Some kind of spring caliper, but you’d need to be able to release it to get it off the rim whilst still being able to reset it to its original position to then measure the gap…

Use a vernier and a little spacer

Small diameter micrometer? damn it waswas, it’s a posh digi. one too 😛

vernier caliper & a dremel

Use a vernier and a little spacer

That is a fine idea!

Will the spacer sit flat against a worn rim? Is the inside of the rim parallel? Does the rim have wear indicators – most do. Mavic’s appear when worn, others disappear when worn.

Cut a piece out and use a ruler?

The example picture posted by jam bo is the answer … an iwanson spring calliper. Cost less than £5 on eBay. Been meaning to get one for ages.

A spoke bent to fit makes a perfect spacer.

aracer – Member

A spoke bent to fit makes a perfect spacer.

This.

Like this:

I wouldn’t bother, I would realize I’m riding last centuries technology and upgrade to discs 😉

They are Mavic Open Pros. What wear indicator should I look for?

Mavic explain how their rim wear indicators work on here;

http://www.tech-mavic.com/tech-mavic/technical_manual/data/docs/products/2_119.pdf

INTERNAL:
Process:
The inside of the braking surface of the rim is machined on both of the wings of the rim.
Principle:
When there is too much wear on the rim, a little hole appears on each of the 2 braking surfaces of the rim. Depending on the ad
justment of the brake pads, it is possible
for the wear indicator to appear on only one of the 2 braking surfaces. In any case,
the appearance of the wear indicator on at least one of the 2 braking surfaces
means that the sidewalls are too thin, and it could be dangerous to continue to use the rim. It should be replaced as soon as p
ossible.
The position of the wear indicator is marked by 2 yellow arrows on the stickers on the rim, opposite the valve hole.
Refer to the chart on the previous page to find out which rims offer this internal wear indicator.
INTEGRATED IN THE PROFILE:
Process:
the wear indicator is an integral part of the rim. There is a groove on the entire circumference of the rim, at the center of t
he braking surfac
Principe :
The groove becomes more shallow as the braking wears down the surface of the rim.
Its disappearance, on one side of the rim or the other, means that
the thickness of the braking surface is too thin and it could be dangerous to continue to use the rim. It should be replaced as
soon

In the past I’ve used the top of the braking surface cracking off and making an aluminium hoolahoop around the axle as a wear indicator 🙂

I too have used that technique.