An Insanely Profound Comparison Of Seat Post and Frame Mounted Rear Racks (get some popcorn…)

Bike racks add a lot of utility to a bicycle by allowing you to carry bags and larger objects that don’t fit in pockets and small frame bags.

Cargo racks attaching to the eyelets near the rear axle and the seat stays are the most common type. They are often found on city bicycles designed for commuting.

The rest of the cargo-carrying options are front racks, baskets, bags, and rear racks that mount directly to the seat post via a clamp.

In this article, I will compare the classic frame-mounted rear racks to the seat post racks.

What Are The Advantages of a Seat Post Rack?

1. Universal Fit

The main selling point of seat post racks is that they can be installed on almost every bicycle.

The requirements for seat post rack installation are:

a. Enough seat post real estate. A short part of the seat post has to be showing because if it’s all inserted into the seat tube, there would be nothing to mount the rack to.

b. Non-carbon seat post. Seat post carriers exert a serious clamping force on the seat post. This makes them non-suitable for carbon seat posts because carbon does not like being squeezed.

Having said that, Arkle’s Randonneur Seat Post Rack works with carbon seat posts according to the manufacturer. The compatibility is the result of an innovative mechanism which doesn’t grip the seat post very hard,

However, you cannot use this rack with carbon seat rails because it grabs them via a quick-release clamp.

Since the vast majority of bicycles answer the aforementioned criteria (non-carbon seat post + enough space), seat post racks automatically become an almost universal option.

2. Easy Installation and Removal

A seat post rack can be easily installed and removed from the bicycle.

Most models come with a quick-release clamp which eliminates the need for extra tools. But even if the rack doesn’t have a quick-release function, you will need only an Allen wrench to install or remove it.

This characteristic is attractive to race-oriented cyclists who need a rack to occasionally to carry light items.

For better or worse, those are the only advantages of a seat post rack.

Common sense may tell you that seat post racks are lighter and cheaper than full-on rear racks, but the market disagrees.

What Are The Disadvantages of a Seat Post Rack?

The main disadvantages of seat post racks are:

1. Low weight capacity

The maximum load capacity of most seat post racks is around 10kg or 22lbs.

Depending on what you’re transporting, this alone could render a seat post rack non-practical for your purposes.

In the table below, you can see some popular seat post racks and their corresponding load capacity. The data comes from the sites of manufacturers and distributors.

ModelWeight capacity
Ibera Seatpost Carrier IB-RA110kg/22lbs
Topeak MTX BeamRack Bike Mounted Rack9kg/19.8lbs
Blackburn Central Seatpost Rack10kg/22lbs
RFR Seatpost Carrier with Rail KLICK&GO10kg/22lbs
RFR Seatpost Carrier KLICK&GO (no rail)10kg/22lbs
Arkel Randonneur Rack6kg/13lbs

Back in the day, when I was exploring the idea of using a seat post rack, my research concluded that the RFR Seatpost Carrier KLICK&GO series would be the most suitable option for me based on the reviews and the price.

However, I abandoned the idea of a seat post rack and bought a frame-mounted one.

After a while, I dropped all racks and switched to Carradice’s Super C SQR Slim bag which is my commute carrier at the moment.

2. High Load Distribution

Since seat post racks attach to the seat, they have no choice but to be significantly higher than the rear tire. The higher the load is over the rear tire, the more it affects the balance of the bicycle.

For that reason, regular rear racks include special pannier mounts positioned lower than the main rails – the closer the load is to the ground, the less it affects the stability of the bike.

Nonetheless, seat post racks shouldn’t feel unstable if the load is stabilized securely and within the limits of the manufacturer.

3. Weak/Inferior Architecture

The joint connecting the seat post rack to the seat post is under great torque due to the long moment arm. This is an inherent architectural flaw of seat post racks necessitated by the very nature of the equipment.

Since seat post racks aren’t meant to connect to the chainstays, it’s impossible to provide additional support points.

In consequence, the clamp area is under tremendous stress which augments even further during riding because the rack is vibrating and thus creating even more torque at the joint.

And since most seat post racks are made of aluminum (inflexible material which accumulates fatigue), some of them brake precisely at the main weld – the weakest point and yet the one under the highest stress.

4. Shaky connection

Seat post racks tend to move to the left or right over time because the connection to the seat post is subject to a lot of stress, and the bolt(s) could untighten.

When that happens the rider can unintentionally push or pull the rack out of position when locking the bike or dismounting.

This problem tends to be more common for seat post racks with quick-release clamps because the shape of the “catcher” offers less friction in comparison to the 4-bolt versions.

Another reason why the 4-bolt version is more secure is the possibility of putting thread-lock on the bolts. (In some cases, it’s done by the manufacturer, and there’s no need to do so.)

5. Easy To Steal

Quick-release seat post racks are easy to steal because they can be removed with no tools. To prevent this from happening, you would have to carry a separate lock that can be simultaneously used to secure the rack and the saddle.

Another option is to take the entire rack with you when leaving your bicycle alone for hours.

The standard bolt version can be stolen too, but at least it requires a tool to take it off. This peculiarity doesn’t eliminate the risk but at least reduces it because fewer people would be tempted by the opportunity.

6. Heavy weight

One may think that seat post racks are light, but that’s not the case at all. They are as heavy or heavier than the regular racks mounted to the frame.

Why? Because seat post racks come with a severe structural disadvantage (long moment arm) and require reinforcement.

Meanwhile, regular rear racks can get away with being lighter because the support legs are in a more mechanically advantageous position.

The table below contains the weight of multiple seat post racks.

Ibera Seatpost Carrier IB-RA1830g
Topeak MTX BeamRack Bike Mounted Rack696g
Blackburn Central Seatpost Rack997g
RFR Seatpost Carrier with Rail KLICK&GO1114g
RFR Seatpost Carrier KLICK&GO (no rail)950g
Arkel Randonneur Rack495g
Average weight847g

Some frame-mounted racks are lighter than the models above and offer greater stability and load capacity thanks to their superior geometry and attachment system.

7. Seat Post Dependence

The seat post and the seat post rack form a union where each party influences the other.

This “relationship” results in the following downsides:

  • Every time you adjust the seat post, you may have to re-set the rack too.
  • If someone steals your seat post, they’re getting a rack as a bonus.
  • The rack occupies “seat post real estate” that may be needed by other accessories such as a rear fender or a light system.

Saddle Bags > Seat Racks

A rear rack is useless without a storing container or a bag. Thus, one has to pair it with panniers, trunk bags, cargo crates…etc.

In consequence, large saddlebags render seat post racks obsolete for the following reasons:

1. Saddlebags combine a rack and a bag (kinda)

A saddlebag can be used by itself because it comes with an attachment system and storage compartments. Meanwhile, racks require you to purchase extra bags which may or may not be easy to attach whereas saddlebags have straps made specifically for the purpose.

2. Light weight

Since saddlebags are a single unit, they have the potential to be lighter than a rack + bag combo.

3. Lower chances of sudden failure

A quality saddlebag is unlikely to tear and drop, especially if the load is properly distributed – the heavier objects are supposed to be closer to the saddle to reduce the moment arm and the stress on the stitchings.

Meanwhile, seat posts racks have been known to fail unexpectedly at the weld because they’re made of aluminum – a material that breaks suddenly due to the lack of elasticity.

Moreover, aluminum accumulates fatigue which is why a seat post rack may work just fine for a while but break after a couple of years.

But even if the saddlebag was to tear, it would be easier to repair it yourself or found a sewer that can do it for you. Conversely, re-welding a broken aluminum rack would be much more difficult because skilled aluminum welders are not easy to find.

Ultimately, it would be easier, cheaper, and safer to just buy a new rack because aluminum can lose strength during re-welding and repair.

My Choice Has Been The Carradice Super C SQR Slim

Around 1 year ago, I purchased the saddlebag Carradice SQR slim. It beats any saddle rack for the following reasons:

1. It’s a rack + a bag.

The SQR bags come with their own mounting system and do not require separate racks.

The rails at the back of the bag prevent it from swinging and give it support.

The Carradice SQR Super Slim in action. In my opinion, this bag is superior to seat post racks.

2. It’s waterproof

The bag is made of Carradice’s favorite material, namely waxed cotton duck which makes the bag waterproof. I can attest to this. I’ve ridden mine in legit storms without worries.

The only way for water to get in would be to throw the bag into the river and submerge it totally.

3. It can hold just as much as a seat post rack.

The Carradice Super C SQR Slim is rated at 10kg/22lbs. I haven’t loaded it with that much, but for six months I was commuting with around 8kg/17.6lbs worth of stuff without problems.

The support rails are made of steel – an elastic material that does not accumulate fatigue as long as the stress is within a certain range.

4. Easy detachment

The attachment mechanism is solid and fast. You can detach the bag a lot quicker than a clamp rack. The re-installation is just as rapid.

What Are The Advantages of a Frame-Mounted Rear Rack

1. Secure Attachment

A regular rear rack that mounts to the frame is significantly more stable than a seat post rack because it usually has four attachments points – the legs are bolted to the rear triangle whereas the arms of the rack connect to seat stays or the seat post. In consequence, the rack stays in place and does sway to the left or right.

2. High Load Capacity

The standard capacity of a rear rack is 25kg or 55lbs. Some models offer a little less while others go up significantly.

Below you can find a table listing the capacity and weight of many frame-mounted rear racks.

I’ve extracted the data from the websites of manufacturers and distributors.

Ibera PakRak Touring Bike Carrier
Topeak Explorer Rack25kg/55lbs625g
BTWIN 900 Ultra-light Rack18 kg/39.6lbs700g
Topeak Super Tourist Uni Disc Rack26 kg/57 lb915g
Portland Design Works Payload Rear Rack Steel35 kgs/77 lbs1360g 
Axiom Journey Tubular Alloy Rack70kg/154lbs700g
Axiom Journey UNI-FIT MK3 ALU50 kg/110 lbs860g
Axiom Streamlined 29ER DLX50 kg/110 lbs760g
Tubus Grand Tour26 kg/57.2 lbs760g
Tubus Fly Classic18 kg/39.6lbs420g
Tubus Logo Classic26 kg/57.2 lbs800g
Tubus Cargo Classic26 kg/57.2 lbs740g
Tubus Vega Classic25kg/55lbs540g
Blackburn Local Deluxe20kg/44lbs990g
Thule Tour Rack10kg/22lbs (front)
Cube Rear Carrier Heavy Duty25kg/55lbs705g
Kross Travel Rack25kg/55lbs858g
Kross Venture40kg/88lbs750g
Surly 26”-29” Chromoly Rear Rack36kg/79.2lbs1260g
Average weight of the rack: 821g

As you can see, the average weight of the racks above matches that of carriers mounted to the seat post. However, frame-mounted rear racks offer a far greater load capacity for the same weight.

And if your goal is to have the lightest rack possible, it’s better to go for a regular rear rack. For example, Tubus Fly Classic is only 420grams and yet it can support 18 kg/39.6lbs.

The lightest seat post rack that I could find is Arkel Randonneur Rack which comes at 495kg, but it can hold only 6kg or 13lbs. The Tubus Fly is lighter and can carry three times more.

If grams aren’t all that important to you, you can go for a heavier rear rack from the list and get even more capacity.

3. Pannier Friendly

Even though some seat post racks have rails designed for panniers, you wouldn’t get the ultimate pannier experience from such a set-up for the following main reasons:

  • The load is up high and has a negative effect on stability.
  • The rack cannot support a lot of weight.
  • Panniers on a seat post rack can never be as stable as on a frame-mounted rack.
  • Many seat post racks do not have rails for pannier installation.

Frame racks fix those problems and allow you to transport significantly heavier loads.

If you want to get the maximum out of your panniers, forget seat post racks and go for the real deal.

4. Superior Architecture

A quality frame-mounted rear rack is very likely to outlive a seat post model because the support structure is in a more advantageous position.

To understand the difference, you can do the following exercise. Grab a dumbbell or bag and hold it with your arm straight and parallel to the ground (like the top position of a front raise).

Chances are that your elbow and front deltoid will start burning very quickly.

Now lift the same weight overhead as in a shoulder press. It will be significantly easier to maintain this position because the torque on the arm wouldn’t be nearly as high.

As a result of their superior architecture, frame-mounted rear racks experience less frequent failure.

Having said that, they could still fail. Racks are among the components that break the most frequently on a tour which is why many cycling tourists rely on steel models – steel tends to bend before breaking and is easier to weld.

Keep in mind that those men and women are carrying massive loads over bumpy roads for extended distances. When you take the high stress and frequency into consideration, it shouldn’t be a surprise that racks fail occasionally.

5. Low Center of Gravity

Unlike seat post racks, frame-mounted rear racks are closer to the tire. This position makes the machine more stable.

Furthermore, some racks have special pannier rails below the top platform. This is done to lower the load even further and free the space up top for lighter items like a mat.

If you plan on running heavy panniers, it’s highly advisable to get a rack with low pannier rails.

6. Higher Re-sale Value and More Demand

A frame-mounted rear rack produced by a reputable company keeps a lot of its value on the second-hand market. And since the demand for frame-mounted racks is greater, you would have an easier time selling it.

What Are The Downsides of Frame Mounted Racks?

1. Anti-Race Looks

Some frame-mounted rear racks are very light and don’t have a negative effect on your time unless you’re a pro. Yet people who want to preserve the “racy” appearance of their bicycles stay away from racks altogether.

I’ve witnessed the hate for racks during group rides. Some cyclists would rather wrap their bicycles in duct tape than install a rack.

Truth be told, people are taking themselves too seriously. Let’s face it – we’re not professional. If a rear rack is adding utility that you need, you might just as well get one.

2. Problematic installation (possibly)

Frame-mounted racks could be difficult to install on some bikes. Here are some common problems:

a. No Eyelets On The Frame

Many modern frames come without eyelets for fenders and racks. There are two main routes to solving this problem:

a. A rack with attachments to the seat stays;

b. A rack that can be mounted directly to the axle;

A popular choice in the second category would be Tubus Disco and Axiom Streamliner as they can be attached to the quick release.

b. No Eyelets On The Seat Stays

Normally, the arms of a rear rack attach to the seat stays. However, many bicycles do not come with threaded attachment points.

There are three main ways to circumvent this issue:

1. Attach the arms to the seat post via a seat post clamp with rack mounts.

This is Sunlite’s rack seat clamp. The lower threading acts as an eyelet for the rack’s arms

2. Attach the arm under the brake (in case of road bikes)

Some racks designed with road biking in mind (e.g, Axiom DLX Streamliner Road Cycle Rack) use a single-arm witch connects to the bike via the rear brake bolt.

3. Attach the arms to the middle fender mount via an adapter/spreader.

If the bicycle has a mid-eyelet for a full rear fender, you can install an adapter/spreader that will provide two extra mounts.

When I was running a rack on my hardtail, I relied on that method.

Rear rack mount adapter

c. Disc Brakes Incompatibility

The rear disc brake could make it difficult to install a regular rack on the bike.

Back in the day, I purchased a regular rear rack without paying attention to this detail. It was impossible to install it because the brake was in the way.

To circumvent this problem, you can do the following:

1. Buy a rack that attaches to the axle.

Most racks that mount to the axle are also disc brake compatible.

Streamliner Disc DLX

2. Buy a disc specific rear rack with “L” shaped lower attachments.

There are racks specifically designed for bicycles with disc brakes. I have one from Kross which has worked great for my bicycle.

Kross Travel Rack – disc brake compatible rack

The purpose of the “L” shaped legs is to create space for the brake caliper.

If you’re going to be purchasing a rack from an offline store, take the bike with you and make sure that the rack fits properly.

If you’re going to use an online vendor, read the reviews to see if people with disc brakes are having issues.

d. Inconvenience when removing the rear wheel

If you run a rack that mounts to the quick release, you would have to take it off when removing the rear wheel in case of a puncture. Many would find this annoying.

e. Problems when running full fenders

If the frame has only one set of rear eyelets, and the rack takes them, it becomes more difficult to install full fenders. Having said that there are ways to make it work.

A common solution is to use the same mount for both the fender and the rack. The downside of this method is that the two components become dependent on each other. If you’re removing the fender, you’re also removing the rack and vice versa.

Another option would be to use a separate attachment system for the fenders such as P-clips wrapped around the seat stays.

Note: P-clips are good only for installing fenders because fenders are fairly light. If your bicycle does not have rack mounts, don’t use P-clamps to mount a rack.

The P-clips are made of soft material and do not provide sufficient support for a fully-loaded rack.

In conclusion

Frame-mounted racks are stronger, sometimes significantly lighter, and have a greater loading potential than seat post racks. For that reason, touring cyclists around the world rely on frame racks.

If your plan is to buy a seat post rack to carry light loads (e.g., a jacket and a lunch), it would be wise to look into larger saddlebags because they could successfully serve that purpose without requiring you to run a rack at all.

Without a doubt, frame-mounted racks are kings when compared to seat post racks.

I hope this post has been helpful. Good luck.

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