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Shimano Ultegra RX800 and RX805 Di2 rear derailleurs



Works really well off-road, though less useful on road, and the limited range will limit its appeal

At every product is thoroughly tested for as long as it takes to get a proper insight into how well it works. Our reviewers are experienced cyclists that we trust to be objective. While we strive to ensure that opinions expressed are backed up by facts, reviews are by their nature an informed opinion, not a definitive verdict. We don't intentionally try to break anything (except locks) but we do try to look for weak points in any design. The overall score is not just an average of the other scores: it reflects both a product's function and value – with value determined by how a product compares with items of similar spec, quality, and price.

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Shimano's new Ultegra RX rear derailleur, available in both mechanical (RX800, £99.99) and electronic (RX805 Di2, £259.99) versions, reduces chain slap noise and eliminates dropped chains. It's ideal for cyclo-cross, gravel and adventure bikes where rough and bumpy terrain can easily unship an unsuspecting chain, but largely pointless on the road unless you're riding cobblestones on a regular basis.

  • Pros: Quieter, less risk of dropped chains
  • Cons: Extra weight, heavier mechanical shifting, uglier

If you're replacing or upgrading a rear mech anyway it could be a smart move, especially as there's little difference in the retail price. But if you've managed just fine up until now you probably still will; it's certainly no game-changing development in the same way SRAM's 1x groupsets have been for mountain, gravel and cyclo-cross bikes. It falls some way short of a dedicated gravel groupset many people were anticipating, but does indicate a growing interest from the Japanese company.

> Find your nearest dealer here

> Buy the RX800 online here

> Buy the R805 online here

What has Shimano done exactly?

Shimano has simply implemented its Shadow Plus technology from its mountain bike groupsets into the new Ultegra RX rear mech. Shadow Plus comprises a clutch-style mechanism housed inside the slightly bulkier body of the derailleur and it's intended to increase the tension on the cage to prevent unwanted chain movement (by preventing the cage moving forwards) to stop the chain flapping about when riding over rough ground or cobbles and slapping the chainstay (noisy) and, in the worst case scenario, dropping off the chainring.

The small grey lever you can see in the photo below is to enable you to disengage the mech. This is to allow you to easily remove the wheel; trying to get the wheel out with the clutch engaged is just going to make your life harder. You could look at it as being an optional feature, turning it on and off as you see fit, but in reality you're going to leave it on all the time.

shimano ultegra rx2.JPG

I tested both a mechanical version with a double chainring (2x) road bike and the Di2 version (below) on my custom built 1x adventure bike. I used both over several months and through lots of miles and as many different riding situations as possible.

shimano ultegra rx5.JPG

Using the mechanical version highlights slightly heavier shifting at the lever compared with a conventional derailleur, but after the first couple of miles and dozen gear shifts you really don't notice it. Shimano has apparently reduced the tension compared to the mountain bike mechs with the same technology, but its latest mechanical shifting is so light and breezy anyway, and that doesn't change drastically with the addition of the clutch. With the Di2 version, there's absolutely no discernible difference at all.

Who is it aimed at?

All the Shimano press for the new Ultegra RX clutch derailleur talks of increasing your confidence over bumpy ground, helping you maintain cadence over the road less travelled, bringing adventure back to road riding. But it's designed primarily for 2x drivetrains (Shimano doesn't yet offer a dedicated road and gravel 1x groupset) and only accommodates up to an 11-34t cassette. Plus it was launched at Paris-Roubaix, the most roadie of roadie races.

Do you even need a clutch-style mech on a road bike? We've managed for a long time just fine, surely?

That was my first thought, and one I've grappled with during this review. I can't say noisy or dropped chains have been an issue on road bikes; in fact, I can't remember the last time I dropped a chain on a road ride. But off-road, it's a different matter: the clutch-style derailleurs have been one of the single most significant developments, along with dropper posts, in the last decade.

shimano ultegra rx6.JPG

Where the clutch-style mechanism makes a lot of sense is on a 1x bike because removing the front mech essentially robs you of a chain retention device. With the Di2 mech fitted to a gravel and adventure bike with a single chainring, the Ultegra RX mech works nicely to prevent unwanted chain movement and minimise the risk of it dropping off.

Riding over lumpy grass tracks, horse-trodden bridleways and down rocky gulleys prove the merits of the Ultegra RX and is where the clutch mechanism comes into its own. The ride was much quieter – no chain smashing the chainstay – and definitely no dropped chains. For cyclo-cross racing too, the benefits are clear. There's less risk of the chain going AWOL when riding through a sandpit or when you remount the bike after gracefully vaulting some hurdles. So clear benefits for off-road riding then, but less appreciated on the road, in my opinion.

Limited range

The Ultegra RX rear derailleurs are compatible with Shimano's existing road dual control levers and will work with cassettes with a largest sprocket between 28-tooth and 34-tooth, and with chainsets from 46-36t up to 50-34t, with a 16-tooth maximum chainring capacity (the difference in teeth numbers between large and small chainring).

> Beginner's guide to understanding gears

They are really intended to be used with a 2x setup – Shimano is a firm believer in the benefits of the front mech still, whereas SRAM is on a mission, it seems, to wipe it out forever. There are pros and cons to a single chainring versus a double, but that's a discussion/debate/argument for another day.


On a double groupset with an 11-34t cassette, the Ultegra RX mech works as expected, much like a normal Ultegra rear mech really. However, I bravely decided to ignore Shimano's guidelines and experimented with the Ultegra RX Di2 mech paired with a single chainring and an 11-40t cassette.

Would it work? Yes! It worked just fine. Look, here's a video and everything. I should probably caveat this by saying Shimano doesn't recommend this, and I've no idea if it voids the warranty (probably), so as they love to say on the telly, don't try this at home kids.


My takeaway from testing the two Shimano Ultegra RX mechs is that they make a lot of sense for cyclo-cross, gravel and adventure bikes, off-road riding basically, where the reduction in chain slap noise and chain dropping avoidance are big wins on rough and tumble terrain. The Shimano recommendations do limit their appeal, especially with wide-range cassette and 1x enthusiasts, but as my experimenting shows you can bend the rules a bit.

> Your complete guide to Shimano road bike groupsets

I'm less convinced of the benefits on a road bike unless you happen to be riding cobblestones on a regular basis.

All we need now is for Shimano to produce the rest of the Ultegra RX groupset for the growing adventure and gravel bike market and SRAM will have serious competition for its dedicated 1x Force and Rival groupsets. Until then, it feels like nothing more than a conservative dipping of the toe.


Works really well off-road, though less useful on road, and the limited range will limit its appeal

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Make and model: Shimano Ultegra RX800 and RX805 Di2 rear derailleurs

Size tested: GS

Tell us what the product is for and who it's aimed at. What do the manufacturers say about it? How does that compare to your own feelings about it?

Shimano says, "ULTEGRA RX brings SHIMANO's pioneering chain stabilizing technology to road cycling, delivering smooth and precise shifting over technical terrain. It expands road bike capabilities, inspiring confidence on any surface and drives the evolution of road cycling to include greater elements of adventure. No longer is the road bike narrowly defined, SHIMANO ULTEGRA RX broadens the notion of drop bar riding and emboldens cyclist to explore further."

And from Wiggle: "With a clutch mechanism that removes unnecessary movement in the derailleur cage and holds chain tension, you can expect precise and accurate shifting from the Shimano Ultegra RX800 Rear Derailleur over rough surfaces.

"Borrowing its Shadow Plus technology from its proven MTB rear derailleur range, the Ultegra RX800 Rear Derailleur fits close to the frame, which not only looks sleek, but helps to prevent it from being snagged on anything and damaged in the process. This combined with the clutch mechanism that can be disengaged for easy wheel removal makes it perfect for for adventure road, gravel and cyclocross use."

Tell us some more about the technical aspects of the product?

Chain Stabilizer mechanism adopted from MTB design

ON/OFF switch

Compatible with current road DUAL CONTROL LEVER

Max. front difference: 16T

Total capacity: 39T

Low sprocket: Max. 34T/Min. 28T

Top sprocket: Max. 12T/Min. 11T

Rate the product for quality of construction:

Usual Shimano top quality construction.

Rate the product for performance:

Works as smoothly as a regular Ultegra rear mech, which is to say it's excellent, with the added bonus of reduced chain slap.

Rate the product for durability:

Through several months of testing neither rear mech has shown any sign of distress.

Rate the product for weight (if applicable)

There's a small weight penalty but not enough to be significant for all but the fussiest weight weenies.

Rate the product for value:

You're paying a little more than a regular Ultegra rear mech but the extra chain tension for off-road riding is worth it, although, as I mentioned in the main review, unless you're riding cobbles every day it's of questionable value for pure road riding.

Tell us how the product performed overall when used for its designed purpose

Prevents unwanted chain movement and minimises noise and the potential for dropped chains.

Tell us what you particularly liked about the product

Less chain slap and better chain retention.

Tell us what you particularly disliked about the product

It's a bit heavier and uglier looking.

How does the price compare to that of similar products in the market, including ones recently tested on

Shimano's Deore XT mountain bike rear mechs are roughly the same price (£89.99 mechanical and £269.99 Di2); SRAM's 1x rear mechs (Force 1 and Rival 1) are £186 and £106 respectively.

Did you enjoy using the product? Yes

Would you consider buying the product? Yes

Would you recommend the product to a friend? Yes

Use this box to explain your overall score

If you want extra chain retention for riding over rough ground and bumpy terrain, and currently use a Shimano groupset, it's easy to recommend. But it's not quite a dedicated wide-range gravel-focused offering.

Overall rating: 8/10

About the tester

Age: 31  Height: 180cm  Weight: 67kg

I usually ride:   My best bike is:

I've been riding for: 10-20 years  I ride: Every day  I would class myself as: Expert

I regularly do the following types of riding: road racing, time trialling, cyclo-cross, commuting, touring, mountain biking

David worked on the tech team from 2012-2020. Previously he was editor of and before that staff writer at RCUK. He's a seasoned cyclist of all disciplines, from road to mountain biking, touring to cyclo-cross, he only wishes he had time to ride them all. He's mildly competitive, though he'll never admit it, and is a frequent road racer but is too lazy to do really well. He currently resides in the Cotswolds, and you can now find him over on his own YouTube channel David Arthur - Just Ride Bikes

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