DRietz

Hey guys, I'm attempting to figure out how to do the calculations, or find a gear calculator online for figuring out junior gearing combinations and their respective rollouts.

For example, I'm quite curious if a 50x13 would bring me in under 26feet with 700x24c tires.

I'm getting a new crankset and will need to block gears out from my 11-25 cassette to start, but eventually when I am monetarily able, I'd like to switch to a cassette that'll put me right in the rollout gear without turning my bike into an 8speed.

Thanks.

thirdgenbird

its in meters, but it should work:
https://www.sheldonbrown.com/gears/

DRietz

8.1 meters with 50x13. Yikes. What a bummer.

Creakyknees

calculators are fine for guesstimating but always always always do a rollout check before you get to the race.

Pump up the tires, no rider on the bike, measure twice.

headshock

i find this most helpfull
https://home.earthlink.net/~mike.sherman/shift.html
in new zealand we are restricted to a rollout of 6.61 metres which is approximatly 21.6 ft
for under 17
i ride either 39/13
or 53/17

chado445510

It does suck, I run a 50x14 now. In order to get the most (as you probably know) you can use 52x14

jsutkeepspining

wow and i thought being restricted to the 50x14 was lame. what do you spin out at like 25 mph. time trials must be interesting haha.


EDit: actually just checked it out, you would go a little over 27 mph in the 50/17 spinning at 120 rpms.

DRietz

A 52x14 works but a 50x13 doesn't? That sucks so hard.

I figure I'll get a 13-26-ish cassette and block one for races. What a bummer.

YMCA

Why don't you just buy a 45t and run 12-??
It's what most other juniors do out there, instead of blocking gears.

carpediemracing 

The correct answer is Creakyknees's answer.

Or, if you want to know before you make changes, measure the circumference of your current tire (pumped up, no weight, on bike - mark where valve is on a floor with a line type pattern like a linoleum or tile floor, roll wheel along line until valve points down again, measure in mm if possible).

Now ChainringTeeth/CogTeeth*circumference = roll out. Chainring/Cog gives you the multiplication ratio the actual gear gives you (45x12 to use YMCA's example). A 45x12 is 3.75, i.e. the cog turns 3.75 times for every turn of the chainring (or crankarm).

Remember that usable ratio relies on tire circumference. One very good rider in CT, many years ago, got DQed in the state TT. He won by a couple minutes but he'd fitted an oversize tire on his rear wheel. He (wrongly) assumed that any wheel/tire would work with a 53x15 for the limit, but when he went to rollout his rear wheel consistently failed by probably half a centimeter (or some minor thing like that). He was duly DQed.

What was frustrating about his experience was that he didn't need that - he was so good that he started carrying a spare tire in the TT because he could have a flat, stop, change it (tubular), keep going, and still finish with a reasonable expectation of winning the TT by a minute or so. He flatted one year and didn't finish, and based on his prior multiple minute winning margin he should have been able to change the flat and still win.

Remember that the gear limit really has to do with your tire's circumference. The gear is simply an easy way to refer to a given gear in a language others understand.

"Dude, what's your biggest gear?"
"7.465 m"
"What?"
"Well, it's 7.452 if I use that other wheel."
"Huh? What are you smoking?"

vs

"Dude, what's your biggest gear?"
"53x15"
"Oh. I'm running a 49x14"
"Oh."

cdr

DRietz

Aki, that's exactly what I was seeking. Thank you very much!

chado445510

yes, i believe the rule of thumb is one tooth in the rear = 3 teeth in the front

YMCA

53x13 = 49x12 (4 teeth)
42x21 = 40x20 (2 teeth)

As you can see, your 3-tooth "rule" is just an avg.

I know on the hardest gears currently used it can be 5 teeth per cog.

Mike_Sherman

Re: Mike Sherman's gear calculator

Hello, I just wanted to spread the word that, because of ISP issues, I have moved the calculator to GitHub.
Because this account is new, I'm not allowed to post a link, but you can message me and I'll send the links.

I'm also working on a new version to try to make the program friendlier on small screens.

-Mike

ToddTheBod

Yes definitely be careful with this "rule". It's really only applicable for stuff in the 84-100" range, applied anywhere else and you're gonna have a bad time

Bikeracer123

52/14 is the closest you’ll get to 26ft and it’s what most of us juniors (or atleast that I race with) run.

Psimet2001 

It really does depend on the wheels/tires.

52/14 is what my son has run for years. It is what I have set every junior I can up with. Getting a junior cassette and getting the right chainring can really help a lot of things. My son has 10 full gears and 2 full chainrings.

Every year at the first few races I have officials sending dads over with that scared look on their face "they said you need to block few out." and everytime there's at least one junior who has a 50t crank and an 11-25 cassette. "Sorry - your limit screw won't let me block out the bottom 3". At least if I am in the shop I can find a longer screw.

Seriously though just put 26' on the ground and check before you get there. With the right pressure in the tires.

caloso

I went the 45x12 route with my son. That allowed me to share wheels between his bike and mine. But it's super close to the limit. 25mm tires might put you over. So use a 44t front ring just to be safe.

ToddTheBod

FWIW, Wickwerks makes 41-33 chainrings specifically for juniors. 41-11 is closer to 26 ft than 52-14, and then lets you use 11-N cassettes.

Doge

My kid started with thin tires and a 53X14 would work. We later changed. I pushed RECON to create a junior (14T) cassette which he did and sells a lot of them. They are solid alloy. They do not shift as well as a Shimano, but are half the weight.
The Euro 15-16 kids race with 52X16.

Doge

Many of the SoCal kids, and ones at nats are riding parent hand-me-downs or getting used stuff on eBay.
I don't, never have used an 11 as an adult and have a 12 on the tandem. They are sold because people buy them, I don't really see the use.