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  1. #1
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    Why no interest in fat tires for Downhill racing?

    I would think a 4" wide tire downhill bike would have distinct advantages over 2.3-2.5" ones in at least some of the races.

    I heard that Sandman has some amateur rider that has been doing fairly well with a Hardtail Fatbike. I'm not sure which tires he's using, since most of the fat tires are made for a supple ride at low pressure, not high speed racing. Nevertheless, even though they're not designed for it, they must be an advantage if an amateur on a hardtail can beat pros on downhill race bikes.

  2. #2
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    I'm not racing DH, but my pug is WAY faster than my HT 29er on some of the rocky, technical dh sections. It was quite surprising. I would think that when full squish fat bikes are developed, they could be more competitive, with the ability to run higher tire pressures to mitigate the increased rolling resistance.
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  3. #3
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    There was interest for a while- hence the 100mm BB, 165mm rear spacing, 3.0" Gazzaloddi and BigHit tires, and the few 40-50mm wide DH rims that cropped up on some downhill bikes a few years ago. Never really caught on, and I don't know enough about the downhill scene to say why.

  4. #4
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    lack of damping comes to mind.

    at speed down hill is the only time I miss squish. Pretty frightening with all that cush and nothing controlling the rebound
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  5. #5
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    Quote Originally Posted by Gigantic View Post
    I would think that when full squish fat bikes are developed, they could be more competitive, with the ability to run higher tire pressures to mitigate the increased rolling resistance.
    spot on so bring on the squishyness!!

  6. #6
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    Racing requires cornering and quick response time. It's not just about plowing over everything, one racecourse designer sees you can do that and they'll throw a nice sharp 90 degree turn to prevent you having any advantage.

    That much heavy rubber that far from the axle made the wheels harder to turn, so they tried 3" tires on 24" wheels for awhile.
    But the extra weight of the the tries was still a bit of an issue, regardless of the air volume they brought.
    Also (as any 29er fanboy will tell you) smaller wheels get hung up on rocks etc a bit more.
    Aaaah, 24x3" gazzis, kujos, intense and arrow tires, the 2.7 highrollers and .243 racing tires.... memories! :P
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  7. #7
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    Quote Originally Posted by Andy FitzGibbon View Post
    There was interest for a while- hence the 100mm BB, 165mm rear spacing, 3.0" Gazzaloddi and BigHit tires, and the few 40-50mm wide DH rims that cropped up on some downhill bikes a few years ago. Never really caught on, and I don't know enough about the downhill scene to say why.

    Better suspension and more travel killed the big tire movement in DH racing.

  8. #8
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    I am not thinking the bigger tires will help at all on absorbing impacts or going over rough sections, other than having more traction while doing so.
    I don't see how bigger tires would not corner better than skinny ones, even with the extra rotating weight that a proper downhill 3.5-4'' tire would carry. The extra weight doesn't affect how fast you can go through a turn, only how hard it might be to turn the handlebars. It also affects the suspension damping, but I'm pretty sure the shock tuners can compensate. In most any kind of racing on pavement or dirt, wider tires will always be able to go through turns faster. The only penalty for the extra weight comes with acceleration or braking.

    I wouldn't expect anyone to run low enough pressures to where the tires would bounce with a properly set up suspension. They wouldn't take the hits at that pressure anyways.
    I don't think you would feel the slight bouncing that comes with higher pressures on a full suspension bike.

    The fact that they went as wide as 3.0x24, and then abandoned it does seem to rule out the idea of anything larger being useful. I think they went with 24" rims for geometry, or the restriction of fitting the bigger tires into frames with the same geometry as the 26x2.0-2.3" tires, not because of weight. most of the time they don't even bother using kevlar beads on the heavy downhill tires. I think for cornering the rear tire having more width than the front was a bad idea anyways for a bicycle. It is necessary when you have a motor to apply power through a tight turn, but can actually hurt cornering while coasting, as most turns are taken on a bicycle.

    With 27.5" and even some 29" downhill bikes making an appearance, the geometry should be capable of full 26x4" tires front and rear, without compromising handling or suspension travel.

    I don't think that all racecourses would favor the fat tires, but certainly some would. I think the ones with loose dirt, or gravel turns especially would. I also think they would be an advantage if there are any fast sections with fast turns through rocky terrain, if the suspension is set up properly for the extra unsprung weight. A sprint up some loose or rocky uphill sections may favor the fat tire if traction was an issue with the skinny ones.

  9. #9
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    I think we need low profiles (probably radial) for fat tyres to work properly at high speed with suspension on rough terrain, otherwise the "bounce" will get you.
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  10. #10
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    I do like the idea of radial tire casings for both the lower rolling resistance it might offer, and possible improvements in grip, but I think bias ply tires could just as easily be made to have a lower profile.
    I would think the first place to see radial bike tires would be on racing road bikes, but they are just starting to use tubeless, since they are more traditionalist than MTBers.

    or, maybe in a tire casing as thin as a bicycle tire, the radial casing isn't viable, or advantageous.

    How confident are you that the bounce we have all experienced with the fat tires is not limited to rigid bikes?

    I can feel a similar bounce in my rigid 29er to a lesser degree, but not on a FS 29er.

    If you have a suspension to absorb the rough terrain, and the oscillations in the suspension, I think the bounce would be eliminated. There just wouldn't be any energy left from the impacts to cause that kind of oscillation.
    even though the tire give isn't damped, the energy that causes the oscillations has to be transmitted through the suspension from the mass that creates it.

  11. #11
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    DH tires have settled around 2.5" for the most part. Some still run a slightly bigger front tire, up to 2.7, and some run a slightly smaller rear tire, down to 2.2, but generally, it's about 2.5. Maybe 10 years ago, the average was about 2.7 or so. Before that, we saw the crazy 3.0 arrows and gazzis.

    Sticky rubber compounds have gone a long way with DH tires. Maxxis minions are still on top as far as what everything else is judged by. Remember that it's about overall maneuverability and speed. A 4.0 tire will be heavier, have a longer distance to the axle, have more mass at a longer distance from the axle due to the rim and other hardware, and generally be slower to accelerate. There's also the contact patch, you don't want a huge contact patch for cornering, because you also want to be carrying speed. A huge contact patch is kind of the opposite of going fast. You might be going fast on your HT/rigid, and you might think it's DH fast, but it's not. Heck, even look at off-road motorcycles. If we were "scaling up" from fatbike tires, they'd be using what, like 7" tires? Some of their stuff is no bigger than 3" though, with big meaty knobs (instead of huge volume with very small knobs).

    One of the things I want to do (apart from get a fatbike next) is built a 29er for all-mtn/freeride/downhill. This won't be an optimal setup, but I want to do it anyway for certain reasons and keeping it light with high end stuff for the "XC/AM" purpose as well. If you think about DH bikes though and 29ers, there are several reasons why it will never work and never be mainstream. I think 29ers and 27.5 will be just about the only good bikes available in the future, and 27.5 might take over DH, but 29 just cant do it. Why? Travel is one reason. Some people say "well, a 3" 29er is like a 5" 26er". While I find that to be somewhat true in terms of speed and not getting slowed down by obstacles, I find it's just as rough of a ride and hard to hang on to as that shorter travel bike, in other words, through all of my 29er experience there is no replacement for travel. 29ers limit the travel radically without going to like 19" chainstays, which also wouldn't work. Then past those geometry problems, there's the wheels. A 26er DH bike is not like a 26er XC bike obviously. The DH bike has far more rotational weight, just like your fatbike compared to a regular 26 XC bike. In this respect, a 29er DH bike with a regular abuse-oriented DH wheel, tires, maybe tube, etc, would just be a dog on a course. Fast once you got up to speed, but realize that DH races are won by guys pedaling their rear ends off. Lots of speed changes and accelerations needed. Lots of quick course changes and inputs needed. Although I love my 29er, they are kind of the opposite of what you want a DH bike to do. My personal idea is more based on the "one bike" mentality and overlap, so I realize it won't be a true DH bike. Lots of people get DH bikes that barely use them or do not live close to places where they could be used frequently. That is another debate or issue, but overall the 29er thing for DH will never catch on IMO. It could take everything right up to, and then go 27.5 for DH, but DH bikes are amongst the most specialized we have in terms of geometry, wheels, etc-meaning they work very well for DH, and pretty crappy at everything else...
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  12. #12
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    Quote Originally Posted by Jayem View Post
    A 4.0 tire will be heavier, have a longer distance to the axle, have more mass at a longer distance from the axle due to the rim and other hardware, and generally be slower to accelerate. There's also the contact patch, you don't want a huge contact patch for cornering, because you also want to be carrying speed. A huge contact patch is kind of the opposite of going fast. You might be going fast on your HT/rigid, and you might think it's DH fast, but it's not. Heck, even look at off-road motorcycles. If we were "scaling up" from fatbike tires, they'd be using what, like 7" tires? Some of their stuff is no bigger than 3" though, with big meaty knobs (instead of huge volume with very small knobs).
    I would never think a hardtail or rigid could even hang with any of the downhill rigs, but somehow Sandman's amateur rider has done so. Certainly he couldn't have been as fast over rough stuff, so he must have made it up somewhere. My guess is he must have been able to take the smoother corners faster.

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  14. #14
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    Quote Originally Posted by Andy FitzGibbon View Post
    There was interest for a while- hence the 100mm BB, 165mm rear spacing, 3.0" Gazzaloddi and BigHit tires, and the few 40-50mm wide DH rims that cropped up on some downhill bikes a few years ago. Never really caught on, and I don't know enough about the downhill scene to say why.
    Yep I rocked them. Monster t, Gazzalodi tires, head wheels. Loved every second!

    It became a light race bs thing. light and skinny became the norm over big hit bikes as thy were called.

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