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Tyres and Rolling Resistance

iwilldoitiwilldoit Posts: 710
edited November 2014 in MTB buying advice
Hi All.

Changed my sons bike from a Giant XTC Jr 24" wheel to a Whyte 604 Compact with 27.5 wheels he's been complaining it's a lot harder for him to go up hills on, his new bike is Nobby Nics as I wanted him to have some grip instead of the std tyres which are a bit slick, the shop we bought it off swapped the tyres for us so don't have the old ones.

How much difference does it make if I put something like a Racing Ralph on the back, how much would this cut the rolling resistance down, or would it be better with a different type of tyre, but we do need some grip as well, he's just started doing XC racing and is doing well, but on the hard pack up hill sections you can see others pull away a little, mind that might also be they have been doing it for years and he's only been mountain biking is 10 months, also I suppose tyre width and weight will come into it.

Posts

  • jimothy78jimothy78 Posts: 1,407
    Firstly, bear in mind that swapping to those bigger wheels effectively harden the gearing of the bike by about 14% (assuming the same range of rings/sprockets, which I realise may not be the case, but you get my drift).

    Secondly, the rims and tyres almost certainly weigh more, and that mass is definitely positioned further from the axis of rotation, which leads to a much higher force needed to accelerate them around that axis.

    Your son will get used to this, but it might take some time before he can exploit benefits of the bigger wheels, and learn to rely on carrying momentum, rather than relying on fast acceleration. Obviously, anything you can do to reduce the weight at the rim is going to be vastly beneficial - racing ralphs sound like a good bet (if the going is mostly firm) - depending on the particulars of the existing tyres and replacements, they could shed a good few hundred grams from the wheels (the reduction in rolling resistance would be noticable, too, but reducing the rotating weight is the major benefit). You could also try tubeless to make further savings.
  • The RookieThe Rookie Posts: 27,748
    Sounds much more like a gearing issue than a tyre issue, with a rolling radius more than 10% larger he will need to be geared down by a gear to get the same gear inch. You need to coach him on climbing technique, getting the gears right and either sitting and spinning or out the saddle. Tyre pressures also play a part, on rougher trails a LOWER pressure gives lower rolling resistance (conforming to the surface better).

    I wouldn't use Nicks for XC racing on hardpack, Ralph's for hardpack, Rons if it's a bit muddier, if its dry and hard then Freds, they will all also be lighter than the Nic's (weight savings really show up on the climbs).
  • iwilldoitiwilldoit Posts: 710
    Thanks for the replies yes think the bigger wheels will play their part in more effort to get moving and more weight.

    Just looked at the gearing the Whyte is Front 42-32-22 Rear 11-34 and the Giant is Front 42-34-24 Rear 13-34.

    Been trying to help him o the up hills, keep trying to tell him instead of moaning about it put the anger into the pedals and different ways of taking his mind off the slog up the hill, mind the bigger wheels come into their own on the other side of the hill, he's flying down them :)

    I will try later seeing what the actual difference in weight there is in the two bikes.
  • The RookieThe Rookie Posts: 27,748
    My personal take on hills is that you should be putting in the same pedalling effort as on the flat and downs, pedalling at your most efficient speed and at a load you can sustain with fatiguing, attacking them can rapidly lead to you slowing down.
  • lostboysaintlostboysaint Posts: 4,252
    iwilldoit wrote:
    Just looked at the gearing the Whyte is Front 42-32-22 Rear 11-34 and the Giant is Front 42-34-24 Rear 13-34.

    In terms of gearing that's about the same for "granny to big sprocket", even allowing for the jump in wheel size. It's probably just a case of him getting used to the size of it, how he's now sat on it and needs to deliver the power etc. It's no different from me riding my Inbred 29 and my Bandit 26, two completely different bikes that need riding with a different technique.
    Trail fun - Transition Bandit
    Road - Wilier Izoard Centaur/Cube Agree C62 Disc
    Allround - Cotic Solaris
  • iwilldoitiwilldoit Posts: 710
    The Rookie wrote:
    My personal take on hills is that you should be putting in the same pedalling effort as on the flat and downs, pedalling at your most efficient speed and at a load you can sustain with fatiguing, attacking them can rapidly lead to you slowing down.

    Your right Rookie my description of putting his anger into the pedals didn't come over quite right, more a case of thinking of keep going a sustainable pace, one thing I have been trying to get him to do is use his gears more and not go straight for the easiest gear straight away and have his legs going a million miles an hour.

    He's done 2 races so far and done quite well a 7th in the first one but he did look a little on edge ( can't blame him, I would of been to the loo a few times before the start :oops: ) and a 5th 2 weeks a go, but did look a lot more like he wanted it.
  • defridedefride Posts: 277
    Also seems as though lower tyre pressures (up to a point) improve rolling resistance. I was surprised this year to see that on the pro xc scene they're riding tubular and tubeless at 1.6 to 1.8 bar (23-6psi). The research suggests that tyres deform over obstacles better and they go faster, don't know how/if this translates when running tubes.
  • Why not try playing his favourite tunes via bar mounted speakers.
  • cyclecliniccycleclinic Posts: 6,865
    I have been running 26" tubes tyres at 30 psi for most of this year including 2" tyres and wider 2.4" which I might try at lower psi's. My tubular 2.0" are run at 28 psi but a change of tubs will allow lower pressures. It really does not slow me down it has had the reserve effect. I don't put it down to rolling resistance although this is important I'm sure. The reason why I am faster at lower pressures is simply comfort. As I ride rigid running 40 psi make the lumps and bumps unbearable and you have to slow down. Dropping the pressures makes the roufgh stuff bearable so you can speed up. Same effect if you have suspension. Grip in the bend and off camber sections is improved also.
    http://www.thecycleclinic.co.uk -wheel building and other stuff.
  • jimothy78jimothy78 Posts: 1,407
    I've noticed myself that riding a hardtail, despite being able to hold my own on smoother trails, I quickly get dropped by my friends on full-sussers as soon as the trail gets rocky and chattery. I put this down to a combination of the two effects mentioned above - without suspension, the only way the rear wheel can get over the front faces of the rocks is by bouncing up over them, which both saps my momentum and shakes me around, making for uncomfortable, slightly chaotic progress.

    Having recently switched to tubeless, I'm hoping the extra deformation of the tyres will help to soften the blow, allowing better control and less loss of speed through deflection.
  • cyclecliniccycleclinic Posts: 6,865
    It will work. Today I race on tubs at 26psi it worked really well apart for the slip sliding arround. Not the right tyres for the day.

    Also bigger wheels require lower gearing to maintain the same speed on a particular gradient.
    http://www.thecycleclinic.co.uk -wheel building and other stuff.
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