How To Know If A Saddle Will Fit Your Bicycle – MTB & Road

As one of the few contact points on a bicycle, the saddle has a crucial role as it bears most of the rider’s weight. It’s also often the cause of discomfort during long rides due to wrong saddle dimensions or wrong bike fit. That’s why bike saddles are components that get replaced quite often.

A bicycle saddle can fit any bike, as long as the seatpost uses the same mounting configuration. Both saddles and seatposts can be easily swapped for specific parts. There are four main saddle configurations and four common seatpost diameters.

So, a bike seat can be changed. In fact, it’s one of the parts on any bike that gets replaced. Most of the time it’s due to ergonomics, sometimes for looks. But there are a couple of factors to consider before swapping.

Can any bike saddle fit on any bike?

Most bicycle saddles are universal, using a dual-rail mount that fits on a standard seatpost. The universally used width between the two rails is 44 millimeters. Though, there are seatposts and saddles using other mounting systems like single rails, pivots and rigid tripods.

As you can see, if a specific saddle will fit a specific bike only depends on the mounting mechanisms of the seatpost.

The real question here is …

Do all saddles fit all seatposts?

Not all saddles fit all seatposts because there are four standardized configurations available: dual rails, single rail, pivotal, and tripod seatpost attachments. As long as the saddle and seatpost use the same mechanism, they are compatible. The dual rail configuration is by far the most common.

There is a method to make dual-rail saddles fit all kinds of seatposts. Usually, that isn’t necessary, but for vintage bicycles or stationary bikes it may be. I’ll describe how I’ve done that further down.

Back to the most common saddles you’ll find: dual rails.

Are saddle rails universal?

Saddles rails are in fact universal in dimension. Most bike saddles use two parallel rails 44 mm apart, called dual rails, that match the universal seatpost clamps. Saddles using one rail, called monorail, are also universal in their dimensions but only fit monorail seatposts. The rails are used to allow the saddle to slide back and forth for a proper bike fit.

Rails make up a big portion of the saddle’s overall weight. Accordingly, there are a couple of different rail materials to choose from, varying in weight and stiffness.

Depending on the material, the rails can also offer slight vibration dampening. That’s less the case with Carbon rails. Those are the lightest but also the stiffest, which makes them a great option for riders for maximum pedal efficiency and minimal dampening.

Metal rails like titanium or alloy rails are the most common ones. They are found on mid-level performance saddles and provide a smoother ride by being not so stiff. Prices are usually much lower but can be double the weight of carbon rails.

So, there’s much more to saddle rails than meets the eye. They are an integral, often unseen part, of a saddle. They are also the part that brakes first.

Can you replace rails on bike saddle?

Generally, rails on a bike saddle can be replaced. Saddle rails are designed to be the pre-determined breaking point, so they are built with repairs in mind. They are also the cheapest part of the saddle, making it last longer with minimal maintenance cost.

The rail replacement can be done in a bike shop for between $ 25 and $ 40 including parts. The rails themselves can be bought or ordered like any other bike part. The exact price will vary depending on the manufacturer and the materials used. Carbon or titanium ones are the most expensive, and aluminum the cheapest. Just make sure the shape fits your saddle model.

If you are keen on doing the replacement yourself, here’s a video of Clint Gibbs showing you how it’s done from beginning to end. (It’s not that difficult)

Seatpost variations

Dual rail saddles

Dual rails are the most common, universal saddle mount. Under the saddle are two parallel rails that are exactly 44 mm apart. The actual rails are longer than the seatpost clamp so the saddle can be slid forward and backward. Most seatposts also allow the saddle to be tilted up or down depending on the type of riding.

The degrees of tilt are dictated by the seatpost while the distance for sliding is dependent on the saddle’s rails.

Dual rail saddles are by far the most common and allow for the most adjustments while being stable and sturdy.

Single rail (monorail) saddles

A single rail saddle is very similar to the dual rail and can only be slid forward and backward as well as tilt up or down. The main difference is that it has only a single attachment point making them more fragile, especially for lateral movement.

Half the rails also means weight savings. Monorail saddles are lightweight and often used for road racing or marathon mountain bike racing.

Due to the smaller footprint, there is also slightly less frontal surface area, which makes them more aerodynamic compared to dual rails. That’s why they are most likely found on triathlon or time trial road bikes.

Interestingly enough, single rail saddles are also commonly found on downhill mountain bikes, like my YT Tues. There it’s aggressively angled and slid forwards to not get in the way. There isn’t much sitting going on anyway with a DH bike.

Pivotal saddles

Pivot-style saddles cannot be slid forward and backward but only tilted up or down. They are also attached at only one point of contact. They are mostly made out of hard plastic, making those kinds of saddles prone to breaking more easily. The actual attachment and adjustment are done by a screw through the top of the saddle.

Tripod saddles

A tripod saddle is primarily used for BMX, Pumptrack, Dirtjump, and Slopestyle mountain bikes. In these disciplines, the saddle is not used for sitting on, but rather as a contact point for the thighs for better control over the bike. In the case of a mistake, when riders hit the saddle by accident, it needs to stand up to potentially a lot of force. That’s why those tripod saddles are rigid in favor of sturdiness, instead of adjustability.

Can you put a regular bike seat on an exercise bike?

If it has a saddle clamp, a regular bike saddle can be mounted on a stationary bike. Generally, that’s the case for most spinning bikes but only for the minority of bicycle ergometers. Saddle clamps usually fit regular dual rail bike seats, which are universal and the most common.

Dual rail saddles are so universal, that they even fit on bikes that don’t even move.

The difference between a spinning bike and a bike ergometer is the overall design. While spinning bikes are more akin to road bikes in terms of geometry, handlebars, pedals and saddle, other exercise bikes are designed for comfort. Often sporting a wide, cushy saddle, upright body position and a screen for stats.

In fact, I swapped regular bike seats on my stationary spin bike using a seat clamp adapter for rail saddles. The one I have came with the spin bike, but those can be had for under $10 on amazon. This adapter clamps around the seatpost (which is just a short tube in my case) and has the standard rail clamps on top to mount any rail saddle onto.

It’s the same solution you would use to make older vintage bikes compatible with modern saddles.

It’s very easy to install, as there is only one hex-head screw nut on the side, that tightens the whole mount.

Swapping the seatpost – what to look out for

Check the seatpost clamp diameter

The current five standard seat tube diameters are 27.2 mm, 28.6 mm, 30.9 mm, 31.6 mm, 34.9 mm, and 36.4 mm. The most common seat tube diameters on mountain bikes are 27.2 mm (standard), 30.9 mm and 31.6 mm (oversize). Older bikes used to have many more seat post diameter options varying in 0.2 mm increments.

In order to measure the diameter of your seat post, you need to dismount it and pull it out of the bike frame. This is done by simply loosening the seatpost clamp. Tip: Remember or mark how far the seatpost was in the seat stay to easily return to the measurement you had.

Simply take a measuring tape to the bottom end of the seat tube to accurately measure its outside diameter. Same for the inside diameter of your bike’s seat stay.

There is no way to make the seat stay diameter of the bike frame any bigger. So a wider 31.6mm seatpost doesn’t fit a 30.9mm seatstay. But there are methods to increase the seatpost diameter to fit a wider frame. For example a 30.9mm seat tube into a 31.6mm seatstay.

This is done by seatpost shims, that go over the seatpost to increase its diameter to fit larger diameter seatpost clamps. There are a couple of brands and sizes to available. Reputable bike component brand Cane Creek has a broad selection of seatpost shims for any size combinations. Their shims are sold for around $10 to $15 on amazon.