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I have a Montra rock 650B MTB. I use 2 lock systems - 1 cable and 1 chain lock for added security. Both are attached to the frame while I cycle. The chain-lock is pretty heavy (around 700 grams).

I was wondering if the mass of the locks attached to the frame affects the bicycle's performance. If so, is carrying the locks in a backpack instead a better solution?

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    Additional weight directly on your bike adds unsprung mass, or at least "less sprung" mass. The "spring" here being you, mostly your legs. If you carry the heavy lock on your back, the bike will be slightly nimbler. So yes, its performance will be reduced with a fixed mount. Unfortunately having all the stuff on your back will reduce your performance - and comfort. Depending on how you use the bike, I estimate this effect to be more important than the small change a fixed mount would have. Do what works best for you. – linac Nov 24 '17 at 13:48
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    Assuming by performance you mean the travel speed of the bike (or the effort you put into riding), as opposed to changes in "handling performance / characteristics", I can't see you experiencing a perceivable difference. My water bottle holds roughly 850ml of water, so 850g plus the mass of the bottle itself - I never notice a difference between riding with or without the full bottle – Mike Nov 24 '17 at 18:27
  • Very minor point will be a reduction in aerodynamics, simply from having another thing on the frame. This will be relatively small impact. – Criggie Nov 24 '17 at 19:13
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    @linac could you post your comment as answer? It's far better than the actual answers and comments should not be for answers anyway. – ojs Nov 26 '17 at 11:50
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Yes your bike will be heavier but it the only performance reduction putting it into your back pack wont change it. Personally I carry mine in a back pack but it is because I cant find a place in my bike where to carry it and bike it self feel more heavy. I can feel how it affects the balance setting it higher and making the bike unstable.

So as a personal chose I prefer to carry by bike lock in a backpack. If you feel ok carrying them on your bike go on. You only problem will be the added weight.

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The worst mass to have on your bicycle is rotating mass, because you must overcome both rotational inertia and "regular" inertia to get the bicycle moving forward. Of course you don't mount the locks on the wheels, so that's not a factor.

If your bicycle had a suspension system, then the next worst kind of mass would be unsprung mass, that being mass that has to bounce up and down on bumps. You have to provide the energy to move the mass up and down, of course. Your bicycle doesn't seem to have a suspension other than the pneumatic tires, so that's also not much of a factor.

If you ride very steadily without bouncing up and down on the saddle, then there might be a slight performance gain from keeping the lock in your backpack. However, if you get off the saddle to stand and pump the pedals, then you might lose performance. Personally I think that any small performance gains you might see from keeping the lock in your backpack would be dwarfed by the discomfort of having a weight in the pack that you might otherwise more comfortably mount on your bicycle frame, so if it were me I would probably elect to keep the locks on the frame.

You could always experiment: try a week or two with the locks on the frame and measure your times on your usual commute, and then try another week or two in the pack. If you do experiment, please report your findings as an answer to your own question. (It's fine to answer your own question, and even accept your own answer if you think that it's the best answer.)

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    If your bike does not have suspension, it and everything attached to it is unsprung mass. I wish I could give a second -1 for perpetuating the rotating mass story even when it's not related to question at all. – ojs Nov 26 '17 at 11:46

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