Cassie Posted August 30, 2009 Posted August 30, 2009 I've noticed on my last few MTB rides...that when I'm in the Big Blade and low gear and really giving it a bit of stick (45 - 55km/hr) - mostly on slight declines /downhills...that my rear is very twitchy....i.e it moves about with each pedal stroke... It my back tyre too soft? I'm running tubeless at ~2bar Is my seat too high, thus causing hips to rock? My middle blade and all gears are not showing this when I put he hammer down in them.... What can it be? I ride a hardtail Merida....
chuck Posted August 30, 2009 Posted August 30, 2009 Depending on your weight the 2bar might actually be a little too hard. I'm 75-80kgs and run 1.8 at the back. At about 2bar my backend starts skipping over the rocks
Johan Bornman Posted August 30, 2009 Posted August 30, 2009 Cassie, a worn or tight headset presents as a bike that "fishtails" i.e. swerve left and right at the back end. This is especially noticeable on headsets that have indexed - motorbikes do the same. However, MTB headserts just about never index, so check for headset adjustment first. But, as you suspect, too-soft rear tyres also create that effect.
Iwan Kemp Posted August 31, 2009 Posted August 31, 2009 1. Headset (Like Johan explained) 2. You mentioned you are running tubeless. Are you using UST, Tubeless ready or std tires? 3. Experiment with Tire pressure. Not sure how you weigh, but I weigh 90k's and run mine under 2bar. 4. Tire tread - some race snake tires don't like fast descents on loose gravel.
Saffa_G Posted August 31, 2009 Posted August 31, 2009 sicne you back end is twitchy you should seek the help of a neurologist ..... they can help with nerve problems! hahaha .....
Cassie Posted August 31, 2009 Author Posted August 31, 2009 Ok, OK Worms & neurologists aside.... I believe the correct terminology is a bike that fish tails under a bit of hard pedalling. So, this only happens on tar road....when I do ~50km/hr on dirt it is usually a lekka downhill and no need for hard pedalling!! I weigh around 95kg's.my tires is converted to tubeless (not UST but the race lites from Merida still). I did notice that they have a seriously thin sidewall...combined with fairlow pressures - maybe it can be that? This stem issue is quite new to me - anyone care to explain that in a bit more detail? Is the stem too tight/ freezing up? Surely that would manifest all of the time and not just under certain circumstances?
Saffa_G Posted August 31, 2009 Posted August 31, 2009 to be honest I weigh around 92kg's 1.83m tall .... my back tire is 3bar and the front i leave at 2.5 bar with tubes in Maxxis Crossmark I am happy with this ... have tire liners too .... So you rear is sliding out when you are peddling liek a beast on a tar road yeah?..
chuck Posted August 31, 2009 Posted August 31, 2009 So' date=' this only happens on tar road....when I do ~50km/hr on dirt it is usually a lekka downhill and no need for hard pedalling!! This stem issue is quite new to me - anyone care to explain that in a bit more detail? Is the stem too tight/ freezing up? Surely that would manifest all of the time and not just under certain circumstances?[/quote'] don't ride on tar - problem solved When you lift the front tire of the ground and turn the handle bar it should a a smooth action. It should not be 'notchy' or sticky. It should also be light meaning if you tilt the bike slightly the handle bar should flop to that side easily. Also check any play on your headset by pulling the front brake with the tire on the ground and rocking the bike backwards and forwards. You should not feel any play. O yes an over tight or freezing headset effect would be most pronounced with small steering movements almost acting like a steering damper.chuck2009-08-31 02:35:41
Cassie Posted August 31, 2009 Author Posted August 31, 2009 Right, it's definately NOT a headset problem....thanks for the info.I suspect it might have something to do with running low pressure on the rear tyre and also having a really thin sidewall??? I have set of maxxis xmarks that I will try under the same conditions... And yes - tar sucks!!! But one need the tar to get to the playground!!
Saffa_G Posted August 31, 2009 Posted August 31, 2009 and up the pressure itchy ass ....... 2bar is too low for your weight
Iwan Kemp Posted August 31, 2009 Posted August 31, 2009 my tires is converted to tubeless (not UST but the race lites from Merida still). I did notice that they have a seriously thin sidewall...combined with fairlow pressures - maybe it can be that? Pretty sure that's the problem. Fit proper UST's or Tubeless ready tires or increase pressure.
Cassie Posted August 31, 2009 Author Posted August 31, 2009 Thanks Crow Will up the pressure till I can get UST's...I've been pushing the limits for a while...
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