MTB Handlebar Diameter Debates - 31.8 vs 35 and more

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sspomer
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Which handlebar diameter do you choose and why? 31.8, 35mm or something else?

Here's a super scientific test from 6 years ago to get you thinking, too. (starts at 7:31)

Poll

My Preferred Mountain Bike Handlebar Diameter is...

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Primoz
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1/23/2026 9:29am

Based on how it's worded, 31,8. Because it can be shimmed to fit a 35 mm stem. 

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seanfisseli
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1/23/2026 9:29am

what are the good renthal 31.8 bars? i want to try those...

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bikelurker
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1/23/2026 9:30am

22.2 is where is at. Also:

Is the difference between 31.8mm and 35mm of the SAME handlebar really that noticeable? I went from 31.8mm Renthal Fatbar Lites (as they come precut to my favoured length) to the 35mm ones (only because I got trapped in the 44mm-same-as-the-fork-offset Fraezen Hype stem, wich is super nice piece of equipment, don't get me wrong) and it feels the same, although I ride with Odi longnecks glued on, wich mute a lot of feedback.

I rode and can definitely feel flex in the handlebar (I've got a trails bmx setup for pumptrack use and the handlebars are light and flexy) and I am also into ATB/alt handlebars, so I've owned some nittos in the past that were flexy enought that you can see them deflect... But my new Renthals just feel like... Renthals!

 

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seanfisseli
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1/23/2026 9:35am
bikelurker wrote:
22.2 is where is at. Also:Is the difference between 31.8mm and 35mm of the SAME handlebar really that noticeable? I went from 31.8mm Renthal Fatbar Lites...

22.2 is where is at. Also:

Is the difference between 31.8mm and 35mm of the SAME handlebar really that noticeable? I went from 31.8mm Renthal Fatbar Lites (as they come precut to my favoured length) to the 35mm ones (only because I got trapped in the 44mm-same-as-the-fork-offset Fraezen Hype stem, wich is super nice piece of equipment, don't get me wrong) and it feels the same, although I ride with Odi longnecks glued on, wich mute a lot of feedback.

I rode and can definitely feel flex in the handlebar (I've got a trails bmx setup for pumptrack use and the handlebars are light and flexy) and I am also into ATB/alt handlebars, so I've owned some nittos in the past that were flexy enought that you can see them deflect... But my new Renthals just feel like... Renthals!

 

favorite bars ever were 22.2 dj bars i put on a rigid dirt tourer. pretty flexy but so comfy.

2
pejzaż
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1/23/2026 10:50am
bikelurker wrote:
22.2 is where is at. Also:Is the difference between 31.8mm and 35mm of the SAME handlebar really that noticeable? I went from 31.8mm Renthal Fatbar Lites...

22.2 is where is at. Also:

Is the difference between 31.8mm and 35mm of the SAME handlebar really that noticeable? I went from 31.8mm Renthal Fatbar Lites (as they come precut to my favoured length) to the 35mm ones (only because I got trapped in the 44mm-same-as-the-fork-offset Fraezen Hype stem, wich is super nice piece of equipment, don't get me wrong) and it feels the same, although I ride with Odi longnecks glued on, wich mute a lot of feedback.

I rode and can definitely feel flex in the handlebar (I've got a trails bmx setup for pumptrack use and the handlebars are light and flexy) and I am also into ATB/alt handlebars, so I've owned some nittos in the past that were flexy enought that you can see them deflect... But my new Renthals just feel like... Renthals!

 

Agreed, I've also had trouble telling the difference between 31.8mm apples and 35mm apples, namely aluminum Renthal apples. Clamp diameter strikes me as a pointless debate since design intent always seems to shine through, unless there is none.

I am curious if anyone has tried and can report on the new 50-70mm Renthals though? The 820mm uncut width is slightly concerning. I want to go a little bit higher than 50mm on my dh bike and I don't feel confident putting any more spacers under the stem, but this means ditching my OneUp v2s which have proved very comfortable

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AndehM
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1/23/2026 10:51am

I honestly DGAF which diameter it is as long as it feels good and the geometry is ok.  That said, the best combination for me is the OneUp v2 / eBar in 35mm rise, which is only 35mm dia.  I have tried Renthal 31.8s and actually prefer their 7 degree sweep, but those things kill my hands so fast... like, I swapped them on, every other component the same, without changing anything else on my bike and my hands were so smoked each ride that I took them off after 2 rides.

I want to see more 7 degree sweep options, not a gazillion more 9/10/12 for the slow arthritic guys.

5
storm.racing
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1/23/2026 11:01am
AndehM wrote:
I honestly DGAF which diameter it is as long as it feels good and the geometry is ok.  That said, the best combination for me is...

I honestly DGAF which diameter it is as long as it feels good and the geometry is ok.  That said, the best combination for me is the OneUp v2 / eBar in 35mm rise, which is only 35mm dia.  I have tried Renthal 31.8s and actually prefer their 7 degree sweep, but those things kill my hands so fast... like, I swapped them on, every other component the same, without changing anything else on my bike and my hands were so smoked each ride that I took them off after 2 rides.

I want to see more 7 degree sweep options, not a gazillion more 9/10/12 for the slow arthritic guys.

I think Burgtec will come out with a 7 degree sweep bar. Gwin said in the Vital interview that they are working on a bar together I believe and he came from almost an entire career of 7 degree on Renthal. 

I hope this is the case as that would be awesome 

4
Primoz
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1/23/2026 11:10am

Well one solution is to roll the bar forwards. Besides less backsweep you'll also get more upsweep which is a twofold factor in opening up the elbows.

I must admit, trying out a 12/8 degree bar was awesome on the way up, but it doesn't work going down. Interestingly a normal 8/5 degree bar is the opposite. Good on the down, uncomfortable going up. 

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bikelurker
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1/23/2026 11:13am Edited Date/Time 1/23/2026 11:30am
bikelurker wrote:
22.2 is where is at. Also:Is the difference between 31.8mm and 35mm of the SAME handlebar really that noticeable? I went from 31.8mm Renthal Fatbar Lites...

22.2 is where is at. Also:

Is the difference between 31.8mm and 35mm of the SAME handlebar really that noticeable? I went from 31.8mm Renthal Fatbar Lites (as they come precut to my favoured length) to the 35mm ones (only because I got trapped in the 44mm-same-as-the-fork-offset Fraezen Hype stem, wich is super nice piece of equipment, don't get me wrong) and it feels the same, although I ride with Odi longnecks glued on, wich mute a lot of feedback.

I rode and can definitely feel flex in the handlebar (I've got a trails bmx setup for pumptrack use and the handlebars are light and flexy) and I am also into ATB/alt handlebars, so I've owned some nittos in the past that were flexy enought that you can see them deflect... But my new Renthals just feel like... Renthals!

 

pejzaż wrote:
Agreed, I've also had trouble telling the difference between 31.8mm apples and 35mm apples, namely aluminum Renthal apples. Clamp diameter strikes me as a pointless debate...

Agreed, I've also had trouble telling the difference between 31.8mm apples and 35mm apples, namely aluminum Renthal apples. Clamp diameter strikes me as a pointless debate since design intent always seems to shine through, unless there is none.

I am curious if anyone has tried and can report on the new 50-70mm Renthals though? The 820mm uncut width is slightly concerning. I want to go a little bit higher than 50mm on my dh bike and I don't feel confident putting any more spacers under the stem, but this means ditching my OneUp v2s which have proved very comfortable

They are higher but they also change the backsweep, are they still Renthals if the geometry of them is different?

As AndehM say, I purchase my handlebars based on the geometry, for me at least, material, compliance, weight... Are of little considerarion, if they do not feel right.

On the topic of handlebar geometry, does people feel like different backsweeps ask for different widths? I conmute in a bike with a 25° backsweep bar (it has a very healthy amount of upsweep too) and it feels as wide at 700mm as a 760mm 7°/5°. I tested some SQLabs at 12° backsweep that feel comfy (but not particularly conductive to good handling) at 720mm... 

...The most extreme case for it being narrow gravel/roadbike handlebars?

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pejzaż
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1/23/2026 11:37am Edited Date/Time 1/23/2026 1:23pm
bikelurker wrote:
They are higher but they also change the backsweep, are they still Renthals if the geometry of them is different?As AndehM say, I purchase my handlebars...

They are higher but they also change the backsweep, are they still Renthals if the geometry of them is different?

As AndehM say, I purchase my handlebars based on the geometry, for me at least, material, compliance, weight... Are of little considerarion, if they do not feel right.

On the topic of handlebar geometry, does people feel like different backsweeps ask for different widths? I conmute in a bike with a 25° backsweep bar (it has a very healthy amount of upsweep too) and it feels as wide at 700mm as a 760mm 7°/5°. I tested some SQLabs at 12° backsweep that feel comfy (but not particularly conductive to good handling) at 720mm... 

...The most extreme case for it being narrow gravel/roadbike handlebars?

Yeah the geometry change is the worst part, 7° fits me the best but I gave up a while ago since 40mm is the highest currently rise you can get. Worse yet, almost everything 60mm+ is 9° or more. I find this strange because taller people will generally want less backsweep. This sorta gets to your point about the relationship between backsweep and width but I would contend the opposite (for aggressive mtb positioning at least). Grip angle increases the wider you spread your hands out, so one should find that as the bar narrows, less backsweep is needed to fit the wrist. Similarly, bar width tends to not differ proportionally because of practical limits, so a small person with a 760mm bar will want a great deal more backsweep than a large person on a 760mm bar.

Maybe I'll wait for these fabled burgtec bars and hope they do an 80 rise like the Bryceland's

t. not particularly slow arthritic guy

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Primoz
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1/23/2026 11:40am

Sweep and width I don't think are as important as what you want to achieve. I find that bringing the elbows inwards negatively impacts my descending. This is done with either more backsweep or less upsweep. Or by rolling the bars back. The opposite is true to raise your elbows in a more active position. 

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AndehM
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1/23/2026 11:55am

The problem with reducing backsweep by rolling a bar forward is it will increase upsweep.  That's a nonstarter for me, as it increases pressure on the outer edge of my hand, which is my problem spot.

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bikelurker
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1/23/2026 12:31pm

And I think that could very well be related to the shape of your hand/grip. When I close my hand, my knuckles follow a diagonal line in relation to the cuff of my shirt (I hope it's clear what I am trying to say) but I know other people have a much more squared grip, so obviously the way your hand interact with your handlebars has huge implications in the way the rest of your body sits in the bike. The more options the merrier I suppose, we use to laugh at how many cockpit brands come every year to the market, but some variety is needed.

I should add that I find most flat handlebars excruciating for long distance riding, so I don't. The better they work for me for handling, the most unconfortable they are

2
Eae903
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1/23/2026 1:00pm
AndehM wrote:
The problem with reducing backsweep by rolling a bar forward is it will increase upsweep.  That's a nonstarter for me, as it increases pressure on the...

The problem with reducing backsweep by rolling a bar forward is it will increase upsweep.  That's a nonstarter for me, as it increases pressure on the outer edge of my hand, which is my problem spot.

What is the direction of your bar rise relative to the ground? I always run my bars with the rise perpendicular to the ground to get the most out of the rise, and I've found that doing that puts the up sweep and back sweep right on the right position. Anything less than that makes the back sweep down sweep

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AndehM
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1/23/2026 1:09pm
AndehM wrote:
The problem with reducing backsweep by rolling a bar forward is it will increase upsweep.  That's a nonstarter for me, as it increases pressure on the...

The problem with reducing backsweep by rolling a bar forward is it will increase upsweep.  That's a nonstarter for me, as it increases pressure on the outer edge of my hand, which is my problem spot.

Eae903 wrote:
What is the direction of your bar rise relative to the ground? I always run my bars with the rise perpendicular to the ground to get...

What is the direction of your bar rise relative to the ground? I always run my bars with the rise perpendicular to the ground to get the most out of the rise, and I've found that doing that puts the up sweep and back sweep right on the right position. Anything less than that makes the back sweep down sweep

Not sure exactly what you mean by perpendicular to the ground.  If I look at the bars horizontally, I have them set so the ends are swept up slightly.  If I roll them so that they're level it feels really awful to me (probably because that's also increasing back sweep).  On the OneUp bars, I find my preference more or less aligns with their markings corresponding to the bike's head angle.

In general though, for bar roll, lever position, etc. I take a page from Aaron Gwin's setup guide and try to do it with my eyes closed when I'm setting up new components, then do slight tweaks after measuring what I ended up with and riding.

1
Mr. P
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1/23/2026 2:50pm

Can we put this tired debate to rest? A 31.8 handlebar can be made as stiff as a 35mm handlebar, and a 35mm handlebar can be made as compliant as a 31.8 handlebar. It's the engineering and characteristics of the handlebar that matters. Pick what suits your riding. 

1
1/23/2026 3:04pm

Pick a handlebar diameter and be a dick about it

1
1/23/2026 6:47pm

35 for dh bike. 31.8 for trailbike.  I run my trail bike at 745 including grips and they are pretty dang stiff  for a 35 bar.  Ride the dh bike at 780 and I doubt I could tell much difference.

Eae903
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1/23/2026 7:03pm
AndehM wrote:
Not sure exactly what you mean by perpendicular to the ground.  If I look at the bars horizontally, I have them set so the ends are...

Not sure exactly what you mean by perpendicular to the ground.  If I look at the bars horizontally, I have them set so the ends are swept up slightly.  If I roll them so that they're level it feels really awful to me (probably because that's also increasing back sweep).  On the OneUp bars, I find my preference more or less aligns with their markings corresponding to the bike's head angle.

In general though, for bar roll, lever position, etc. I take a page from Aaron Gwin's setup guide and try to do it with my eyes closed when I'm setting up new components, then do slight tweaks after measuring what I ended up with and riding.

I mean the direction of the rise separate from the backsweep and upsweep. So on my bikes if you were to draw a line in the direction of the rise of the bars, it would be vertical, straight up and down relative to the ground. Here's a picture of my bike, it might be a bit hard to see but if you look at the bar the rise is vertical, rolled forward compared to the fork. 1000024037 0.jpg?VersionId=2ZeNKS5KBtZgADcWI neMfT2d

Simann
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Murfreesboro, TN US
1/23/2026 7:14pm

 I've got around 20 different carbon bars in my shed, used them all. The new V2 OneUp carbon is a bit too flexy for me, the V1 is the sweet spot. The Enve M7 is noticeably stiffer than the OneUp V1 or V2, so much that i've ditched it for the M6 31.8. Renthal and Deity are well constructed, but again, like the M7, too stiff for my old wrists. Aluminum bars feel like metal and resonate like metal, so i've not ridden them in years. 

TLDR: OneUp V1 carbon 35 and the Enve M6 31.8 are the sweet spot for me. 

2
Big Bird
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1/23/2026 7:44pm

1 1/8" Stele please. 

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Eoin
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1/24/2026 12:46am

I've got like 4 sets of 35mm stems and bars that I took off new bikes, but continue to run an ancient thomson 31.8 stem setup.

Laughing at the "can't feel the difference, I'm on renthal" comments. I'm currently on renthal alu, by far the harshest bar ever, I'll never understand how they got so popular. It happens to be in the exact dimensions I needed so feels wasteful to replace...

3
bikelurker
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1/24/2026 1:13am Edited Date/Time 1/24/2026 1:13am
Eoin wrote:
I've got like 4 sets of 35mm stems and bars that I took off new bikes, but continue to run an ancient thomson 31.8 stem setup.Laughing...

I've got like 4 sets of 35mm stems and bars that I took off new bikes, but continue to run an ancient thomson 31.8 stem setup.

Laughing at the "can't feel the difference, I'm on renthal" comments. I'm currently on renthal alu, by far the harshest bar ever, I'll never understand how they got so popular. It happens to be in the exact dimensions I needed so feels wasteful to replace...

I'm on renthal and I can't feel a difference because they are harsh AF is fine for me, no worries 🤣

1
Snakes
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1/24/2026 1:21am
Primoz wrote:

Based on how it's worded, 31,8. Because it can be shimmed to fit a 35 mm stem. 

What to use/how to shim carbon handlebar? It is a genuine question, I have one 31,8 bar precut to my preferred length but the stem is 35mm.

Primoz
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SI
1/24/2026 6:05am Edited Date/Time 1/24/2026 6:08am
Primoz wrote:

Based on how it's worded, 31,8. Because it can be shimmed to fit a 35 mm stem. 

Snakes wrote:
What to use/how to shim carbon handlebar? It is a genuine question, I have one 31,8 bar precut to my preferred length but the stem is...

What to use/how to shim carbon handlebar? It is a genuine question, I have one 31,8 bar precut to my preferred length but the stem is 35mm.

Intend sells a spacer. I have a similar solution as well, but only 5 sample pieces for now. 

https://www.intend-blackline.com/products/handlebar-sleeve/

2
J26z
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Middletown, NY US
1/24/2026 8:28am
AndehM wrote:
Not sure exactly what you mean by perpendicular to the ground.  If I look at the bars horizontally, I have them set so the ends are...

Not sure exactly what you mean by perpendicular to the ground.  If I look at the bars horizontally, I have them set so the ends are swept up slightly.  If I roll them so that they're level it feels really awful to me (probably because that's also increasing back sweep).  On the OneUp bars, I find my preference more or less aligns with their markings corresponding to the bike's head angle.

In general though, for bar roll, lever position, etc. I take a page from Aaron Gwin's setup guide and try to do it with my eyes closed when I'm setting up new components, then do slight tweaks after measuring what I ended up with and riding.

Eae903 wrote:
I mean the direction of the rise separate from the backsweep and upsweep. So on my bikes if you were to draw a line in the...

I mean the direction of the rise separate from the backsweep and upsweep. So on my bikes if you were to draw a line in the direction of the rise of the bars, it would be vertical, straight up and down relative to the ground. Here's a picture of my bike, it might be a bit hard to see but if you look at the bar the rise is vertical, rolled forward compared to the fork. 1000024037 0.jpg?VersionId=2ZeNKS5KBtZgADcWI neMfT2d

This is the reference/starting point where bars are made/ where the sweep is measure from.  So the Oneups are exactly and 8/5 here.  The more you roll back, the less the upsweep, you can say backsweep won't really change.  

IMO  anywhere between rolled back to level (no upsweep), and angled up aimed at you navel ( while off the saddle in "active mode") is ideal.

 

2
1/24/2026 9:49am

Got both, can’t tell the difference in ride feel, but prefer 35 cos they look cooler 

2
1/24/2026 1:22pm

31.8, because that’s the clamp diameter for my lights.

2

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