I bought a Chromag stem in 35mm rather than the 31.8 that you often find on mountain bikes. I got it because it was a smoking good price but then later discovered what I thought was the length of the stem was actually the bar diameter.

Is there much benefit to running a 35mm diameter stem/handle bar?

I’m gonna give it a try along with a deity 50mm rise set of bars on my do anything Chromag Sylus. It should make it feel a bit more like the old BMX bars I used to love but I am wonder about what trade offs there will be with the higher, stiffer bars.

Stem length and rise and bar rise, sweep, etc., all have a significant impact on bike fit and handling.

If your bike fit was good before, adding significant rise will move you to a more upright position with your weight farther back on the bike and that might not be great.

Disclaimer: Most of the following comes from reading and talking to people as I don't ride hard enough for most of it to matter.

No one is breaking 31.8mm bars (at least riding
- maybe in a really horrific crash). 35mm bars aren't made because they're stronger.

If you're using metal bars, you'd probably never notice a difference. As far as I can tell, aluminum bars are all stiff enough they're effectively completely rigid.

Where people argue about it is with carbon bars. One of the benefits of carbon bike parts is that they can be built with a bit of flex (compliance) that helps soften some of the sharpest impacts, which helps reduce rider fatigue.

In handlebars, a little bit of vertical flex is great because it softens some of the jarring impacts from bumps or landings.

But horizontal flex (theoretically) isn't great, because it means there's a little mushiness or play in your steering.

People who like 31.8mm bars say the horizontal compliance isn't an issue and the vertical compliance is a big benefit.

People who like 35mm bars say they don't care much about vertical compliance (What do you have that front shock for? Huh?) but don't want any horizontal compliance.

It seems to be kind of a low-key holy war.

There are some manufacturers trying to bridge the gap, e.g., OneUp Components makes a 35mm carbon handlebar that has an oval cross section near the stem that they claim gives the vertical compliance of a 31.8mm bar and the horizontal compliance of a 35mm bar.

I put one of the OneUp bars on my bike partially because why not and partially because the stem I wanted in the length and rise I wanted I could only find in 35mm.

I went ahead and swapped out the stem and bars and so far I like it. The lower bars definitely made cornering more confidence inspiring but hitting a few jumps in the front yard with the new bars and stem felt good.

Hardtails forever!