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I want to ask a physics-related question, I'm a male, 162cm, height 50kg/110lbs (I'm healthy, but too light!).

Right now I'm building a Surly Cross Check and I'm confused what wheels I should purchase. So I have two options: Alex DM18 (cheap) and Alex DH19(more expensive). They have similar inner rim width (17.9mm an 18mm). I really want DH19 but I'm little bit overbudget on other parts. I plan to use my bike for commuting and a little bit offroad. If I use the bike for offroad, do DH19 rims will stay true longer? Or because I'm thin, would using DM18 have insignificant difference?

Edit: the plan is to build an ss cx bike, no consideration of cassette/drivertrain

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    Depends more on how well assembled the wheel is, and maintenance it receives than the rim quality. – mattnz Mar 10 '15 at 3:03
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You get what you pay for. The DM18 is a cheap rim with good enough reviews rating it good value for money. The DH19 is a better rim - only you know if you can or want to stretch your budget, however "Quality is remembered long after price forgotten" comes to mind. Are you prepared to regret not spending that extra money every time you go for a ride?

Provided the wheel is built well and maintained, the cheaper rim will stay true (its the spokes that keep the rim true, not the rim). The DH19 has eyelets for the spokes - meaning the rim will handle more abuse and last longer (without eyelets the spoke holes tend to fatigue and crack). However, as you are only 50kg, I expect the cheaper rim will easily handle the riding you are suggesting. If you were 150kg, I would steer you to the DH19.

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I have a hardtail MTB with much older Alexrims AD-something (the numbers on the decal are gone now) They are doublewall, 32h, but have no eyelets on the spoke holes. I have had them for at least five years of moderate XC, and road use.

Since they where laced somewhere near 2010 they never went out of true, and I know because I use V-brakes. My weight ranges from 150 to 175 lbs (68 - 80 Kg) and add to that roughly 8 lb (4 Kg) of gear, water and tools. I'm an aggresive descender (for XC) and even perform jumps and drops on this bike.

What I'm trying to say is that even though there are quality diference between the two rims that you are comparing, the "lower" one is not bad at all, so, if budget is a limitation, rest assured that in this case, the low priced one will perform well.

I agree with Mattnz in that the quality of the wheel depends way more on the quality of the workmanshipwhen lacing and truing the rims, and that providing the adecuate maintenance is key.

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As for the primary question - yes if you are lighter you put less stress on the wheels.

As for the secondary question of building a SS CX bike.

I am going to assume you don't CX race now. If you do race and really want to race this bike SS then just let me know and I will delete this answer.

Most CX racers use clincher or tubeless for lower pressure.

Be aware the SS heat is brutal. SS on sand, flat, and uphill takes a lot of power. The single speed heat will also have all categories of riders (cat 1 - 5). Typically it will only have cat 3 and better riders.

The Cross Check is true CX design but you don't see it in races as it is heavy. It is a great commuter bike. Since you are light a heavy frame effects you more than someone that weighs more. And extra weight is more penalty in SS.

Yes the Cross Check is a commuter you can race CX but if your end goal is a race bike then that is not a good project frame.

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