According to the specialized website, the 54cm crux 2021 comes with 700x33c cx tires.


And, its bb drop is 69mm and its bb height is 283.5mm.

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That means its tire height is 41.5mm?

My current 700x25c's tire heigh is 25mm and I also ride a 700x44c and its height is about 40mm.

So, did I make wrong calculation? or CX's tire is taller than normal road bikes?

The calculation is

( ( 283.5 + 69 ) * 2 - 622 ) / 2 = 41.5
  • This is an interesting question, but how important is the height of the BB above ground? To my knowledge, ground strikes by your cranks aren’t that common in cyclocross. – Weiwen Ng Sep 13 '20 at 18:50
  • @WeiwenNg The BB height is directly related to center of gravity and also off-road ability. It is one of the key factors to distinguish the different kinds of road bike, isn't it? I am just collecting the different BB heigh of a cx, gravel, climb, aero or endurance road bike. I need correct BB height to clarify the differences among them. – Cray Kao Sep 14 '20 at 2:00
  • 1
    BB drop is a clearly defined parameter. Lower Bb drop = lower center of gravity. That said, in general, BB drop doesn’t vary a huge amount over most drop bar bikes. CX race bikes may have less BB drop (I.e. higher Bb, so higher pedals and center of gravity) to reduce pedal strikes. Everyone else will probably have 70-80mm. It varies more when you compare drop bar to mountain bikes. – Weiwen Ng Sep 14 '20 at 10:13

BB height is a measurement that depends on too many ill defined parameters to be useful.

A rough approximation for three height of a tyre is to take its nominal width. That leaves us with a discrepancy of about 8 mm. Spezialized seem to have used either a height from the tyre manufacturers data sheet or actually measured themselves.

Tyres deviate from a perfect toroidal shape. For example, a fully inflated tyre on a narrow rim might be much taller. Rim and casing will determine the actual cross section.

There is also a layer of thick rubber to roll on and, in the case of the CX tyre knobs.

Lastly, for a real measurement one has to consider tyre drop* of the tyres. For road riding up to 15% is typical. CX tubular tyres might be depressed by 50% however.

Finally, Spezialized's road section might use different conveniens l conventions than their CC section or MTB section.

Overall there is so much guesswork in interpreting that value that you might only find out that BB height means by asking Spezialized themselves.

  • Tyre drop is a measure of the reduction of a tyre's extension in radial direction under a radial force oriented towards the hub.
  • So, the formula of the calcuation is correct, isn't it? – Cray Kao Sep 13 '20 at 13:59
  • I guess the bb height includes the hight of "mud" during the cx racing. It is the most possible what I can think of now. – Cray Kao Sep 13 '20 at 14:03
  • Yes, your calculation is correct. I don't think mud is included. My guess is the height of the tyre in CAD from the manufacturer's model. – gschenk Sep 13 '20 at 14:39
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    By “depression”, I assume you mean tire drop, as documented by Frank Berto? This is the vertical distance a tire compresses under a specified weight. It’s not a very common term in the industry to my knowledge, and I don’t know if it’s an accepted standard concept. renehersecycles.com/tire-pressure-take-home – Weiwen Ng Sep 13 '20 at 18:48
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    Berto, Heine, and Silca were recently discussed in the following question and answer: bicycles.stackexchange.com/a/71997/30402 – gschenk Sep 13 '20 at 20:54

There are several things that may be at play here.

Tires bulge like a balloon out of the rim. Thus, tire height in general is more than tire width because tire is higher than a half circle. So it is untrue that a 25mm wide tire is 25mm high too.

Firstly, most road tire purchasers look at tire weight and make a purchase decision based on the tire weight. Thus, your current 700x25c tires may actually not be 25mm wide but they may be 23mm wide instead. By selling a 23mm tire as a 25mm tire, the manufacturer is able to make it lighter than its competitors. Soon every manufacturer starts to do this to gain the same benefit of light weight, and we have lots of tires sold with incorrectly marked size. This would mean that 23mm tire is 25mm high, i.e. tire is 8.7% higher than it is wide.

If you take this effect into account, 33mm tire should be about 36mm high.

Cyclocross tire manufacturers do not have as large incentive to sell tires with incorrectly marked size, because cyclocross tire purchasers do not make decision on the weight of the tire.

Secondly, cyclocross tires have knobs. They do not contribute to width but contribute to height. If a 33mm wide tire which is 36mm high has 5.5mm high knobs, the tire total height is 41.5mm.

  • 3
    I metered the tire, and it is actually 25mm height. – Cray Kao Sep 13 '20 at 13:57

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