BC's restrictions on gearing - what gears would give 5.1m roll out?

[TPMB12]

This is only an approximation I'm looking for as tyres will affect this.

I'm wondering if anyone can tell me what commonly found gears in kids bikes (20" wheels in my sons case) will conform to the British cycling gear restriction for an under 8 cyclist? The gear / tyre combination must not exceed 5.1m when you measure one complete rotation of the pedals when wheeling backwards.

Assume 20" hybrid wheels like kids Kendal small block. It's a frog 52 bike BTW.

I'm hoping I'll not have to restrict it by one less gear.

BTW I'm planning ahead as I've found out the local youth cycling club takes them a year younger than I had been led to believe. Our son's a keen cyclist and I intend to support that.

[Jakob W]

According to the tech spec the Frog 52 comes with a 32 chainring and an 11-32 cassette, and with ETRTO 40-406 tyres. That gives an effective wheel diameter of .486m, and thus, if my calculations are correct (.486π x 32/11), a top gear development of 4.44. You should be fine with a 36 chainring or smaller (though, depending on how oversized the tyres are, a 36-11 gear would be getting close).

[citoyen]

Can't really add anything to Jakob W's post, except to say that I entered the numbers into bikecalc.com and they corroborate everything he says.

Worth trying it for yourself - https://www.bikecalc.com/gear_meters_of_development

[Brucey]

this is my favourite gear calculator

http://ritzelrechner.de/

calculates in gear inches or development (amongst other things), and  allows for many different tyre sizes too.

cheers

[rogerzilla]

It's just gear inches* x pi/12 x 0.3048 (metres in a foot)

*which most of us understand if we've ever ridden fixed or singlespeed

True story - Greg LeMond used to fit a chainring with one tooth more than everyone else (53 instead of 52, probably) for junior races.  The rollout test was sufficiently imprecise that he was always allowed to compete.  The judges didn't look at the rings and sprockets, they literally rolled the bike backwards one pedal revolution.  He wasn't actually breaking any rules.

[TPMB12]

It's got 47-406 tyres on at the moment which gives a slightly higher figure. Still within the rules. In fact the highest gear is well below the limit.

TBH I'm not really interested in trying to get as close as possible because it's all about fun at 6 years old. Enough time to get truly competitive in a marginal gains kind of way IMHO.

[mzjo]

It's just gear inches* x pi/12 x 0.3048 (metres in a foot)

*which most of us understand if we've ever ridden fixed or singlespeed

True story - Greg LeMond used to fit a chainring with one tooth more than everyone else (53 instead of 52, probably) for junior races.  The rollout test was sufficiently imprecise that he was always allowed to compete.  The judges didn't look at the rings and sprockets, they literally rolled the bike backwards one pedal revolution.  He wasn't actually breaking any rules.

This is how it was done in my young day (1970's). The machine examiner had a distance marked out on the ground, you started with the pedal pointing down at the first mark, then wheeled the bike backwards towards the second mark at which point the pedal would once again be pointing at the mark (or have already passed the vertical if the gear was a lot smaller). Something that I didn't know and which would undoubtedly have allowed me to start with a 49/15 instead of 49/16 - since the exact gear was supposed to be 48/15. It didn't make any difference in my one race attempted as I was shelled out the back on the first hill and never got close to getting back on again!

[citoyen]

This is how it was done in my young day (1970's).

Exactly how it is still done now.

[Brucey]

....  He wasn't actually breaking any rules.

he was, he just didn't get caught....

an ideal apprenticeship for life as a professional cyclist....

cheers

[yoav]

You can also restrict the gears by screwing down the limit screw so that the mech cannot change to a higher gear. That is also allowed.

[drossall]

Something that I didn't know and which would undoubtedly have allowed me to start with a 49/15 instead of 49/16 - since the exact gear was supposed to be 48/15. It didn't make any difference in my one race attempted as I was shelled out the back on the first hill and never got close to getting back on again!

But the exact gear wasn't 48*15, it was 86.4". That's only 48*15 on 27" wheels. Since in practice everyone was riding 700Cs, and probably tubs, the real wheel diameter is less. I'm not sure by how much, and it's not very much, but your gear was only 2% higher than 48*15 would have been so, as long as your wheels were 2% smaller, you really were legal. In reality, of course, the depth of your tyres (whatever their notional size) and, theoretically, the pressure at which they are inflated both affect your actual wheel diameter, and therefore your gear in inches.

[IanDG]

Only way to be certain is to do your own 'roll test'. It's a long time since I rode gear restrictions (1979) but at the time we all rode different (but acceptable) chain ring/sprocket combinations

[giropaul]

As a schoolboy and junior I often used Barnum tubulars. They were very low profile and worth a tooth or so

I don’t think I’d paid enough attention in Maths to realise that if the gear rolls out on the limit then there’s no advantage on all the others whose roll outs are the same - no matter what the ring and sprockets sizes.

In The Netherlands and Belgium youngsters years ago had to ride on single freewheels with specific tooth numbers. Tyres might have made a slight difference in that situation.

[drossall]

I don’t think I’d paid enough attention in Maths to realise that if the gear rolls out on the limit then there’s no advantage on all the others whose roll outs are the same - no matter what the ring and sprockets sizes.

Marginal gains - you believe that you're stealing a march on everyone else, so you go that bit better, even though it's nonsense

I often wonder why people didn't pay more attention in maths and physics. Maths helps you win a junior road race. Physics tells you why breaking speed limits by 5% makes a 10% difference to everything that matters about the subsequent accident. And so on.

[Brucey]

the problem with the rollout test as it is usually implemented is that it relies upon the person doing the test to say if the crank is really pointed at a particular spot on the ground or not. Rather than have an argument, they often say "it does" when in fact it clearly doesn't.

 Errors are substantial; if the wheel size is a skinny 700C and the gear is about 72" (as in many TTs with a medium gear regulation) then the gearing between sprocket and chainring is about 3:1. This means that if the crank points 1/2" away from the required spot on the ground, the bike is actually travelling about 1-1/2" further with each turn of the pedals.

 BITD it was common for folk to be allowed to ride 42/15 gearing in medium (72") gear events; even with quite a skinny 700C tyre this is well over 72" and folk with (more accurately determined) 72" gears, (such as 44/16) would be at a significant disadvantage on tailwind sections, having to pedal about 2% faster.

If I were responsible for measuring rollout, I'd explain well in advance that I'd set a spirit level on the crank at the start position and if the bubble were one way after a 226.2" rollout they would be allowed to ride and if it were the other way they wouldn't. Simple as that; next to no interpretation required, so no wriggle room and no arguments. Anyone can do the same test at home, so no excuses either.

cheers

[rogerzilla]

But the ground has to be perfectly level at the start and end of the rollout!

[Brucey]

But the ground has to be perfectly level at the start and end of the rollout!

no it doesn't; it just needs to be the same gradient at the start as at the end, i.e. reasonably flat, rather than exactly level.

cheers

[Jakob W]

Isn't the spirit level method independent of any slope? The bubble is oriented to the local gravity vector, which should be the same at beginning and end of the rollout...?

[DuncanM]

No, because the theory the cranks from parallel to the ground, to parallel to the ground, assuming that the ground is flat and measuring the angle using gravity. If the gradient at the start and the end are different, then the "parallel to the ground" bit is not a good assumption.

[Jakob W]

Yeah but 'cranks horizontal according to the level' to 'cranks horizontal according to the level' is also a 360° rotation, no?

[yoav]

Does anyone in this thread actually do gear checks on a regular basis?

[Brucey]

Isn't the spirit level method independent of any slope? The bubble is oriented to the local gravity vector, which should be the same at beginning and end of the rollout...?

it is, but the tangent angle of the tyre contact may be different at one full turn of the cranks (meaning that the distance travelled by the wheel centres is not quite the same as the distance between the start and finish tyre contact points..... and if the ground is ripply the distance travelled won't be quite right for this reason either. The latter effect (alone) will always result in the gear appearing slightly smaller than it really is, but may be overwhelmed by the former effect which can introduce an error in either direction.

Yeah but 'cranks horizontal according to the level' to 'cranks horizontal according to the level' is also a 360° rotation, no?

yup, it is, hence a slight slope is OK but ripplyness might not be.

In reality everyone would be judged by the same yardstick (ripply or not) at the event and what you would keep schtum about is that actually you have a small leeway so that you don't accidentally disqualify gears that are actually spot on but that you measured wrongly. You would be able to get the accuracy much improved vs traditional rollout test, anyway, quite easily.

When riding/loaded the wheel doesn't travel exactly the distance suggested by the circumference of the tyre (because the tyre deforms in a way that doesn't conserve tread length ) and indeed the deformation will vary with the tread thickness and carcass construction too.

Under torque load there is always some slippage of the tyre too, so the harder you pedal, the smaller the gear gets....

cheers

[giropaul]

Does anyone in this thread actually do gear checks on a regular basis?

I’ve done a few this year. The circuit has a nifty u-shaped trough that pins together and is marked for all the age group roll-outs.

Is it 100% accurate? I doubt it, but it’s certainly near enough. It’s a children’s bike race we’re checking, not a precision weapon!

[yorkie]

Does anyone in this thread actually do gear checks on a regular basis?

Yes, weekly or more often in the summer, less frequently at other times of year for both track and circuit racing. Mainly using a marked channel as per Giropaul's post above. If the bike is close to the limit, I will give it another go to make sure, but that is very rare - usually the gears are way over!

[yoav]

Me too, and I just don’t see what the the problem is. Gear check fails are mostly because the parent doesn’t know or understand the rules. Once it it explained to them they are fine. Deliberate cheating is rare (in my experience).