Twitchiness at 40+ mph

[dogman dan]

That's good to know. It looks like a good bike, but if made to be too light, it could have been an easy to bend frame.

Maybe it's just this simple, the new tires at that speed introduced forces your cheaper shock just can't deal with. Or, it's simply designed to steer fast. One of my shocks has a tiny bit different dropout position, and it's a twitchy ride at any speed compared to all my other shocks.

You want a better shock eventually anyway, right?

Meanwhile find the fastest speed you are comfy with. 35 mph is still booking along pretty good.

 

[dominator]

Yea I'm considering putting my small block 8 tires back on but I'm pretty lazy to undo all the electric tape i normally dont go 40mph all the time either mostly cruise at 30s mainly because the wind pushing in my face gets cold lol so going fast it just gets worse

 

[TMaster]

Your stem is really short and that is most likely causing the twitchiness. I don't see how anyone didn't notice that. I can't tell how long your handle bar is, but it looks short. That will effect it also.

I would suggest putting a longer stem on, and if it is too high up, flip it over so it is angling down. A lot of the 29'ers have the stem flipped from the manufacturer. The geometry on that 29'er is really angled! That top tube would drive me crazy. heh

I have a 120 or 130mm stem on my mountain bike. I also have a crappy cheap 2002 suntour with the oil leaked out. I don't have any twitch going over 40mph. The longer stem and handle bar will have more relaxed steering. I find myself very comfortable with speeds in the 30-35mph range even sitting up.

 

[Paul_G]

Found an interesting article about heavier tires and shimmying in this post http://endless-sphere.com/forums/viewtopic.php?f=3&t=57471 , I did recently upgrade my tires to schualb big apple tires, was hoping for more grip but I think this is when I noticed the shimmying / twitch more, its not much though I can deal with it, just tough to ride one handed requires 2 hands on the grips for sure at 40mph, but I may be asking for too much.

There is no frame flex though for sure.

I'll say it once more.....tighten the spokes to the max and its reduced. The new tires loaded the wheels more. tighten the spokes till they sing very high notes when tapped. Just a few off key and you wobble and flex.

This worked well on my Optibike as well as my Bomber, the Stromer I also have is tight enough there is no flex at the moment.

I also reduced vibration on the Bomber by balancing the rear wheel with lead weights, that made an amazing diff at high speeds....smooth as silk. But thats a diff thing as flex

 

[CEGB]

A note of caution, don't just arbitrarily tighten spokes. There is an optimum point for the tightening of spokes. A point at where they are sitting in elastic tension so that they can comfortably deal with the variations in tension that occur as the wheel revolves, has torque or lateral loads applied to it, and cope with high peak loads without being permanently deformed. If you tighten them to much, you risk pushing them beyond the elastic phase into plastic deformation. This, ironically, will make them loose again. It will also cause them to harden, since they are being taken beyond the yield point for the material. The next step is fracture and failure of the wheel, followed by expensive dentistry.

You might also want to consider the ability of the rim and hub to cope with the addition forces.

 

[dogman dan]

Well for sure, don't run too loose spokes.

Interesting about the stem. The twich could be getting started by what the rider is doing with the steering. A longer stem might result in less overcorrection?

Playing with the head angle and trail might be interesting, if you can find a taller fork for cheap for example. That one fork of mine with a funny trail sure rides a lot more sensitive. At 20 mph it took getting used to!

Your rockshock looks like it's got enough trail though.

 

[speedmd]

This bike looks to be a very short wheel base and has a tiny head tube. Made with very light tubing also. Make sure the head set is set on the tight side. Make sure the front wheel bearings are also set tight. If the spokes are properly snugged up and the fork is working well, it is what it is. You could try thicker oil in the fork. Not worth putting any more into it IMO. A risk going as fast as you have it going. Extending the rear would help, but nothing will help stiffen up the short distance between the head set bearings / steerer tube problem. I don't see even a stiffer steerer tube or dual crown fork helping much in this situation. Try the stem position also, but have my doubts you will get it settled/ comfy at higher speeds.

 

[dominator]

I may try a longer stem, the only reason I liked it this way was so that I could ride my bike sitting upright, the angle isn't that bad it looks bad in the picture because of the kick stand, the front of the bike is a few inches off the ground. I don't think I feel comfortable enough to tighten my spokes myself though, I tested them all by hand and not one is loose that is for sure, I will try switching back to my old tires and a longer stem first I think, those are the cheapest quick fixes, just don't want to be dumping much more money into this build since it is costing a lot already and I don't really ride it that much. Thank you everyone though for all the good information and suggestions, I will refer back to this thread as I try all your suggestions.

 

[100volts+]

Someone on youtube did a video explaining that the best head angle for any bike is 0 degrees. Does that sound right?

 

[speedmd]

It is caster angle that effects steering stability and feel. Some refer to it as trail, but they are not the same. For higher speeds you want a longer wheelbase. Any head angle can work as long as you have a proper caster angle (around 81 degrees) and sufficient wheelbase. You will be able to ride no hands and steer with balance alone at high speeds without wetting yourself. Steeper caster angle will turn easier with balance, and shallower will want to stay straight / upright.