Stem shims and school maths

[Titiritero]

Thinking in getting a Moto bar, mostly because of the big rise. Problem is that they mostly come in 22.2mm clamp size.

I have a 25.4mm stem, and some spare 31.8 to 25.4 shims. Using my school maths, I get that:
31.8mm /2 /3.14 = 5.0611mm radius
25.4mm /2 /3.14 = 4.0425mm radius
22.2mm /2 /3.14 = 3.5332mm radius

Which means the shims I have (25.4 to 31.8 ) need to cover a 1.0185mm difference, which is exactly double the difference of 22.2 to 25.4 (0.5092).

And that leads me to think I could use just one shim instead of 2 to fit the 22.2m bar in my stem.

If you were able to follow until here... am I correct?

 

[stephan]

I know the example is a seat post - but the sizes are the same:

Shim Size Required I Guess:

ID= 22.2mm
OD= 25.3mm

https://www.unicycle.co.uk/seatpost-shim-25-4-to-22-2mm.html

Here is a split shim for easy install for MTB bars:

Gusset Handlebar Shim

<p>Allows you to fit smaller diameter bars in your larger stem. <br></p> <ul> <li>Black anodised alloy</li> </ul>

winstanleysbikes.co.uk

I would buy the correct size so you have a shim on each side with the same material, unsure if only one shim would put extra stress of the softer material either bar or shim. (also it wont work as the radius with one shim would knock it off)

 

[CassidyAce]

Thinking in getting a Moto bar, mostly because of the big rise. Problem is that they mostly come in 22.2mm clamp size.

I have a 25.4mm stem, and some spare 31.8 to 25.4 shims. Using my school maths, I get that:
31.8mm /2 /3.14 = 5.0611mm radius
25.4mm /2 /3.14 = 4.0425mm radius
22.2mm /2 /3.14 = 3.5332mm radius

Which means the shims I have (25.4 to 31.8 ) need to cover a 1.0185mm difference, which is exactly double the difference of 22.2 to 25.4 (0.5092).

And that leads me to think I could use just one shim instead of 2 to fit the 22.2m bar in my stem.

If you were able to follow until here... am I correct?

Er, no. (Not if I follow you correctly.) Pi doesn't come into it.

The radius is just half of the diameter so, for example, if the diameter is 31.8mm, the radius is 31.8mm / 2 = 15.9mm.

However, if you want a shim to fit a 22.2mm bar snuggly into a 25.4mm stem, to get the correct thickness of shim, you need to work out the difference in diameters, then divide by 2.

(25.4mm - 22.2mm)/2 = 1.6mm. The 22.2mm --> 25.4mm seatpost shim (and handlebar shim) mentioned above should be 1.6mm thick.

 

[stephan]

Er, no. (Not if I follow you correctly.) Pi doesn't come into it.

The radius is just half of the diameter so, for example, if the diameter is 31.8mm, the radius is 31.8mm / 2 = 15.9mm.

However, if you want a shim to fit a 22.2mm bar snuggly into a 25.4mm stem, to get the correct thickness of shim, you need to work out the difference in diameters, then divide by 2.

(25.4mm - 22.2mm)/2 = 1.6mm. The 22.2mm --> 25.4mm seatpost shim mentioned above should be 1.6mm thick.

Yea and have one shim on each side at approx 1.6mm thick on both, I agree! I would just buy the correct shims for the job, they are not expensive. "However, if you want a shim to fit a 22.2mm bar snuggly into a 25.4mm stem, to get the correct thickness of shim, you need to work out the difference in diameters, then divide by 2"

 

[Titiritero]

Thanks to both! You are right, pi was not needed, I mixed up diameter with overall circumference. However, my theory is still right, since I would need 2x1.6mm shims, which is the same as one 3.2mm shim of those used for a 31.8 stem. Although I agree that a two side shim application would be better than one sided...

 

[stephan]

Back in late 70's early 80's I used Zinc plate to make my BMX seat post shims, back then things like shims were hard to get, Zinc is soft and corrodes approximately 100 times slower than other metals such as alloy and steal, so great material for shims (stopped the alloy post seizing into the Crome-Moly frame). However I don't suggest making one out of Zinc, plenty to buy from a whole seller those day that are accurate in size.

 

[Dr Dremel]

Thinking in getting a Moto bar, mostly because of the big rise. Problem is that they mostly come in 22.2mm clamp size.

I have a 25.4mm stem, and some spare 31.8 to 25.4 shims. Using my school maths, I get that:
31.8mm /2 /3.14 = 5.0611mm radius
25.4mm /2 /3.14 = 4.0425mm radius
22.2mm /2 /3.14 = 3.5332mm radius

Which means the shims I have (25.4 to 31.8 ) need to cover a 1.0185mm difference, which is exactly double the difference of 22.2 to 25.4 (0.5092).

And that leads me to think I could use just one shim instead of 2 to fit the 22.2m bar in my stem.

If you were able to follow until here... am I correct?

How would that work? Even half of a 22.2 - 31.8 shim will not fit into the 25,4mm clamp of the stem. It still has 31.8mm diameter. And even if it would fit, it does not magically wrap around the handlebar, so it would not make contact all around.
Or was this a purely mathematical question if the one 31.8 shim has the same volume as 2x 25.4 shims?