Isaac Kelvin review

German design meets Taiwanese manufacturing in an ultra-stiff package.

Our rating 
4.0 out of 5 star rating 4.0
GBP £1,754.00 RRP

Our review

Beautifully built, but extreme stiffness counts against it on UK roads.
Skip to view product specifications

The Kelvin combines excellent Taiwanese maufacturing with German design and that classic German pursuit of frame rigidity. It’s stiffer than a very stiff thing, but if that’s your bag, this could well be your bike.


Although Isaac have been based in England since 2001, Isaac frames are designed and developed in Germany to exacting standards and subjected to independent German testing body EFBe’s fatigue testing protocol. However, like many others in this price range (with the exception of Cannondale’s SystemSix), they are actually manufactured in Taiwan. The Isaac Kelvin is one up from their lowest priced Pascal model.


This 2007 Kelvin weighs in at 8.76kg (19.3lb), a little lighter than last year’s model. The frame is manufactured from a mixture of carbon grades and is laid up in the mould entirely by hand. Visible sections under the Isaac graphics and on the fork have a coarse decorative weave. Unusually, to maximise stiffness the fork steerer tube has a larger diameter at the base than at the top – a feature that is only normally seen on some ultra high-end frames. The long head-tube provides a slightly more upright riding position without resorting to using headset spacers, which can rob some of that precious stiffness. The bike is available in six sizes, from 48 to 60cm.


The Isaac would normally come with a ‘standard’ spec based on Shimano 105 for £1,674, though ours had an upgrade in the form of an Isaac-made stem, closely resembling the beautiful, cold-forged and CNC-machined, German-made Tune. The anatomic handlebars give a relatively upright riding position when you’re on the tops or hoods, but the drops are deep enough to provide a low racing position. The gear package, based on Shimano 105, was curiously difficult to index which we later found to have been caused by slight misalignment of the gear hanger – easily corrected in a matter of seconds, once the gear mech was removed, using a Park hanger alignment tool.


Tests have shown that the positive effects of a stiff frame are wasted when it’s partnered with a wheel of insufficient lateral stiffness. So it was a relief to discover that the modestly priced Mavic Aksium wheels work very well with the Isaac, with a remarkable ability to feel lighter than they are when climbing. Having said that, we noticed a further improvement to responsiveness by fitting Fulcrum R1 wheels that set the benchmark for stiffness. The Continental Super Sport 23mm tyres rise to the challenge when exploring the Isaac’s handling limits on descents and grip well on damp surfaces.


While the Isaac Kelvin warms to the kind of smooth road surfaces enjoyed by our German cousins, we found ourselves consciously avoiding any road surface irregularities because both frame and fork react to every blemish. On the flipside, however, the Kelvin feels outstandingly rapid when spooling up the cranks to get up to speed, and the longer than average head-tube helps in part to minimise torsional losses, making it remarkably stable in blustery conditions. Time triallists, however, might wish for a more stretched out, lower riding position.


Isaac are very good at making stiff and efficient bikes and we love the oversized tubing and one of the very best finishes in the business, but we can’t help feeling that they’ve gone too far in their quest for über stiffness with the Kelvin. That said, it’s worth considering if you are tall and heavy and have the need for something as utterly dependable as a frame designed in Germany and built in Taiwan.

Product Specifications


Name Kelvin (07)
Brand Isaac

Bottom Bracket Height (cm) 27
Weight (kg) 8.76
Seat Tube (cm) 51
Seat Tube Angle 73.5
Shifters 105
Standover Height (cm) 81.5
Top Tube (cm) 55
Trail 6.3
Wheelbase (cm) 101
Rear Wheel Weight 1705
Available Sizes 48cm 51cm 54cm 55.5cm 57cm 58.5cm 60cm
Front Tyre Ultra 2000
Front Tyre Size 700x23C
Rear Tyre Ultra 2000
Rear Tyre Size 700x23C
Year 2007
Rims Aksium Race
Rear Hub Aksium Race
Bottom Bracket 105 Hollowtech II
Fork Offset 4.5
Brakes 105
Cassette Shimano 105 10spd
Chain HG 5600
Chainstays (cm) 41
Cranks 105 Hollowtech II
Fork Carbon
Fork Weight 417
Rear Derailleur 105
Frame Material Carbon
Frame Weight 1414
Front Derailleur 105
Front Hub Aksium Race
Front Wheel Weight 1335
Head Tube Angle 72.5
Headset Type Integrated
Sprocket Size Tested 10spd - 12/13/14/15/16/17/19/21/23/25