The bike Weight and Performance gain for Lightweight riders?

Question

I keep hearing people saying:

The bike weight isn't a problem, rider is.

My girlfriend is only 48 kg, and a mtb hardtail usually 13-15kg (full suspension would be more like 18kg for an older one), which is already 25% of the total mass.

Is there any huge difference going from 15kg bike to 7-8kg?

I would like to acknowledge the points that others have made:

• Cycling/pedalling skills (sorry, not applicable in this question, but good to know)
• Weight/Cost/Performance dilemma (choose 2 out of 3, and this question chose Weight+Performance)
• Power output / Acceleration and Speed difference with respect to Bike Weight's difference

Edit: It is performance vs bike weight, please avoid adding other unknown parameters. It is worth mentioning those for the benefits of other cyclists, but please do not focus on unknown variables.

Edit2: I agree that in real world, you need to take into account other variables. However, my approach is focusing at the bicycle (not cyclist, and in my subjective view, I certainly cannot tell my gf to get fit to ride the bike I made for her...), then whatever components is not feasible for "real-world", replace it with inexpensive alternatives.

Answer 1

In context of this kind of discussion - weight and cost are the same thing - the more you spend on a bike, the lighter it is (with diminishing returns) and you cannot talk about weight without talking about how much money you have and are prepared to spend (sometimes not the same). The context of this answer is a targeted weight of a 7-8kg MTB.

No weight does not matter - or more accurately, it matters far less than most people imagine it does. If you are trying to build a 7-8kg MTB, a hair cut will be providing cheaper $/gram than bike upgrades. For most people, losing 1000g in body weight is cheaper than losing 1000g from your bike, as is gaining a little bit more fitness. Do you ever arrive home with water in your drink bottle, or snacks in your pocket - that 100mL of water is 100g you spent $100s to get the super light carbon bike.

A well coached rider on a heavy bike will out perform an unskilled rider on the lightest bike on any mountain bike trail. Learning to ride will increase your enjoyment, endurance and speed far more than spending money on a lighter bike, and unlike a lightweight bike, it's something that lasts a lifetime. For a vast majority of riders I have met, a weekend coaching session would provide immeasurable improvement in riding (speed, endurance and enjoyment) compared to the spending money saving weight - yet many people never spend a dime on coaching after forking out $1000s on a 'faster' bike.

Clearly at some point - you are a finely tuned elite athlete with no more body weight to lose, you have been coached till your riding form is used in reference videos, you have trained till there no more training you can do, a light bike becomes the only way to go faster. There is also the mid way - you want to go as fast as you can and a lighter bike will make an instant improvement, why not - if you have the cash, there's no reason not to.

However, other things come to play. They do not build light bikes with cheap bearings and poor quality shifting. Much of the gain attributed to weight is actually improved efficiency in the drive train, better geometry and in the case of MTB, shocks that get more energy into forward movement. Lighter bikes are usually less durable than slightly heavier bikes - sponsored riders do not care about it, but they are not the ideal bike for a weekend warrior

I am by no way advocating heading out and buying the heaviest bike in the shop. What I am advocating is looking at your own financial situation, your riding desires and being absolutely honest with yourself as to why you are aiming for a 7-8kg MTB. Probably the best advice I was once given (as a non competitive weekend warrior) was "Buy the heaviest bike your ego will let you ride".

Answer 2

Boring answer: it depends, mostly on the type of terrain your GF will be riding.

Scientific answer:

Head on over to bikecalculator and start punching in your numbers.

When we take the reduction in total energy spent (Calories) as "increased performance" we see that for your GF (48Kg, 150Watts) on a MTB on a flat road the difference in energy spent per Km between a 8 Kg and a 16Kg bike will be less than 2%.

The same bikes with your GF climbing on a 30% grade (pretty steep) has a reduction in calories between the 16Kg bike and 8Kg bike of almost 12.5%.

Note that "Performance gain" can be measured in various ways. Total energy spent is just a simple first assumption that probably gives a good indication how much "reduction in tiredness" your GF can expect after a long ride. For serious racing, other factors are more important. If you are intereseted in this, you can look into the decrease in energy required to accelarate your bike, the reduced rotating masses, different wheel sizes and possible losses in suspension.

Be aware that this is serious weight weenie terrain where a lot of manufacturer pseudoscience/voodoo is practiced. See the various questions on the whole 26"/27.5"/29" wheel size debate for examples and decide for yourself on what info can be trusted.

Non-scientifc remarks from a practical biker's view

This is probably not helpful for your question, but it is, I think, very relevant for other people seeking bike advice on this site. A lot of people will probably disagree with these points:

• The difference between a 8Kg and a 16Kg MTB is huge. Quite probably both bikes are not intended to be used in the same terrain, or even in the same decade and therefore any one-on-one comparison is not valid.
• Getting the lightest bike also often means fragile or less durable components. Pro riders don't care if their bikes last only 1 season, or less.
• This is the real world, "other unknown parameters" as you put it, are always important. You cannot ignore the $/Kg value and the diminishing returns you get for pouring more $ into the bike. On top of that, a high $ bike equals high $ maintenance.
• In the end, what do you ride for? Personally I ride for fun, and having to worry about damaging my new 3k$ bike on a rock or root, or obsessively think about shaving 15g of my bike by installing a carbon bottle cage, in which I then will place a 0.8Kg water bottle, is not fun at all. Getting the latest and greatest gear each year (13 speed sprockets with 26.5" wheels anyone?) and in the process only dropping tons of cash just because otherwise you could be at a tiny disadvantage, is also not fun. For these reasons, I ride a 8 year old 13Kg second-hand 26" bike, with all standard robust components that are easy to get, cheap to replace and are guaranteed to be available for many years to come. The bike cost me $200, I am not afraid of it getting damaged or stolen and therefore I have tons of fun. Does that mean I'm slow? Not at all, but if someone does pass me it is just because they are in better shape or more skilled. Not because of my bike.

Answer 3

Yes weight matters. It matter more on hills and stops and go. On flat once you get up to speed it only has a small effect in rolling resistance. Spinning weight - weight in the wheels matter more - like a 2x.

But it is relative. Take the bike weight plus rider.
From 100 kg to 90 kg is 10%.
From 50 kg to 40 kg is 20%.

For sure going to 7-8 kg will have huge difference in cost. Even on a small frame you are going to need to go carbon pretty all the way (from handle bars to wheels). Even consider a carbon fork over a shock.

I have full carbon frame and carbon fork single speed medium and it weighs 18-19 lbs. Even with carbon wheels I am not sure it would make 16 lbs.

Answer 4

big. difference.

Let say the GF (48 kg) can output 256 watt of power. That means on bike A (16 kg) her power to weight ratio would be 4 w/kg while on bike B (8 kg) it'd be 4.57 w/kg.

Compare that to random guy (64 kg) who can produce 320 w. On bike A he'd have 4 w/kg and on bike B it's 4.44 w/kg.

The gain is bigger for the GF than the random guy (0.57 w/kg vs 0.44 w/kg).

Answer 5

@blam

No, I don't acknowledge spinning is 2x. Any rim mass added/removed only matters 2x during changes in kinetic energy, i.e. during accelerations only, and not for the vast majority of riding we do, which is mostly steady state or involves such low rates of acceleration that the difference in energy demand is completely negligible.

Don't forget that any extra rotational kinetic energy a heavier rim possesses also means the wheel doesn't decelerate as quickly either, meaning one can coast a little more.

Claiming wheel mass matters 2x is misleading, because, well, it doesn't. The 2x only applies when accelerating and only to mass added at the outer rim edge, and we get some of that energy back when decelerating. During the vast majority of time we are riding we are not accelerating, or the changes in speed happen so slowly (even when racing) that the difference in energy demand is miniscule.

Answer 6

Weight on the rider and same weight on the bike give different results. I rode with a 8 kg backpack on my 10 kg roadbike and all was fine performance-wise. I climbed very well and my average speed was good (on a 50 km ride). But put those 8 kg on the bike and the outcome will be totally different. You will become a lighter rider on a much heavier bike and pedalling will be considerably harder. Weight on the body help you push harder on the pedals and the bike responds. Weight off the body and on the bike is opposite.