10

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.

  • 4
    Well, yes, you're right. That saying is true but I think it came about when you think of the huge cost of ultra-light bike parts, where most of us could make the same weight saving (often considerably more) just by going on a diet! You just need to recognise that the saying is a generalisation and doesn't really apply to your 48kg girlfriend. – PeteH Dec 2 '14 at 20:00
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    That saying is intended to remind clydesdales like myself that watching your diet and shedding a pound off your waist is a more practical approach to increasing speed than say, spending $200 on a carbon handlebar to shed 40 grams off of the aluminum one that came with the bike. It in no way applies to your 48kg girlfriend on a mountain bike. – Scott Hillson Dec 2 '14 at 20:05
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    While I agree with the points made earlier, this not just about bicycle bang for buck, it is also about the relationship. I think you'll get four lots of kudos for the price of one. 1: She gets a nice new bike. 2: It's expensive. 3: You built it for her. 4: She gets to ride with you. Do it! – andy256 Dec 2 '14 at 23:51
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    What else does she carry? It might be cheaper and easier to strip her existing bike of the lights (during the day), pump, toolkit, spare drink bottle and anything else that you can carry instead. Around town I often swap panniers with my gf because she carries 10kg+ in her pannier and I often have ~2kg including the pannier. – Móż Dec 3 '14 at 1:33
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    If we are putting aside all other factors, of which some are important (e.g. bike fit) and others less so, then in essence the biggest differences will be in her ability to: - ride up steeper hills roughly 10% more quickly at same power - accelerate a little quicker, but unless racing to gain position at start of a race, probably no big deal - ability to carry the bike more easily when that's required, such as up difficult terrain, or when putting on the car or up/down stairs. – alexsimmons Dec 4 '14 at 22:01
13

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".

  • Even for me, riding with a laptop in backpack (about extra 3kg), there is a huge difference. I climb hills much slower, extra sweat and etc etc. and 25km is just as far as I can go (i usually on trail 40-50 km). My gf is entirely different, she's a bit weak as a normal cyclist's standard. So I was thinking shedding 7kg off a bike would make a lot difference for her – Nhân Lê Dec 2 '14 at 23:28
  • I go faster and longer on a light bike - I am not disputing that - what I am saying is how much faster and how much longer is debatable - An extra 3kg halving a riders endurance is not typical . – mattnz Dec 2 '14 at 23:54
  • mattnz, sorry if the question is ambiguous. I'm not asking about money. I was asking about the performance regarding weight. Would it be huge? just so-so? or nil? I understand it is for the benefits of others, but I think this answer would best fit with a question "How should I improve performance beside reducing the bicycle's weight?" – Nhân Lê Dec 3 '14 at 7:24
5

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.
  • I can see that yours is very similar to my gf's bike. Well, it cracked and I just have to start from scratch for her. As much as I have agreed with you, but in this case, why not make the best bicycle for her to ride for years than buying second hand and upgrade it over the months? That's my thought, though. What do you think? – Nhân Lê Dec 3 '14 at 10:43
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    When you say "make the best bicycle for her" you have reduced the problem to the fundamental question (what is The Best Bicycle?), which is great! But what is best for her? That is a question only you can answer, as we don't know your GF, or the type of riding she will be doing. All we can do, unfortunately, is provide background information and specifically for my answer, give some counter arguments for the much heard: "Get the lightest, newest bike you can find!", as this is not required to enjoy the sport. – biker12 Dec 3 '14 at 11:50
  • a 7KG will not last years........ – mattnz Dec 4 '14 at 2:45
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.

  • 1
    Blam is right regarding the spinning weight (so the weight of the rims, tyre (and inner tube if not tubeless) and spokes) making the biggest impact on performance. I believe the hub isn't as big an issue as it's at the centre. – jonifen Dec 2 '14 at 20:55
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    @jonc Exactly. The hub is spinning at the same rpm but not the same velocity. If I was an ant riding on the wheel at the top of the cycle I would be traveling exactly twice the velocity of the the bike. If I was on top of the hub my speed would not change as much. – paparazzo Dec 2 '14 at 21:19
  • Rotational inertia of wheels is at least 2 orders of magnitude less important with respect to the power demands than the overall mass of the bike + rider and other factors. Rotating mass is way, way, over estimated by many in it's importance to performance. Indeed it's so small it can be effectively ignored. This item summarises the physics: biketechreview.com/reviews/wheels/63-wheel-performance – alexsimmons Dec 4 '14 at 21:57
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    @Blam - you clearly are not reading the item posted are you? It is not empirical data. It's the physics model of cycling applied to different scenarios. Simple fact is your assertion that the spinning weight of the rims, tyres (and inner tube) matters 2x is quite simply wrong. It's clear you haven't understood just how small the rotational inertia of wheels is, and how little energy is required to change that rotational inertia, even when you make very large changes to a bicycle wheel's rotational inertia. – alexsimmons Dec 7 '14 at 22:50
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    Yes, I understand a wheel has both linear and rotational KE as I clearly demonstrated by showing the calculations above. But it's massively dwarfed by the linear kinetic energy of the entire bike + rider. So much so that claims that differences between two wheel's rotational inertia is a big factor in performance is highly misleading. Why do I have to stay off a post? I could just as easily ask people don't write nonsense in the first place instead. – alexsimmons Dec 9 '14 at 20:58
3

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).

  • now this is more scientific. Like it. – Nhân Lê Dec 3 '14 at 7:10
0

@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.

0

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.

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