We review the Isaac Muon Ironman triathlon bike

Isaac Muon

Dutch firm Isaac has certainly delivered a distinctive looking and focused aero alternative to the established elite. Does it put the negative spin on your bike splits that its name suggests though?


The Muon frame isn’t brand new, but you wouldn’t realise that immediately by looking at it. The deep triangular tapered fork legs sync into a cut-out in the down tube, and the whole frame uses geometric triangular and kite-shaped aero sections rather than smooth teardrops. The head tube is kept short for a low position while the big hunchback top tube has a rubber plug for the vertically inserted gear and rear brake cable routing. The top tube is aggressively triangulated and tapered to blend into the geometric seat tube with wheelhugger cut-out.

The similarly profiled seatpost comes in two versions, depending on use. The Type A triathlon version we tested gets a forward flare at the top, as well as a sliding rail for the saddle clamp to allow aggressive forward seat positioning, while the Type B version reverses the top for conventional time trial positioning. The press-fit bottom bracket is asymmetrically shaped for maximum stiffness, with super-deep chainstays taking your torque to the rear wheel.

The replaceable horizontal slotted dropouts also let you fine-tune tyre-to-frame clearance. The front brake is conventionally front mounted on the fork, while the rear brake cable loops out from the down tube into a bolt-on mast ahead of the unmasked U-brake mounted under the chainset. It’s not the most aero method of attaching anchors, but it’s a blessing in terms of braking control, maintenance and easy fat-wheel fit. The powerfully built frame isn’t light, though.


Isaac supplies the Muon in various complete bike formats, or you can go down the à la carte route as we did with our sample. The new generation, conventionally cabled 11-speed Shimano Dura-Ace transmission helped drop the overall bike weight to a competitive level, but we really missed the cow-horn shift capability when swapping over from Di2 bikes.

The Vision tri-bars also caused distributors Jungle some set-up issues but they felt great once they were on, and for once the extension length wasn’t baboon-specific if left uncut. Vision’s flexible armrest plates are always a welcome sight on firmly focused rides like the Muon too. In contrast, the big broad Isaac box stem might not have helped comfort but it was a vital link in the precise and well-balanced handling character of the bike.

While most manufacturers list wider-cassette-compliant 11-speed compatibility, actual availability is still proving an issue. That meant the Isaac ended up rolling on light, responsive and precise feeling DT Swiss Dicut wheels.


As it turns out, the wheels weren’t just a great aesthetic match to the Muon but they also provided a very good parallel for its overall character. The acceleration-aiding low weight and high-tension direct-pull spoking helped offset the heavy frame weight of the Isaac, and meant there was no loss of punchy power delivery pulsing down the big chainstays with every press of the pedals.

The big stem and firm bars mean you’ll have no trouble recruiting your shoulders into any climbing or corner-exit stomp either. In fact you’ll have to feather your ferocity occasionally to keep the rear tyre connected. While it responds to the spurs when needed, it’s still a mount that tends to sit at a given speed – particularly on slight upward gradients – rather than gradually creeping faster and nudging for the next gear up as you get into your stride.

Interestingly, despite its obvious drivetrain stiffness, the Muon always seemed to ride better being spun at a high cadence rather than crowbarred along in a big gear, but we’ll admit that we’re unsure why. One possibility is the excellent handling, which has a surefooted stability and assured authority that refused to get flustered however fiercely we spun the pedals.

Having experienced some gusting and tramlining with the DT Swiss wheels on our default Scott Foil test bike, we were impressed with how obedient and well behaved the Muon made them feel. We certainly didn’t have any worries about reaching for gels or taking an aero tuck down sweeping descents, even in blustery, slightly sleety conditions that had us backing off on the less confident bikes in the test.

The handling also helps to keep your mental fatigue in check over longer rides too, but that’s tempered by a decidedly firm ride quality that becomes more and more intrusive as the miles increase. It’s by no means as punishing as the P5, but it is enough to make you aware of the quality of your chamois on rough road surfaces.

+ Distinctive looks, excellent handling, powerful feel
+ Great for riding low and aero; easily maintained brake

– Heavier than expected frame that is also a bit too firm-riding
– Frame isn’t as aggressively aero as some of its rivals