Richard Mille

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How much RM035 really weighs

I've seen greatly varying accounts of how much the RM035 weighs. In this forum, 35g is mentioned. On various versions of the announcement I've seen both 41g and "under 20g". On Richard Mille's web site, the specs for RM035 list the exact weight of the movement, along with other minutiae, but no mention of the total weight of RM035. This is surprising, since the RM027 is mentioned as being 18g with the strap, and 13g without the strap.

Fortunately, it's possible to figure a fairly precise estimate of the weight of RM035, establishing it is around 20g, and certainly much lighter than the 35g mentioned here.

It's possible to make this estimate based upon measures of the RM010. Here's how I did it ...

First we need to know the volume of the case metal for the RM010. Using a precise gram scale, I weighed my titanium RM010 and found it to be 65g with the kevlar strap and titanium deployant (actually 64.99g was the weighing). I have a friend who weighed his white gold RM010 and got 165g. Thats's a difference of roughly 100g (assuming his 165g weighing with a less precise balance is accurate). Since titanium has a density of 4.5 g/cc and 18K white gold has a density of 16.9 g/cc, it's easy to compute the volume of the case material to be 100g/(16.7 g/cc - 4.5 g/cc)=8.2 cc.

I should mention, this volume computed is the total volume of all the titanium metal on my RM010, including both case and buckle (my friend's RM010 has a white gold case and white gold buckle, but everything else is the same on our RM010s).

Let's assume that the RM035 has a case that is no thicker than the case of the RM010. It wouldn't make sense for it to be thicker if they are trying to reduce the weight! The density of the magnesium-aluminum alloy on the RM035 case and buckle is 1.8 g/cc. So, if the RM035 had the exact same dimension case as the RM010, the RM035 would be lighter by the following amount: 8.2cc(4.5g/cc - 1.8 g/cc)=22.1 g.

But, we have to adjust for two things. The RM035 is actually a little thinner (12.25mm vs 13,84mm) than the RM010 and the RM035 does not have a sapphire display back. The 2 sides of the RM035 are roughly 13.84mm - 12.25mm=1.59mm=0.159 cm thinner. Assuming a case thickness of roughly 0.2 cm (2 mm) and a length of each side roughly 5cm (it's 48mm but curved), then the thinner RM035 is also this much lighter: 2 (1.8 g/cc)(0.159cm)(0.2cm)(5.0cm)=0.57 g.

More of a savings results from not having the sapphire display back. The RM010 has a sapphire display back that is 1.0mm in the center and 1.7mm at the edges. With a little integral calculus it follows that a sapphire back with a uniform 1.39mm would have the same volume. The display back is roughly 28mm x 30mm. Sapphire is 4g/cc, so its mass is roughly 4g/cc(0.139cm)(2.8cm)(3.0cm)=4.7g. Assuming once again a case thickness of 2mm on the RM035, it has a mass of magnesium-aluminum case in place of this sapphire case back weighing a total of 1.8g/cc(0.2cm)(2.8cm)(3.0cm)=3.0g. So, not having the display back means a savings of 4.7g - 3.0g=1.7g.

Much more savings comes from the strap. The RM035 has about the same polyurethane strap that the RM027 uses. Since the RM027 weighs 18g with that strap and 13g without that strap, the polyurethane strap weighs 18g - 13g=5g. I weighed an old one of my RM010 kevlar straps (identical to the one on my RM010) and it came to 10g. So, using the polyurethane saved 10g - 5g=5g.

Further savings comes from not having the titanium deployant which weighed in at 7g.

The movement of the RM035 is essentially a RM010 movement with a few modifications to make it lighter. The main modification is removing the rotor, which I've heard weighs about what a US nickel weighs, or 5g.

Putting all this together, we can estimate the weight of the RM035 from the weight of the titanium RM010 as follows:

65g (Titanium RM010) - 22.1g (case material) - 0.57g (case thickness) - 1.7 (no display back) - 5g (lighter strap) - 7g (no deployant) - 5g (no rotor)=23.6g.

So, the "less than 20g" is posible if there are 3.6g of further savings in the movement rework. But, it should be quite clear from the above estimates that 35g and 41g are too heavy for RM035.

In any case a weight of 23.6g is quite impressive for the RM035. That's about 1/3 the weight of a RM010! You'd think that Richard Mille would want to mention that in its press releases, instead of simply mentioning the weight of the movement.

Greg

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