Twitchiness at 40+ mph

D

dominator

10 W
Joined
Nov 27, 2013
Messages
75
Location
Westminster, CA
Here is my bike:



So I thought getting new wider road tires would help and it did, but today I tried for the hell of it locking my front suspension and the bike is now much more stable at 40+ mph... The only reason I got this bike was for the front suspension, question is should I upgrade to a better suspension fork or just go with a carbon fiber one since it seems the suspension is causing twitchiness at high speeds.
 
It can be done, but dual crown forks tend to be longer, so your geometry will be awful.

Also, speak to the frame manufacturer, they may well advise against it. That headtube is pretty short, and it's often said that dual crowns place an awful lot of stress on that area.

Also, steering lock is somewhat restricted, and the top and bottom tubes will easily damage the stanchions if you crash, potentially writing off your frame...
 
Try tuning the suspension to get a better high speed ride. Stiffen the front rather than fully lock it? Or is the front too soft because your rear shock is too stiff? Or is it hardtail? The pic is cut in half for me.

I can't say exactly what you need to do, but I think you just need to tune the shocks different for an Ebike.

It may just flat out be too short a bike for 40 mph. A longer bike will love the speed.
 
dogman said:
Try tuning the suspension to get a better high speed ride. Stiffen the front rather than fully lock it? Or is the front too soft because your rear shock is too stiff? Or is it hardtail? The pic is cut in half for me.

I can't say exactly what you need to do, but I think you just need to tune the shocks different for an Ebike.

It may just flat out be too short a bike for 40 mph. A longer bike will love the speed.
Click to expand...

Yea I tried turning the suspension dial the opposite direction to the max will ride it tonight see how it does, I really hate to lose the suspension though but if I have to I will
 
Suspension dial? The lockout adjustment on the top of the fork crown? Look at the bottom of the fork leg by the axle for a rebound adjustment lever that turns by hand. Turtle= slow spongy rebound, Rabbit= fast hard rebound,try 1/2 way between to start and go from there.
 
Did locking the front fork mean the front of the bike rode higher than normal? That would change the head angle of the bike and the associated stability.
 
Maybe that is what you need, taller. Bear in mind, the MTB frame was never intended to go that fast really, with it's slightly short wheelbase. So bolt on rear dropouts that lengthen are becoming a common modification for the cromotor crowd.

Also, if anything is moving in the back, it will amplify the frame whip and the high speed wobble.
 
Lol no not in my pants ;) this is a hard tail it only has front suspension and locking it doesn't change the height from what I can tell, i still havnt had a chance to test ride it yet with the change in suspension, the bike has two dials on the fork one to lock it and the other to change the preset weight on the springs.
 
Take the bike to 30-40 MPH and wiggle the bars...feel the bike flex?

Now tighten the spokes to the max setting or even a tad more and try again, you will be a happy camper with a grin as a lot of flex is in the wheels/spokes and is now reduced but if you want 0 flex buy my Bomber I have for sale...lol
 
It looks a poor design. The triangle is tiny and way it pinch's too the tiny head tube offers next to nothing. To stop the twisting you have just the wheel on the ground, but through suspension.... that's not going to work. It can dance all over the place. Even with it locked you still have a large fork length levering on the area in question. With a good weight in the frame resisting a change in direction.

It just looks soggy.
 
I don't think its the spokes though I looked at them and its all tight, with the suspension locked its rock solid turning the handle bars also at 40+ mph, the battery is an HPC LiNmc battery 90v 13ah. The bike is a Trek Xcaliber 8 2014 model, I was told the geometry on these bikes was really good, but I guess not for 40+ mph =).
 
Rockshox XC32's on 29" rims aren't exactly going to be the last word in composure and stiffness. The XC32 is RS' entry level fork. Its going to be hard pushed to cope with ordinary riding with anything like the competence of high end or even mid range forks, so hammering down the trail at over 40mph is asking a great deal of its limited abilities. Try a different set of single crown forks, you might find them a revelation.
 
CEGB said:
Rockshox XC32's on 29" rims aren't exactly going to be the last word in composure and stiffness. The XC32 is RS' entry level fork. Its going to be hard pushed to cope with ordinary riding with anything like the competence of high end or even mid range forks, so hammering down the trail at over 40mph is asking a great deal of its limited abilities. Try a different set of single crown forks, you might find them a revelation.
Click to expand...

Does the same apply if I'm just riding on the street not on trails?
 
dominator said:
CEGB said:
Rockshox XC32's on 29" rims aren't exactly going to be the last word in composure and stiffness. The XC32 is RS' entry level fork. Its going to be hard pushed to cope with ordinary riding with anything like the competence of high end or even mid range forks, so hammering down the trail at over 40mph is asking a great deal of its limited abilities. Try a different set of single crown forks, you might find them a revelation.
Click to expand...

Does the same apply if I'm just riding on the street not on trails?
Click to expand...

Sadly, yes. Mediocre damping and stiffness remain mediocre even on tarmac, and the extra length of 29" forks allow more flex than would be comfortable given the duty being asked of them. You're carrying around a lot of extra weight, and putting a lot of additional load on the forks. It could easily be that they're just being pushed to hard. You might also want to consider new headset bearings, they can have a dramatic effect in stability when they're worn or indexed.
 
The twitchiness is better now that I tightened the suspension, perfect no, but much better. I dont know enough about maintenance to change bearings may ask my lbs, but the bike is only about a month old too with roughly 65 miles on it.
 
With only 65 miles the bike would have needed to be beaten to death. The headset bearings will be fine. The forks however, will be very tight. Give them a few miles to break in and they will settle down to more normal behaviour.
 
Remember, this bike may have been designed twitchy, so you can make fast steering inputs on a twisty single track.

Now you have it hauling ass, in the 40 mph club. I'd be seriously squinting at the rear dropouts, with a bolt on,2" extension in mind. That lenthener, which may also allow you to adjust the bikes head angle, should improve things for hauling ass. It could also include a pinching dropout on the left side with ease.

But if you can see the frame flexing a lot, it's too thin walled and weak. If that is the case, you are looking at a do over.
 
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.
 
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