Giant Propel Advanced Disc

The Giant Propel Advanced Disc is an efficient, firm-feeling road bike with aerodynamics designed specifically with disc brakes in mind. It's not the lightest bike available for this kind of money but it's fast whether you're soloing off the front or sprinting for the line.

• Pros: Stiff and efficient, strong aero credentials, excellent disc brakes
• Cons: Not especially lightweight, you're tied into using Giant's handlebar and stem

Rim brake versions of the Propel have been in the Giant line-up for years, but this is the first time that disc brakes have been added. Although rim brake Propels are still available, they're all based around the previous frame design; the new version is disc brake only.

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The Propel Advanced Disc might not lurch forward like some lightweights do when you accelerate out of a slow turn, but that's not what aero road bikes are all about. It's designed with efficiency in mind, both in terms of aerodynamics and stiffness. The aero side of things is difficult to discern (I'll cover aerodynamics in a bit) but this bike certainly feels stiff when you stamp on the pedals, with very little flex to speak of even when you're standing up and giving it everything you have.

This stiffness extends to the front end with a head tube that features Giant's Overdrive 2 design with a 1 1/4in upper bearing and a 1 1/2in lower bearing. Giant's own alloy Contact SL Aero handlebar doesn't flex much when you haul on it either, which is notable because some aero options do thanks to their slim profile.

You'd have to say that the Propel Advanced Disc provides quite a firm ride, and that might or might not be to your taste. By that I don't mean that it's uncomfortable – it's not that – but it is, yeah, firm. This isn't a gran fondo or endurance bike, it's a race bike (even if you don't race it) and that comes through in the feel.

That said, I got on fine with Giant's Contact Forward saddle with a composite base that flexes enough to take the edge off things, and Giant's own 25mm Gavia Race 1 tyres are set up tubeless so you can run them at lower pressures than you otherwise would without the danger of pinch flatting if you hit a pothole.

The Propel Advanced Disc treads the middle ground in terms of manoeuvrability, flicking around ably when you want to move within a group or change your line on a fast descent, and it isn't too much of a handful in crosswinds compared to other aero bikes I've ridden, partly thanks to Giant speccing a shallower-section (42mm) SLR 1 Aero Disc wheel at the front than at the back (65mm).

In terms of aerodynamics, Giant says that the Propel Disc had a detailed development phase involving its engineers and experts at the Aero Concept Engineering facility in Magny-Cours, France. They used a moving mannequin and spinning wheel on the bike in the wind tunnel in order to get accurate data.

The result is a bike that features a new truncated ellipse aerofoil profile. You probably know the deal by now because it's a formula followed by other brands: designers come up with an aerodynamically efficient shape and chop the trailing edge of the profile off square to save weight, improve handling and stick within UCI rules, while maintaining most of the wind-cheating performance.

This aerofoil profile is used for the fork legs, most of the frame elements and the seatpost. Giant uses other features that we've come to expect of aero road bikes over the years, such as a hidden seatpost clamp – it's a wedge system that sits within the top tube/seat tube junction – a seat tube that's cut away around the leading edge of the rear wheel, and a horizontal top tube.

One other key feature is Giant's own aero handlebar and stem system that leaves no exposed hydraulic brake hose and just 5cm of gear cable showing on either side (between exiting the handlebar and entering the stem). I'd explain how it works but a picture paints a thousand words...

Essentially, Giant specs an aero handlebar (the dimensions of the tops are 56mm x 26mm) and has added a composite cover to the top of the stem. The gear cables and brake hoses run under there, then through some rubber shrouding that sits behind the headset spacers and into the frame. The stem cover is held in place by 2mm hex bolts that are a little fiddly, but nothing you can't handle.

The other important fact regarding aerodynamics is that, as mentioned, the new Propel framesets are disc brake only. Giant says it found that with proper integration a disc brake design can be more aerodynamically efficient than a rim brake bike.

'This is because the location of traditional callipers (either in front or behind the fork crown/legs) creates 'dirty' air,' says Giant. 'Opening up the fork crown area (by placing the disc-brake callipers down at the hub) means that the air hitting the new disc-brake calliper has already been disrupted by the leading edge of the tyre/wheel. This effect is further enhanced by an asymmetric fork that helps smooth out airflow over the calliper.'

There are three Propel Disc platforms and various builds, and here are the main features of each:

• The top-level Propel Advanced SL Disc has a frame and fork made from Giant's Advanced SL-grade carbon composite, an integrated seatpost and a Contact SLR Aero composite integrated handlebar and stem. The fork is a one-piece construction.
• The Propel Advanced Pro Disc has an Advanced-grade carbon composite frame and fork, a non-integrated seatpost, a Contact SLR aero composite handlebar and a Contact SL Aero alloy stem. The fork is full-carbon but it's not a one-piece construction.
• The Propel Advanced Disc, which is the one we have here, is also made from Advanced-grade composite, but the fork steerer is alloy, as are the Contact SL Aero handlebar and stem.

Putting the integrated seatpost to one side, the frame and fork profiles are identical across the range. Giant doesn't say that the Propel will save you a certain amount of time over a given distance, but does say that it is the fastest disc brake aero road bike it has tested at all wind angles except zero (when the apparent wind is coming from straight ahead).

The only times you'll get a zero apparent wind angle are when the air is completely still, when you're riding directly into a headwind and when the wind is coming from directly behind you. Giant's engineers reckon that this is rare and claim that the Propel is the fastest bike in its category in real world conditions. Giant also says that the Propel has a lower drag coefficient at a wider range of yaw angles than its non-disc brake predecessor.

Geometry

The Giant Propel Advanced Disc is built to an aggressive geometry, which is what you'd expect of a bike designed for speed. We have the M/L model here with a 545mm seat tube, a 570mm top tube and a 165mm head tube. The stack is 562mm, the reach is 398mm, and both the head angle and the seat angle are 73 degrees.

The height of the front end is boosted by a whopping 5cm of headset spacers. Chances are you'll want to drop that, at least a little, by chopping down the fork steerer. This requires that you take the top cover off the stem in order to access the headset's top cap. You can only refit the cover if the top of the steerer sits flush (or nearly flush) with the top of the stem – but you can run the stem without the cover for a while if you just want to try out a couple of different handlebar positions for size.

While we're talking about such things, the aluminium alloy Giant Contact SL Aero stem is 110mm on M/L versions of the Propel, and it will soon be available aftermarket (£150) in 80-140mm lengths. You don't get a choice of angles so it's just the number of headset spacers that determines handlebar height. It's down to the individual dealer whether they'll swap to a different length stem if you want one. You don't need to disconnect the cables or brake hoses in order to swap the stem, by the way.

The unique clamp shape means that with this stem you're tied to using a Giant handlebar for the duration (it's 42cm wide on our M/L size bike) because nothing else will fit (the Giant Contact SL Aero bar on the Propel Advanced Disc is alloy, although a composite SLR version is available in 40cm, 42cm and 44cm widths for £249.99). The cynical among you might figure that Giant does this to keep you true to the brand. Maybe, but on a bike that's all about aerodynamics it also makes sense to use components that have been specifically designed to work together as a complete system to minimise drag.

That's fine if you get on well with this bar, and I did. I liked the ergonomic shape of the drop and didn't find my wrists hitting the tops while I was down there. I also found the deep, aero section tops really comfortable during climbing.

However, you can't alter the angle of the handlebar and if you don't like the bar's shape or level of stiffness you're stuck, so make absolutely sure that the Contact SL Aero bar works for you before handing over your cash.

The unusual shape of the handlebar means that although you can fit a rubber-backed bike computer mount with O-rings, you can't clamp an out-front mount to it. Instead, Giant gives you an out-front mount that fixes to bolts that hold the stem's front plate in place. It's a neat solution but the mount currently has inserts only for Garmin Edge and Giant Neostrack computers and Giant doesn't have plans to extend that to cover a Wahoo Elemnt Bolt, for example. The last we heard, Giant was going to speak to RaceWare to see if there was enough demand for it to produce other inserts, and you can buy third party out-front mounts that fit to stem bolts for other devices.

Wheels and tyres

As mentioned above, Giant specs its own SLR 1 Aero Disc wheels, the rear one with a 65mm-deep composite rim to reduce drag, the front one with a shallower 42mm-deep rim for greater stability in crosswinds. Makes sense. The rims are the same as on Giant's top-level SLR 0 wheels but with lower end DT Swiss hubs. Both the front and rear wheels are 12mm thru-axle.

I went on a couple of really windy rides during testing and only noticed a significant effect on handling compared to shallow section rims when descending at [checks Strava] about 40mph with the wind gusting across me. Of course, some people (and their bike positions) are more susceptible to being thrown off-line than others.

The SLR 1 Aero Disc wheels and Giant's own Gavia Race 1 tyres come set up tubeless. Periodically top them up with sealant and you should experience far fewer punctures than normal. There's enough space for you to swap the 25mm tyres for 28s if you fancy putting more air between yourself and the road.

Ultegra groupset

The Propel Advanced Disc is built up with a second-tier Shimano Ultegra groupset which I won't cover in depth here, but it essentially performs like top-level Dura-Ace with the addition of a little weight. The 52/36-tooth chainset is matched to an 11-28-cassette, which doesn't give you the widest range of gears ever but it's spot on for a bike of this type, and the Shimano PressFit bottom bracket hasn't creaked at all during testing.

The Shimano Ultegra hydraulic disc brakes put in a reliable performance in all weather conditions. Giant specs 140mm rotors front and rear (it does this across its road range) which deliver marginally less stopping power than 160s and will heat up quicker, but I've got along just fine without giving them a second thought.

Good value?

Of the aero road bikes we've reviewed here on road.cc recently, Bianchi's Aria Disc was £3,350, another model for which the manufacturer doesn't publish any specific aero figures (whatever those figures are worth!). The Aria Disc has a carbon frame and the version we reviewed had a Campagnolo Potenza groupset, although it's also available in a Shimano Ultegra build for £3,150.

The rim brake Orbea Orca Aero M20 Team that we reviewed was built up with an Ultegra groupset and DT Swiss PR 1600 Spline 32 wheels, and was priced at £3,632. On the face of it (and this is always dangerous), the Giant is better value for money.

> Buyer's Guide: 15 of the best & fastest aero road bikes

Overall, Giant presents a compelling package here. The Propel Advanced Disc is fast and efficient with excellent disc brakes and very good wheels, unlike some rivals that come fitted with place-holder wheels that you'd want to upgrade in order to realise the bike's full potential. It does have a firm feel and you can't switch to a different handlebar and stem combo, but if you're happy with those things then this is a bike that'll give you a boost in your pursuit of speed.

Verdict

Aero road bike that offers speed, efficiency and a strong spec