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I am looking forward to getting a new mountain bike and in various reviews sites they are really focused on if a bike have conical head tube or not. In many cases it is the first thing listed as pro (or cont in case the bike don´t have it).

It suppose to make the handling easier. But I don´t really see how that works. It is just another commercial trick or does is really affect the handling a lot ?

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Forks flex mainly at the lower head-tube bearing race (this is true even with traditional road forks that are curved to rake them), so increasing the diameter of the steerer tube at that point will stiffen it.

With some bikes, you may actually want that flex, as it will dampen some vibration. With a suspended mountain bike, obviously this is unnecessary and probably makes the suspension dynamics harder to dial in. And increasing a tube's diameter is a better way to stiffen the tube than increasing its wall thickness.

  • Properly pre-loaded headset bearings must eliminate any play between fork and bearing race for any headset standard. – Yury Rudakou Oct 19 '17 at 11:29
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    It's not a matter of play—it's a matter of the fork/steerer-tube material flexing. As I understand it, the flex gets concentrated there because of the discontinuity in the shape of the fork. – Adam Rice Oct 19 '17 at 13:32
  • And because the lower bearing acts as a fulcrum. – Adam Rice Oct 19 '17 at 13:49
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Assume we have a tapered headset with 1.5" lower bearing and a straight 1 1/8" headset. Let's compare two nearest thrust ball bearings from the manufacturer's spec (SKF ): 30x47x11 (dxDxH) and 40x60x13. For 30mm bearing the fatigue load limit Pu=1.6 kN, for 40mm (the nearest to 1,5") Pu=2.32 kN. So the tapered headset can carry more weight or it will work much longer in the same conditions as straight 1 1/8" headset. Additionally, the bigger lower part of headtube allows to increase the downtube diameter and make more stiff front-end. The stiffer front-end gives more precise steering and increases the pedaling efficiency as well.

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    Do the loads at headset ever come near the fatigue limit? How does stiffer front-end give more precise steering and increase pedaling efficiency? – ojs Oct 19 '17 at 11:32
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    @ojs Assume 100 kg rider rides off saddle on rough road. The estimated load is P=100 kg * 9.81 / 2 = 490.5 N ≈ 0.5 kN. For dynamic loads estimation the dynamic load coefficient should be applied. It is 2..4 as we learned in college. So Pd (dynamic) = 0.5 * 3 = 1.5 kN. It is close to Pu for 30 mm bearing. – Yury Rudakou Oct 19 '17 at 12:04
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    @ojs When you pedal you connect to handlebars and transfer the power through headset and head tube. Less flex in the front-end - more power is transferred directly to rear wheel. When you corner at high speeds the headtube and fork flex under lateral load. So less flex in the front-end - more responsive steering. – Yury Rudakou Oct 19 '17 at 12:13
  • @YuryRudakou Doesn't this assume that all the load from the rider is taken up by the single bearing. The suspension, tires, wheels and arms of the rider will help to absorb a lot of that load. Also, the entire weight of the rider doesn't go into the headset, much of that weight is going to be placed near the rear of the bike as the rider is standing on the pedals. The speed of the bike would also have a large effect on the amount of force applied to the headset. I'm not sure what the proper number is, but I think it's a lot more complex than what your calculations are estimating. – Kibbee Oct 19 '17 at 12:55
  • @Kibbee I've shown that loads may come near to limits. When you ride a road bike on cobbles at 40 km/h the loads can be even more than limits. I agree that it's complex task. – Yury Rudakou Oct 19 '17 at 13:05
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Its an incremental improvement over a straight steerer. A great bike from 2010 with a straight steerer will still outperform an average bike from 2017, and a tapered head tube is just one of 1000 small incremental improvements.

Today you cannot buy an average let alone a great bike with a straight steerer, so a starting point for a quality check is if its got a straight steerer, its probably a BSO. However, you need to consider far more than this one item to know if the bike is any good.

  • Cannondale Trail 2017 uses a straight steerer. Same goes for Trek X-Caliber. They aren't race machines, but I wouldn't qualify them as BSOs. Not even GT Zaskar Comp 9R comes with a tapered head tube, although the Zaskar Sport GTw 27.5 comes with one; and Zaskar is anything but a BSO. – Cristian Ciupitu Oct 20 '17 at 0:48
  • @Cristian: Edited to say probably. I need to stop talking in absolutes as there is always someone who calls me out on a trivial detail. – mattnz Oct 20 '17 at 2:11
  • It seems that the web page for Zaskar doesn't mention explicitly a tapered head tube, but it says "Headset: FSA No.42E ACB, Integrated, 1.5" Lower, 1-1/8" Upper". So after all even the entry level Zaskar has a tapered one. – Cristian Ciupitu Oct 20 '17 at 13:56
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You mean a so-called tapered headtube probably? Should be stronger in theory because there's more material on the lower part. Geometry-wise it shouldn't affect handling as all measuerements stay the same (at least, I assume downtube length gets adjusted so all angles stay the same). In theory it might be somewhat stiffer; that seems to be where most of the debate revolves around. But I'd love to see actual measurements and blind A/B testing on that. Let alone it makes for a true improvement when riding. As usual: maybe if you are a pro and you can feel the smallest detail you could notice a difference. (apart from that: not all forks are equally stiff anyway and I don't think there was ever a huge debate in finding the stiffest one out there, so it's a bit funny now that suddenly would matter a lot).

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    Blind A/B testing seems to show that tapered headtubes are much better. The guy hit 18 trees with the regular head tube but only 14 with the tapered. However, we think he might've started peeking round the blindfold during the second run. – David Richerby Oct 18 '17 at 12:23

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