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Men's Mountain Bike Undershorts 900

Sale price: $20.00 Regular price: $39.99

The B'Twin 900 men's bib shorts are very comfortable thanks to an ergonomic pad with gel inserts. Mesh fabric lets moisture escape. To be worn underneath a pair of MTB shorts (size buttons).

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Size Guide
Sale price: $20.00 Regular price: $39.99

Free shipping over $35

Final Sale: No returns or exchanges

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San Francisco, 735 Market St. Options

Select an available color and size to see options.

Model: 8354334

Product Features

A photo of the Men's Mountain Bike Undershorts 900 in use

Men's Mountain Bike Undershorts 900 | frequent mountain biking in warm/hot weather (rides of 3 hours or longer).

The B'Twin 900 men's bib shorts are very comfortable thanks to an ergonomic pad with gel inserts. Mesh fabric lets moisture escape. To be worn underneath a pair of MTB shorts (size buttons).

Sitting comfort

Ergonomic pad with gel inserts and 3 levels of density for added comfort.

Moisture wicking

The body stays dry as the fabric absorbs & diffuses moisture, drying quickly.


Elasticated fabric keeps the undershorts in place.

Compact design

Attaches to shorts with buttons at the waist.

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Men's Mountain Bike Undershorts 900,black

Product Customer Reviews

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Men's Mountain Bike Undershorts 900,black
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great value

Size and construction good

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Perfect match to ST900 shorts

Comfortable and perfect match to ST900 shorts. Thinner material than normal lycra shorts and a bit see through, so can't wear by themselves, but just fine with shorts.

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Just what I was looking for.

Very comfortable, would recommend.

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Good product

Not much to say about a pair of under shorts other than they fit and are comfortable over a couple of hours

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excellent value

I am very pleased with the service Decathlon provides and the quality of their products.

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Sizing is Rediculous - otherwise great shorts

As others have said, the latest (2021) version of these are sized/cut for a child with a beer belly! OR a stick insect. Dont buy without trying on in store. Likely uou will need to size 2 times up and take the waist in. Decathlon, PLEASE fix the weird 'large waist/ultra skinny legs' fit - its mad! I have a 32" Waist, and like the pair of these I got years ago bought Large, which used to be for 82-89cm and fitted well32-34". The latest version Large MT900 now state on the label they are for 99-106cm (38-41")!! However, despite the claimed size I could barely get these on, they are so tight around the thighs and bottom, my legs started throbbing the legs were soooo tight. And I have a slim athletic shape legs/bum - not a 'Chris Hoy' figure. riends keep telling my I need to eat more and put on weight, so its not my shape, but these new cut of the shorts. The waist might just at a push stretch to 41" - but there is no way anyone with that waist would get them above their knees. The shorts themselves (my last pair) are excellent otherwise - the pad is fabulous, comfy, not bulky, light, doesnt get smelly. Such a shame they have totally ruined the cut. I am going to have to try an XL (for a slim build & 32" waist!) in store and hope I can then take the waistband in - as they are otherwise great undershorts. Maybe these were a mis-manufactured batch - however I also have the excellent and great value MT500 undershorts - and also an old pair - and its exactly the same issue, infact, even worse on 2 pairs of those I bought. You would need absolutely pipe cleaner legs, no bottom and a huge waist for the cut to have the correct snug but comfy fit.

Really good undershorts

Comfy for long rides

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Nearly perfect

Very good fit, nice flexible material, pad comfortable with no rough seams. Only slight negative (though not enough to make it 4 stars) is that the padding is fine but may not be enough for a day in the saddle in rougher terrain. But hey, I'm being picky.

Look after your balls

Good all round product

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Do not purchase without trying on in store first

I have had good experiences with Decathlon products before so decided to try the under-shorts. Unfortunately none in store, I had to order online. I told Decathlon I was 36" inch waist (I am in fact 34") and they said I was size large. Product arrived and they are VERY snug, I am athletic and not a large build but these would only fit those with a stick insect type figure. So size up ! I also found there is a gripper on the legs, which I specifically asked about as I do not like them, again I was given bad information by Decathlon customer service. Advice; only buy Decathlon products in store where you can see and try on before buying and if out of stock, go elsewhere.

Inner shorts

Good fit and style. Plenty of padding. To wear under mtb style shorts.

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Best padded cyclewear I have had

I've tried padded shorts and padded undershorts from other shops but these are the only ones that I find comfortable. My biggest problem with other makes is chafing at the groin/very top of the leg - these are sufficiently wide and padded to avoid that.

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900 mountain bike shorts

Bought for partner but returned. They looked great, fitted well/snug but he found them uncomfortable with the gel pad seating. I guess it’s something you get used to.

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Size guide wrong

Never actually worn them as I used the size guide to order them and when they arrived it turns out the size guide was wrong and couldn't even get them on. However they do look really comfy.

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Gel under shorts

Comfortable good fit, well made.

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Size Guide

Size Guide Visual

Additional Product Information


Main fabric

  • 86% Polyester cationic
  • 14% Spandex

Padding - Inner fabric

  • 100% Polyamide

Padding - Outer fabric

  • 77% Polyester cationic
  • 23% Spandex

Care tips

Machine wash at 86°F. Synthetics cycle. Turn the article inside out. Use a minimal amount of laundry detergent. Do not use fabric softener. Line dry in a warm, ventilated area. Do not tumble dry.Do not dry clean. Do not bleach. Do not place on a heater to dry.

Product tests

Our team of product designers based at the B'Twin Village in Lille, France is continuously working to make cycling more comfortable for our users. Our mission is to create products that meet the specific cycling needs of our users and to eliminate obstacles to a good cycling experience. We then test our products in the field, as we are convinced that only by subjecting them to real world conditions can we continuously improve our products.



Superior quality. Unbeatable prices.

For 40 years, Decathlon has delivered the best value in the retail sports industry by offering high-quality, sustainable and cost-effective products.

Decathlon makes the difference
  1. product-research

    Research and development

    delivers high-quality cost-effective products

  2. State-of-the-art production processes

    minimize manufacturing costs

  3. Optimized supply chain

    reduces retail prices

  4. laptop-checkout

    Minimal retail footprint

    lowers overhead and indirect costs