I have an under 16 doing their first sanctioned race at the weekend which means a gear distance of 6.93 metres

Current set up is 50/34 & 11/28 – keeping the front chainset if a swap the cassette for a 14/28 will this pass or will i need to then restrict down to the 16th cog? Ive used a couple of calculators which seem to give conflicting information

You’ll need to physically measure. The ratio side of it is fairly straight forward, but once you factor in tyre circumference, it throws a lot of things out. Even going from (say) a vittoria to a Conti tyre of nominally the same size can put you over.
I’ve DQed kids before now, bu usually turned a blind eye for lower level racing and small mistakes.

There are (IIRC) some commisaire guidelines for how to do the gears, at normal regional and lower racing, you can lock out with a longer limit screw, for nationals, you’ll need the correct sized cassette.

IIRC the recommended ratios are usually 52×16 for U16 and 52×14 for U18 so your 50×14 will be massively on the large side. 50×16 will leave your rider at a slight disadvantage.

Assuming 700c wheels you’ll need to limit the rear mech to 16T which is 6.6.
Youth gear restriction of 6.93 is based on 52×16 on 700 x 23 wheels.

thanks guys – dont suppose anyone has a lionk to a shop that has a 16t cassette currently in stock – as usual needed urgently!

Shimano make a 16-27 Ultegra cassette, but they’re not cheap. Why not just set the limit screw on the rear mech?

What event is it? Ot it might be easier to get the nod from the organiser (or commisaire) as to what will be required, locked out with the limit screw or proper cassette.

And are you 10 or 11 speed? (Don’t think 11 speed has a 16 up cassette available yet)

I might have some junior ultegra cassettes in the loft, 10sp mind. Likely 14t, but your welcome to them if any use.

11 speed, shimano…

Just set the limit stop on the RD. You will need to measure the exact roll out distance for the tyres you are using and at the pressures. All closed circuits will have these recorded with markings, and bikes do get checked regularly.

Junior cassettes are expensive because there is low demand. The 50 makes it a little more complicated, as the original numbers were set for standard 52 chainsets.

BTW:
52/16 = 3.25
34/11 = 3.09

Not a huge amount in it. If you have a smaller block, you can lock the front derailleur and run 11-25 or even 11-23. It is his first event, after all.

https://www.britishcycling.org.uk/zuvvi/media/bc_files/officials/Youth_Gear_Restrictions_-_Riders__Parents.pdf

BRITISH CYCLING GEAR LIMITS
CATEGORY GEAR LIMIT EXAMPLE FOR 700c WHEELS and a 23mm tyre.
Age category for the year. Roll out metres Approx Gear Inches Chain ring/Sprocket combinations that work.
Youth E Under 8 at 1st Jan 5.10 metres 64 inch 39×16 / 41×17 / 43×18 / 48×19
Youth D Under 10 at 1st Jan 5.40 metres 68 inch 41×16 / 43×17 / 46×18 / 49×19
Youth C Under 12 at 1st Jan 6.05 metres 76 inch 39×14 / 42×15 / 45×16 / 48×17
Youth B Under 14 at 1st Jan 6.45 metres 80 inch 39×13 / 42×14 / 45×15 / 51×17
Youth A Under 16 at 1st Jan 6.93 metres 88 inch 39×12 / 42×13 / 47×15 / 52×16
Junior Under 18 at 1st Jan 7.93 metres 100 inch 52×14

Setting the limit screw will be fine, that’s what most people will be doing. It’s an early season U16 event, the scrutineer will mainly be checking to ensure that the bikes are safe – nothing loose, bent or broken and brakes working OK.

You can check the roll out yourself if you’re really worried. Two bits of tape 6.93.m apart, front wheel axle over the first bit, cranks vertical, roll the bike backwards until the cranks have gone round once and see if you’ve passed the 2nd bit of tape.

11S shimano, you’ll probably only be looking at BBB cassettes. Can’t find anything else.

Where is the race? I am at a couple at the weekend and may be able to lend you a wheel with 16-27 cassettes on. I have both 10 and 11 speed (two sons currently racing U16!!).

Edit: – or do as the other suggestions and lock out the offending gears (perfectly acceptable).

Hope he gets on ok today. If he has never done one before, it is all about the start. Tell him to get as close to the front as possible on the line (don’t be intimidated by anyone) and clip in as quick as possible then pretend it is a 2 lap race. They always go off like maniacs but if you can hold on for the start you stand a good chance of finishing with the bunch.

They will ease the pace off a bit after the first couple of miles.

Let us know how it goes.