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I'm looking to buy a preowned Voodoo Marasa 2014 hybrid bike (full spec available here).

The bike comes with the original 700 X 45C tyres, but I'm wondering if it is possible to replace them with something thinner, and if so, whether it is worth it.

From the very limited understanding of bikes that I have, I gather that thinner tyres mean less friction, and so you go faster. I do most of my riding on the road or cycle paths (almost never off-road), so a thinner tyre seems like a more sensible option, but I don't know whether the benefits would be very noticeable or cost effective.

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    The benefits are often overstated, but you should be able to go somewhat narrower. – Daniel R Hicks Jan 24 '17 at 23:17
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    Have a browse of the existing questions - listed on the right side of screen under "Related" heading. Generally you should buy the bike you want to ride, and not one that needs a lot of changes. Swapping out tyres is not a big deal, they are consumables after all. Note that bike does appear to be a low-spec BSO, and given you do road/paths, why buy a MTB with heavy and useless suspension? – Criggie Jan 24 '17 at 23:21
  • @Criggie besides being a rather cheap bike where one typically cannot expect great value, it seems rather well suited for the intended use. I cannot see any suspension forks on it. The tires on the picture also seem not very knobby. – gschenk Jan 25 '17 at 14:25
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Rolling resistance depends much more on the tread, the shape of a tire, and the suppleness of its side walls. A few points to explain this:

A tire with a very pronounced tread will roll less smoothly over hard roads and requires more energy than a slick tire of same width.

A ballooning tire that is considerably wider than the rim will have to deform more as it rolls.

A stiff tire dissipates more energy for these deformations.

If you like to reduce rolling resistance, you might try to get a slick tire, or a tire with small treads. (There is by the way no aquaplaning concern with slicks for bicycles.) This will bring by far the greatest gain you may get. Marginal improvements may be had if you get a tire that is only slightly wider than the rim and is supple. However, the trade-off for the latter are a higher risk of flats, less comfort, and considerably higher costs per tire.

Also note, in general a tire that is narrower has to be used with higher pressure in order to avoid pinch flats.

Estimate

If you like to calculate how much a reduction of rolling resistance may bring in a best-case scenario you could use this calculator. Enter your weight desired power. Also increase the cross sectional area to something closer to 0.7 m^2 to reflect the more upright posture on a hybrid. The rolling resistance coefficient 0.004 assumes very well running tires. Calculate the speed once with this value, then double and tripple it.

Example calculation: For 100 kg weight, at sea level on a flat, and 200 W power I get 9.15 m/s, 8.53 m/s, and 7.82 m/s for coefficients 0.004, 0.008, and 0.012, respectively. For lighter riders rolling resitance matters less.

Further reading:

Sheldon Brown, tread patterns

ibid, tire width and pressure

Jan Heine, suspension losses

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I had a mountain bike and replaced the chunky off road tires with thinner commuter bike tyres and the difference was very noticeable. You can get thinner tires (not like super road bike thin, but thinner than standard) for hybrids pretty cheaply, so price shouldn't be an issue. If you're going to replace them anyway, why not?

Only disadvantage I can think of is that if you're cycling for exercise it will hinder you there by making it easier. I used to be puzzled why I'd see guys riding chunky mountain bikes to work along the pathed bike paths, but then I realised they were doing it to get fitter and stronger.

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    A lot of the difference is due to the texture of the tires. Even at the same diameter, a smooth tire is way better than a knobby on the road. – Batman Jan 24 '17 at 23:45
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Briefly:

  1. Yes you can install a much thinner/narrower tire on your rims

  2. It is not worth it (compared to buying a similar but narrower tire) because:

    • Adjusting inflation properly on your current tire will be more effective for:
      1. Cost
      2. Performance (compared to a similar narrower tire)
  3. Unless the tires are worn out or damaged, then consider:

    • A better quality tire of the same size or larger will generally be faster based on:
      • Suppleness which is not how thin/narrow the tire is but often the thinness/flexibility of the tire materials.
      • Proper inflation which is always a factor for a tire of any quality, suppleness, or width.

Ugly (but useful!) Details:

It is very rare for a tire to be too narrow for a rim like yours, though a tire section as narrow as 18mm might not be ideal for that rim. However...

I strongly believe you should only replace your tire with a superior tire for your purpose and avoid the false notion that a thinner/narrower tire will be faster.

Try your existing tires at various ideal inflation levels for you (discussed here) to get a sense of how your current tires can perform. Appropriate pressure is almost always lower than the maximum pressure listed on the tire.

Ultimately, find an affordable tire that will flex to absorb irregularities in the road surface but with a tire sidewall or casing that is tough enough for your riding locations. (Some folks content with really rough roads, road debris or even thorns.) The most "supple tires" (Great modern keywords for product searches!) are usually tough enough for typical uses.

Sometimes you must give up some flex to purchase a tougher tire or to allow it to endure more mileage. However, with any tire you can add more pressure to help avoid things like "pinch flats" or tire trouble in rough terrain like rocks, unexpected curbs, and pot holes.

Even seemingly smooth roads and cycle paths are not uniform marble-like surfaces. If you choose a tire that has supple sidewalls (a better type of thinner tire!) and you inflate them so that your weight and load will allow them to absorb the bumps properly, you'll actually have a faster tire no matter how wide it may be and you get advantages of a bigger more secure contact patch on the road.

Some people might mention the extra weight of a larger tire, or rotating mass that might affect acceleration when speeding up or when stopped. In my experience, those optimizations count less than the comfort or ability of a tire to mitigate road irregularities or even road buzz and vibrations.

(Note: Knobby tires and some significant tread patterns may affect how a tire handles on the road but I'm assuming a mostly smooth tread.)

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