Are Lighter Bikes Faster?

It’s hard not to be impressed when picking up a featherweight sub-8kg mountain bike but does really make a difference on the trail? Michael Hanslip takes a look at the benefits of putting your bike on a ‘dirt diet’.

Light bikes are faster, right? Plenty of money is poured into developing bikes and components that are lighter whilst still retaining exceptional stiffness and strength. Thanks to the use of composite materials and the continued refinement of manufacturing processes, it’s almost inevitable that the latest gear just keeps getting lighter. Weight loss is quite tangible and easy to measure—it’s easy to gauge so plenty of people get obsessed with making their bikes lighter. Weight weenies sink lots of cash into making their bikes lighter.

It’s logical to think that weight makes a difference. If it didn’t we’d do just as well pedalling around on something that weighed as much as a motorbike whilst wearing a pair of steel capped Blundstones. Surely manufacturers wouldn’t pursue the ‘lighter is better’ dictum if it didn’t matter. But how much difference does it really make? If you currently get around on a 13kg mountain bike, how much faster would you go on one that weighed half a kilo less?

While we could attempt to answer these questions by riding around a test track with a power meter attached – adding and subtracting weight along the way – there are too many variables that could influence the accuracy of any results. However, it’s entirely possible to calculate the results through physics and the use of mathematical models. It’s the same methodology used in devices that estimate your power output by simply looking at your speed and the forces that you are working against. Taking this approach allows us to see the effect of incremental changes in your bike weight, without the results being blurred by other random influences.

For most of our examples we’ve relied on a web-based tool that you’ll find at Analytic Cycling (www.analyticcycling.com). Designed by mathematician and cyclist Tom Crompton, this tool lets you enter the data and find out exactly how fast a cyclist will go for a given power output. Using this, we ran through a number of different scenarios.

For these examples we used a fairly average mountain biker as our model; a 75kg rider on a 13kg mountain bike. Of this weight, 3.5kg is in the wheels/tyres and there’s 9.5kg in the rest of the bike—so the total rider and bike mass comes to 88kg. For our calculations we’ve taken an educated guess at the rolling resistance and frontal area figures (your frontal area directly relates to aerodynamics). The real life figures are sure to vary but we’ve kept these forces constant throughout our calculations—this allows us to focus solely on the differences that come about through weight loss.

On the Flat

Let’s say our 75kg pilot is trucking along a flat, hardpacked fire trail and putting 250 watts into the pedals. In this situation our test dummy, let’s call him Fred, will be maintaining a constant speed of 7.76 meters per second, or 27.9km/h. Now Fred’s scored some overtime pay and lashed out on a new carbon frame—it’s 500g lighter than his old one and looks pretty cool to boot! Surely it’s got to make him faster? Running the same model again, now with a total rider and bike mass of 87.5kg, the result is the same. On the flat, small changes in weight make little or no difference to your speed. Even if we drop a whole 10kg, the speed only rises to 7.86m/s or 28.3km/h. So a 65kg rider (who’ll find it tougher to put out 250 watts) will have little advantage on the flat, although being smaller will also decrease your aerodynamic drag. That’s why big and powerful riders typically do so well in flat terrain—at least they also provide a good windbreak when sitting on!

Climbing

Of course weight plays a bigger role when you are fighting gravity. We ran our rider up a nasty hill; five kilometres long at a ten per cent gradient. In this instance our mate Fred may well notice a performance improvement for his carbon investment; he’ll get to the top of the hill 10.36 seconds faster and be in front by 26.26 metres. So buying a lighter bike can really make you faster! Is it enough to really matter for the expense? Well that’s up to you.

Let’s bump things up a level and get Fred onto a 9.5kg hardtail—how good is that going to make him look! Ignoring any efficiency gains from eliminating the suspension, he’ll now be a whole minute and 11 seconds in front of his old time! Pretty cool hey? Still, Fred’s skinny 65kg riding buddy (Jimmy) will be a further 2:11 up the road, even though he’s riding a 13kg clunker! It’s also worth noting that the steeper the trail, the greater the difference—if you regularly find yourself scaling 20% grades, losing a little weight is always going to help.

Wheel Weight & Acceleration

We often hear that it’s better to save weight from the wheels and other rotating parts of the bike. So does it really make a difference—what if Fred spent his overtime pay on fancy wheels and lighter tyres rather than that carbon frame? Well according to the model it would have made no difference—this contradicts the usual mantra of ‘weight on the wheels is more important’. Once up to speed and riding at a constant pace, removing 500g has the same effect whether it’s taken from a static or a rotating part of the bike.

Weight loss is supposed to help in acceleration—think of a powerful truck versus a small car at the traffic light derby. To test this we put Fred in for a 250-metre standing start sprint at the beginning of a cross-country race. During this effort he pumped out an average of 350 watts with a peak output of 500 watts. With his 500g lighter frame, he was ahead by 36.7cm, or 0.045sec. Interestingly, in this case reducing his rotating mass did offer greater benefit, but only just. Using his heavy old frame and removing 500g from the wheels put him ahead by 47.8cm, or 0.062sec. If we started this XC race up a 10% grade, at the end of the 250-metre sprint, the lighter frame would be ahead by 81.4cm or 0.073sec while the lighter wheels would be ahead by 88.7cm or 0.087sec.

So there are measurable gains in reducing rotating mass but they’re ever so small and they only become apparent when accelerating. When you hear it said that saving a gram on the wheels equates to four grams on the frame, well that’s stretching the truth. In most cases it’ll be more like a gram on the wheels equalling somewhere between 1.1 and 1.5 grams on the frame—it depends greatly on where the weight is placed and how hard you accelerate. As you go further out from the hub, any weight change has a greater influence; that’s why fitting heavy tyres to a 29er will make it feel more sluggish than heavy tyres on a 26er or a 27.5 bike. Any difference becomes more apparent as the rate of acceleration increases; it’ll be more noticeable when going from a near standstill on a switchback climb than sprinting on an open fire-road when you are already carrying some momentum.

Micro-Accelerations

Mathematical models such as Analytic Cycling base their outcomes on perfectly linear power delivery—in reality this simply isn’t the case. In researching this article, both Raoul Luescher (Director Luescher Teknik) and Josh Poertner (Technical Director at Zipp Speed Weaponry) pointed out that even during steady-pace riding, the unevenness in your pedal stroke means that you are constantly accelerating and decelerating (sometimes referred to as ‘micro-accelerations’). This phenomenon may explain in part why lighter wheels can ‘feel’ more responsive even though the actual speed difference is comparatively tiny. More elaborate computer modelling can take these micro-accelerations into account. According to Josh Poertner, these tests have actually shown that really light wheels can be slower for some riders when climbing—the total opposite of what you’d expect. In these cases, a slightly heavier rim can be more efficient as the added inertia smooths out the lull in their power delivery. MTB wheels and tyres are never as light as their road-going cousins, so this is unlikely to be a consideration that Fred needs to worry about—lighter will generally offer better off-road performance, so long as you haven’t compromised stiffness, reliability or traction in the process.

While there’s no denying that weight loss can make you faster, we also need to keep the advantage in perspective. When compared to the other forces at play, weight can become a fairly insignificant factor. Lower weight will only help when climbing, accelerating or physically lifting the bike. Reduced rolling resistance will assist all of the time; whether you’re climbing, descending or stomping along the flats. Once your speed rises above 20km/h, wind resistance is by far the biggest factor. On level ground at 30km/h, over 60% of your effort goes into fighting wind resistance, and overcoming rolling friction takes around 35% (compare that to climbing a 10% grade and in that situation resisting gravity consumes 82% of your power). We’re not suggesting you fit triathlon bars to your MTB but it shows that in many cases, weight loss is the least of your worries.

The Intangibles

In addition to the small but measurable performance differences, there’s no denying that a lighter bike feels different, and for the most part it feels better. Further to this, lighter rims always make the bike feel better than the same weight loss from the cranks. The wheels are out at the extremities and lightening them makes it easier to throw the bike around. It becomes easier to lift the front wheel over obstacles and directional change is easier (a good thing in singletrack). The cranks however are smack in the middle of the bike and have little influence on how the bike feels when moving around on the trail.

Similarly, a light seatpost and saddle combination allows the bike to swing more easily when you’re out of the saddle. In this situation they move further than any other piece of the bike, so weight loss in this area tends to be quite noticeable. Ditch your seat bag; instead put your tube and tools in your pocket—the bike will feel better even though the whole package weight has barely changed.

Unsprung Weight

These days, lots of people choose to ride full suspension bikes and even the hard-core XC racers generally run a suspension fork. As a result, the concept of unsprung weight is relevant to mountain bikes. Even in the bad-old rigid bike days, unsprung weight was kind of relevant as your limbs acted as suspension.

Unsprung weight is the total mass of those components that are not riding on the springs. In any vehicle (car, bus or bicycle), less unsprung weight provides better traction, a smoother ride and puts less load on the suspension system itself. In short, it allows the wheels to better follow the terrain. On a mountain bike, unsprung components include the wheels, brake callipers and fork sliders. With rear suspension you can also factor in the cassette, rear derailleur, half the shock plus some of the chain and swing arm.

Substantial changes in unsprung weight will permit a lighter suspension tune (particularly with the rebound settings). It can help to keep your tyres on the ground in rough terrain and improve the handling overall. However, beyond looking at your wheel and tyre choice, there isn’t much room to play with unsprung weight (things like derailleurs and chains don’t vary by much). Light tyres can make a bike feel better but it’s simply not worthwhile if it comes at the expense of reliability and traction. It’s certainly a false economy to choose superlight minimal tread tyres if your technical skills hold you back during the descent.

Sprung Weight

It follows that if unsprung weight is all the stuff below the suspension, then the sprung weight is everything above. If the wheel is the most important part of your unsprung weight, then the rider is by far the most important component of sprung weight. In our test case, Fred is 85% of the total bike-plus-rider weight. Even with a 60kg rider on board, the ratio remains similar with an 82-to-18% split.

So, before you get too excited and throw your bike headlong into a ‘dirt diet’, consider that just about anyone this side of a world-class professional can benefit from dropping a little body weight. Making your body lighter doesn’t just affect the total vehicle mass; it can also make you – the motor – more efficient.

Removing weight from a bike only takes money. Fred’s 13kg bike probably cost around $3,000 and dropping 500g may set him back $1,000. A bling-tastic 10kg racer may be more like $8,000 and spending even more might get it down to 8kg. So for roughly $10,000 extra, the bike could lose around 5kg. That’s a fair bit of cash to drop the bike weight by 40%. More realistically, it’s a huge expense to remove less than 6% of the total vehicle mass. Even if the whole bike weight could be removed that’s still only 15% of the total!

Without wanting to offend our mate Fred, there’s a fair chance that he could stand to lose a few kilos. By reducing the beer and pizza intake, Fred may even save some money and dropping 5kg of body mass will certainly cost a lot less than ten grand. With a 5kg weight reduction, Fred will crest the 5km-long 10% grade 90 seconds faster. Factor in the general health benefits of losing some latent flab and he’ll get up that hill much faster. If this newfound fitness allows him to increase his power output by 10%, he’ll be four minutes faster up the hill and a whole second faster in the sprint!

Mountain biking, as the name implies, involves a lot of riding up and down hills. Every time you go up, you are working against gravity. A lighter bike will make the ascent easier. On the way back down, less weight can allow your brakes and suspension to perform better. Abrupt directional changes also become easier and it takes less effort to lift over obstacles. However, this all pales into insignificance when compared to losing some body weight. So stop obsessing like a weight weenie and ride your bike more—it’ll make you lighter, fitter, faster and you’ll become more technically proficient in the process. If only losing body weight was fast and easy as buying a bling new carbon trinket!