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I have a new 2021 Cube Nuroad Race gravel bike with Shimano GRX, 11-34T, 2x11.

I am used to riding a road bike, and find on the new one I don't need the easiest gears on the hills, but could use a tougher gear on the downhill as I can't get any power down when descending. (I have switched the 40mm tyres for 32mm tyres while the weather is still not half bad in the UK).

I have never messed about with the cassette on a bike, switching gears etc, and was wondering how to go about it. Do I need to buy a whole new cassette? What cassettes are suitable and compatible?

Thanks

Edit:

Some useful suggestions. For more information, it is not just pedalling down serious slopes that I run out of gears, but on not very steep descents and occasionally on the flat if my speed is up. Though it seems what I need is to switch out the chain rings.

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    Some additional relevant info: the bike appears to have GRX 600, which implies a 46/30 crank. – Weiwen Ng Sep 11 at 11:27
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    Do you need to get on the power because you are racing/fast group ride? Or for longer interval sessions on rolling terrain? At the kind of speed you spin out a 46/11 an aero tuck is probably just as fast as pedalling in 90% of situations – Andy P Sep 11 at 12:14
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    In a 46x11 gear, you hit 31 mph at 90 rpm. I agree with @AndyP that you're probably better off getting into an aero tuck. – Adam Rice Sep 11 at 14:06
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    There are a lot of ways to make one's posture more aero without riding like Chris Froome. For example, getting very low on the drops, pulling my knees in, pushing my arse aft, I cannot pedal efficiently anymore, but am much more aero while as safe as I can be on my bike. – gschenk Sep 11 at 19:21
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    We have a few of these questions on the site - the answer often comes down to "Spin faster" - 46/11 is only slightly lower than a typical road set of 50/12 and gives 54km/h at a cadence of 100. Unless you have very good high cadence pedaling, training to spin is a better option than pushing bigger gears. – mattnz Sep 11 at 19:50
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You cannot get a higher gear ratio by replacing the cassette. The 11 tooth sprocket is the smallest you can get on a compatible cassette. (Systems with 10 tooth sprockets exist but use a different freehub design).

The issue is that you have 46/30 tooth sub-compact chainrings (I looked up specs here). Which are giving you the low ratios. This is common on gravel bikes as they have larger diameter tires run at lower pressures and are designed to negotiate rougher surfaces. A road bike typically has 50/34 or 52/36 tooth chainrings.

If you want higher gear ratios you have to get larger chainrings. GRX 2x11 cranks only come in the 46/30 size. You may be able to find larger rings made by another manufacturer that fit the GRX crank, or you get a whole replacement crank with larger rings.

If you still want to know how cassettes fit on hubs see this Park Tool video. There are a few standards for the freehub that the cassette fits on. Shimano's HyperGlide II is dominant and also used by SRAM. Campagnolo has their own standard. Systems that permit 10 tooth sprockets like SRAM XD and XDR are becoming more popular. In addition to the freehub attachment the cassette sprocket tooth range has to fit inside what the rear derailleur can handle. This includes the max sprocket size and 'total capacity' (difference between chainring tooth counts plus difference between largest and smallest sprocket tooth counts).

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    A crank with bigger rings might not clear the chainstay or the tyre, though. Better check. – Carel Sep 11 at 11:40
  • @Carel good point, thanks – Argenti Apparatus Sep 11 at 11:41
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    A minor quibble: the GRX 600 crank only comes in a 46/30 ratio. The 800 crank comes in 48/31, which will steepen gearing slightly. I suppose the OP could buy the 800 chainrings; I haven't yet seen 3rd party rings for GRX (whose bolt circle diameter is smaller than Shimano's road cranks), but they should come. Nonetheless, new chainrings won't be cheap. – Weiwen Ng Sep 11 at 12:46
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    Gabaruk already make GRX chainrings. Best inquire there directly for compatibility since they might only have 1x rings. – gschenk Sep 11 at 12:53
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    @gschenk good point, but you're right that Garbaruk currently only offer 1x GRX rings. They do offer aftermarket rings for Shimano road double cranks, so they may soon put out some for the GRX BCD. Because of the issue with designing the shifting ramps (and getting around Shimano's, SRAM's, Campy's, and anyone else's patents), I have a feeling it's a lot easier to design a 1x ring than a 2x. – Weiwen Ng Sep 11 at 18:52
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FWIW, there is one company that makes a 9-34 11 speed cassette, but that cassette only works on XD hubs. It's unlikely on a stock bike like that but if the wheel set allows you to switch an HG driver for an XD one, you can get one higher gear for about the same price as switching cranks.

Personally, I think 46/30 and 11/32 is an ideal road riding setup if you live in hilly country.

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I am used to riding a road bike, and find on the new one I don't need the easiest gears on the hills, but could use a tougher gear on the downhill as I can't get any power down when descending.

Don't pedal down hills.

Seriously, when riding downhills your main concern is air resistance.

For example, a bicycle speed simulator I wrote in Matlab tells that in certain downhills (minus 5% grade), I get 57.1 km/h speed when not pedaling, i.e. coasting.

For the same downhills, if I manage to reduce my frontal area by 20%, I get 63.8 km/h.

If I manage to pedal with 150 watts extra power, but the rapid pedaling creates air swirling that increases the air resistance by 10%, I get 58.4 km/h speed.

Most of the benefit of pedaling went away due to the air swirling increasing air resistance. In fact, by pedaling I get 2% extra speed and by optimizing my position to minimize frontal area I get 12% extra speed.

The position that minimizes frontal area is not a very efficient pedaling position.

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    There are various hills of various slopes and various curvatures of the road. You cannot simply say do not pedal, many downhills are not steep enough for high speed coasting. Moreover, I do not believe your air swirling at all. I am not an aerodynamicist, but I do fluid dynamics and turbulence. The effect of the pedaling feet will be very small if not negligible completely. – Vladimir F Sep 11 at 15:41
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    Also, the answer does not actually answer the actual question at all, it is just a rant. – Vladimir F Sep 11 at 15:43
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    @VladimirF I agree that this answer isn't the most productive of all the answers. It does raise a valid point, though: you can get quite fast just coasting, although the same point was already raised by Andy P and Adam Rice in comments on the original question. – Weiwen Ng Sep 11 at 18:50
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    And comments are the appropriate place for such recommendations. There are enough reasons conceivable why the OP might want to gearing that makes a difference downhill. – gschenk Sep 11 at 19:17
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    @mattnz Those comments are upvoted.(also by me) and well visible and placed where they should have been placed = unlike this answer which is just off-topic and deserves only a dv. – Vladimir F Sep 12 at 6:41

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