012, 11:37

Hi to everyone at DHBC. We have been out to the velodrome 3x now and Ruby is inspired to begin track cycling with DHBC.
Big thanks to Lizanne last night for answering our questions and warmly welcoming us to the fold.
Ruby has joined DHBC/Cycling NSW this morning, a precious email silver licence to suffice until her membership card arrives in the snailmail.
Whilst she will avail herself of a club "loanbike" for her first couple of tries, Dad (Alan) has been thoroughly researching the marketplace for a suitable track bike of her own.
One big question though..... The gearollout of 6m for U/15 is a complicated calculation. I've poured over the Cycling Australia gearchart. Plugging in the wheel+tyre diameter to the spreadsheet varies the chainring/cog combos, expensive proposition if you buy the wrong size chainring.
What is the precise diameter of a 700c wheel running a 23mm width tyre?
Plug 700mm into the gearchart gives one set of gear options under 6.0, plug 690mm and it gives another.
Can anyone suggest the best chainring/cog options. Im a dinosaur, always measured gears in inches, my reckoning is to provide her bike with the capabilities to ride 66/72/76. Is 76 equivalent to a 6m rollout ?
regards
Alan + Ruby

Lizanne
Posts: 1178
Joined: 15 Sep 2010, 13:58
Location: Wolli Creek

Postby Lizanne » 30 Aug 2012, 11:56

i found it!
yay i knew this existed. the reference chart and calculations that will be used by Cycling Australia.

I've just had a quick look but it looks fairly straight forward

http://www.nsw.cycling.org.au/default.asp?Page=45674

i think Colin is the guru on jr roll outs (so have a chat to him next week). and you can also e-mail officials@cycling.org.au

P.S welcome to the club! it's great to see junior numbers, especially female junior numbers growing.

christian
Posts: 837
Joined: 30 Oct 2007, 19:21
Location: Earlwood

Postby christian » 30 Aug 2012, 12:01

It's never quite that simple. The actual wheel diameter will depend on things like tyre brand and tyre pressure. The commonly accepted circumference of a 700x23c is 2096mm, so the diameter is 667mm. If in doubt just measure it.

RR-M
Posts: 25
Joined: 27 Aug 2012, 11:20

Postby RR-M » 30 Aug 2012, 12:37

Hi L + C
well yes the CA gearchart does appear straightforward if you know the precise diameter of the wheel+tyre you are running, difficult if the bike is still on the drawing board! C mentions the diameter of the most common wheel+tyre is only 667mm that makes a huge difference to the green squares (<6m rollout) in the CA gearchart spreadsheet.
Plug 667mm into the spreadsheet and the most flexible chainring size is 40 with a 14T sprocket for racing, 15 or 16T for training, 17T for cadence training. 40x14=5.99m rollout or a 75 inch gear. 40x15 =5.59m/70" 40x16=5.24m/65.6" 40x17=4.93m/61.7".
All well and good mathematically and research shows a mechanical advantage for youngsters spinning up a smaller chainring, 40T compared to say a 48T. Here's the rub though, try finding a 40T chainring, its a rare beast.
44/46/48 are common but with a 667mm wheel+tyre diameter the gear options are compromised significantly.
Maybe CA should up the U/15 rollout slightly to match more readily available chainring sizes, particularly after they announced the U/17 will be upped to 7.0m (prev 6.5m) from Oct 1 2012.
Wheres Einstein when you need him :)

christian
Posts: 837
Joined: 30 Oct 2007, 19:21
Location: Earlwood

Postby christian » 30 Aug 2012, 12:47

Not being able to get a small enough chain ring is probably due to 144 BCD cranks. You may need to go to 130 BCD road cranks. I'd look at going with a larger sprocket, 48T chain rings are the most common size, 48/17 will give you 5.92m roll out. Generally you change chain rings for changing gears as its easier and requires less tools. So to train on a smaller gear, or to race in the wind you could go 47/17 or 46/17. If you want to get an odd chain ring you can try Cycle Underground http://cycleunderground.com.au/contact.htm and get them to make you one. Its cheaper then you'd think.

RR-M
Posts: 25
Joined: 27 Aug 2012, 11:20

Postby RR-M » 30 Aug 2012, 13:31

Hi C, cycle underground a great tip ta. The 144v130bcd argument is valid too, Im considering a suginoRD 130bcd 1/8" chain crankset for this build http://www.benscycle.net/index.php?main ... cts_id=528 it comes with a 48T chainring but 44/46/48/50T all available. Its sets up with a 45mm chainline measure, most research Ive done says 42mm most common chainline I wonder how significant that is.
Aim here is to build the best fitting bike with quality components to a medium budget, give ruby the best chance of success on a free rolling bike, good BB, wheel hubs, crankset. Design and component selection crucial, best bang for the bucks. Im a techhead, love this stuff :D
Alan 4 Ruby

christian
Posts: 837
Joined: 30 Oct 2007, 19:21
Location: Earlwood

Postby christian » 30 Aug 2012, 13:59

The chain line is important if you want things to last and you also want a quiet bike. The disadvantage of a 130 BCD crack is for larger chain rings it isn't going to be as stiff as the chain ring edge is further from the bolts, although unless Ruby is a sprinter she is not going to notice this just yet. I'd be inclined to go with a track specific crank as you know the chain line will be correct. I'm not sure what difference going from a 40T to a 48T ring will make. Its not that much extra mass, you should be able to offset it by using light rims and tyres. There will be more chain so maybe a little more drag there but I've also heard that smaller cogs and rings cause the chain to bend more creating more drag due to the plates of the chain having to rotate more. Not sure if anyone has actually measured this properly. No doubt a mechanical engineer can weigh in on this one, Alex?

timyone
Posts: 4380
Joined: 22 Nov 2006, 20:29

Postby timyone » 02 Sep 2012, 00:33

far out being a junior looks complicated, I seem to just have trouble working out what to wear, I have a red bike, and not a lot of red clothing. Black goes with red, and red goes with red, but I don't really own any all black..


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