Chrina Rims - too tight for tyres

[Des49]

Remember ............ at the risk of repetition of this subject ............. Chrina rims vary.
I had a "pair" that were different ERDs. One tyre was ok on one rim, but NOT on the other.

Yup, and this mug purchased 4 rims in one go and all are the tightest rims I have ever experienced. So tight as to be unacceptable for some users.

Hopefully this thread serves as a reminder and warning to others.

[Gattonero]

Remember ............ at the risk of repetition of this subject ............. Chrina rims vary.
I had a "pair" that were different ERDs. One tyre was ok on one rim, but NOT on the other.

Do you have a repeatable and precise way to measure the ERD?

[Mick F]

I use the Two Spoke Method.

Get two spokes, cut off the hooked ends and superglue the nipples on.
Thread each spoke in opposite holes and measure the gap between the two.
Do this on a few places round the rim.
Add the average gap to the sum of the two spokes, and that's your ERD.

The two Chrina rims were different by a few mm. Sorry, can't remember the figures, but it made the difference between each rim quite noticeable when fitting tyres. One easy, the other tough.

[Gattonero]

I see.

Could be the combination of thickness & force the eyelets were pressed in, and roundness of the rim.
Did you measure the ERD in two opposite patterns?

[Brucey]

FWIW I've built quite a few chrinas and none of them were more than a small fraction of a mm out of round. However they did vary in ERD, and with it the fit of the tyres varied too.

The reason this happens is entirely because the extrusion was not trimmed to the correct length when the rim was manufactured. The acceptable 'correct length' varies with the rim design; a shallow-welled rim (at top limit) oughtn't be allowed to be quite the same OD as a rim with a deep well.

I now have a 'rollout gauge' which allows me to accurately measure the rim circumference (even if the rim isn't quite round) and this can give a super-accurate ERD for building, too, again even if the rim isn't quite round.

cheers

[Gattonero]

I know it does not provide a solution, but this just makes me think that you get what you pay for.
Having built lots (which means 3-digit figures) of Mavic, Ambrosio and Rodi, I never had any discrepancy that was ever worth mentioning. In fact, I cannot recall any of those rims being unreasonable tight with any tyre, or to give any noticeable difference with the spoke length.

[Brucey]

I've had both Mavic and Ambrosio rims that were large/tight. Not many, but some rather than none.

You will only notice the variation in spoke length if you are fussy about spoke length.

cheers

[Gattonero]

Well, if a rim has an ERD difference greater than 3mm (<1.5 or >1.5) then it's not me being "fussy" when the spokes do not thread as much as they should.
If a rim is so inconsistent in ERD that the usual spoke length, typically ending with +0.5/1.0mm of spoke poking out a 12mm nipple when on full tension, then it becomes 1mm under the nipple end, I call it "not good for a decent mileage".
Surely it's not a disaster, but it's not optimal either.

[Brucey]

a 3mm variation in diameter would be enormous; it would mean that the rim would have been cut wrongly by +/-5mm and that there would be a 10mm variation in the optimum bead length for the tyres that fit.

ERD variations that accompany variations in tyre fit are typically smaller than that.

cheers

[NATURAL ANKLING]

Hi,
3mm is a mile as said.

How do you know you have a good quality rim.....................

Circumference is accurate but you have to do your own nipple add ons, no hardship.

I am finding tables not accurate, can anyone recommend some?
Think I will consult table - two spoke measure and circumference in future.

[NATURAL ANKLING]

Hi,

I now have a 'rollout gauge' which allows me to accurately measure the rim circumference (even if the rim isn't quite round) and this can give a super-accurate ERD for building, too, again even if the rim isn't quite round.
cheers

I am curious which bit you are measuring? So I can do it
Recessed rivets means you are then making adjustments on erd circumference to get it accurate?
Ta,

[Brucey]

basically you measure the rim circumference by rolling it out on a flat surface (needs to be about 2m long for 700C). Then fit a nipple to the rim and measure the depth from the rim edge to where you want the spoke to finish (eg to the bottom of the nipple slot).

Then

(rollout/pi)- 2x(depth measurement) = ERD

an example; rollout = 1998mm, depth = 18mm.

1998/pi = 636mm. 636mm - 36mm = 600mm, so the rim is 600mm ERD.

It is important to realise that this method works very accurately even if the rim is not round. Even a slightly deformed rim can measure 2-3mm out on diameter, and even multiple diameter measurements cannot eliminate such errors entirely.

The largest source of error with this method is potentially in the depth measurement; a 0.5mm error in the depth measurement will lead to a 1mm error in ERD. By contrast a 1mm error in rollout will only generate a ~0.3mm error in ERD.

cheers

[NATURAL ANKLING]

Hi,
OK, that's what I thought you would do it.
Measure a circumference then adjust with a depth measurement.
Ta,

[fausto99]

Have only skimmed this thread, so aplogies if I missed something. Are these rims welded or pinned? If pinned, has anyone tried to split the join and then filed a few mm off? This would be a better method of reducing the circumference rather than reducing the diameter of the shoulders.

[Mick F]

Pinned.
I had a wrecked Chrina rim, so had a play with it.

The pins are plain steel, and mine were a little rusty, so it was difficult to pull the joint apart even though I'd cut it out of the rim.
I would be possible to pull the joint apart with a brand new rim, but from what I remember, the pins stuck out of the joint by an inch or so. You'd have to pull quite hard, and you could damage the rim ................ but if something is assembled, it can be disassembled .......... so I don't see why you can't do it.

However, I wouldn't touch a Chrina rim with a barge pole any more. I've had a few, but the last lot were the worst.
Two different ERDs, and one had the brake area "lumpy" giving a pulsing to the braking. It eventually wore in ok, but it was terrible before it did.

I've since splashed out with Ambrosio Excellence. Absolutely wonderful rims, perfect in every way. £50odd each though, but you do indeed get what you pay for.