## Metres of development with wider Rims

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JBC2
Posts: 14
Joined: Sat Jan 13, 2018 4:51 pm
What is the rollout impact for these newer wider rim wheels?

Reading on another thread that 23mm Conti GP4000s inflate to 25mm at 90psi on Zonda C17's.

Albeit minimal the change from 23 to 25mm will impact the roll-out distance.

An expensive mistake to make if you are supporting a Junior rider who has a gear restriction in the UK and Europe and you find the combination takes you over the white line!

I have sent plenty of Youth riders fail gear checks using standard 25mm tyres, BCF regs are designed for 23mm max.

Many thanks

basilic
Posts: 653
Joined: Wed Jan 05, 2011 8:05 am
Location: Geneva, Switzerland
No experience, sorry.
What matters is the diameter of the wheel. Can you measure that with precision? Then multiply by pi (3.14) to get wheel rollout. Then multiply by chainring/cog teeth ratio to get development.
If going from a 23mm tire to 25mm increased radius by 2 mm (probably not, as the wider rim will mitigate that), circumference would increase by 12.56mm. With a rider sitting on the bike the tire height might decrease by 15% or so, so effectively only 10.x mm? depends how they measure it. On a wheel circumference of 2110 mm it's just about 0.5%.

NovemberDave
Posts: 169
Joined: Sat Oct 09, 2010 11:42 am
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You have to measure to be safe.

Perhaps counterintuitively, tires are not shorter on wider rims. They are generally very slightly taller on wider rims.

The rough conversions we've measured on A LOT of rims and tires in the 14 to 20mm internal rim width and 22 to 28mm tires is that for every 2mm of internal rim width gain, a tire will gain 1mm of inflated width and .5mm of inflated height. That would imply that a given tire on a 14mm internal rim would gain 1mm of height when put on a 20mm tire, which would add roughly 6mm to the rollout measurement.

You have to measure to be safe.

JBC2
Posts: 14
Joined: Sat Jan 13, 2018 4:51 pm
Very interesting feedback Dave and exactly covering my concern.

BC regulations are not likely to change so I would imagine this situation is one that is going to catch out a lot of parents/riders this and for seasons to come.

mattr
Posts: 3458
Joined: Fri May 25, 2007 6:43 pm
Location: The Grim North.
I know when i used to do bike checking even some of the larger 23 tyres would fail, then pass if you lowered the pressure.......

wingguy
Posts: 3672
Joined: Thu Mar 08, 2012 11:43 pm
JBC2 wrote:
Mon Jan 15, 2018 10:49 am
BC regulations are not likely to change so I would imagine this situation is one that is going to catch out a lot of parents/riders this and for seasons to come.
Well, rollout's always been a bitch hasn't it? As always, it'll only catch you out if you haven't tested and measured the wheel/tyre/pressure combo you take to the commissaires.

Worst case you end up needing to source some 21 or 22mm tyres to make it right. Annoying, but not the end of the world.

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