yaw&roll instability of small quad (atv)

[ivymike]

I picked up a cheap used quad for my son, and spent a little time getting it running last night. actually, I made a bit of a late night of it - couldn't find wire the right size to force the congealed former gasoline out of the jets, and I didn't have an appropriate solvent.

Anyway, test driving this morning I noticed that the vehicle has an interesting steering characteristic. It steers opposite the way it is tilting. This means that with a high CG (like with me on it), a little tilt to one side or the other causes the vehicle to veer in the other direction, which causes it to tilt more, which causes it to veer more, etc. Quite easy to roll it if you have any speed at all...or should I say difficult to keep it upright above 20mph.

It would seem that the short tie rods are angled somewhat steeply downward from the stubby little pitman arm, and so when you lean left the tie rod becomes more horizontal and the left wheel turns right. Now the effect is likely exaggerated with me on it vs. my son, since I'm 220lb and he's 37 lb, and I'm a little less than twice his height. I can barely fit on the seat without my knees rubbing the handlebars.

Anyway... since it's such a sketchy ride for me, I was starting to think about simple mods to make it more stable. lowering the static height seemed one way (making the tie rods more nearly horizontal in the static condition).

Any thoughts?

 

[ScottyUK]

Not solving your problem, but there were a few nasty accidents reported over here in recent years where an ATV had reared up so violently that it flipped over backward - a combination of poor throttle control and adverse ground conditions I imagine. The riders were suffered severe neck injuries, at least one being paralysed. I think these would be much more powerful machines than the one you've bought for your little lad but you might want to check if your machine exhibits any such tendency.


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If we learn from our mistakes I'm getting a great education!

 

[ivymike]

nah, it'd have trouble getting out of its own way. it also has a throttle limiter screw which prevents pushing the throttle control lever beyond a set point, and I've turned that guy in a bit to reduce max acceleration (top speed is still way faster than I'm totally comfortable with).

Back to possible solutions, I think another option would have been to have a forward-facing pitman arm instead of the current rearward-facing one, with tie rods that connect in front of the steering axis instead of behind. That should give the opposite swerve behavior, making the vehicle try to correct a tilt instead of trying to exaggerate it. I'll have to look at whether the steering knuckle design would allow me to swap sides (can probably turn the handlebars and downward bar around)

 

[Latexman]

Not being knowledgable in ATV suspension and handling dynamics, I don't have any advice on that, but I did have a personal, profound ATV experience and advice on something else.

A neighbor's (two doors down) son (age 11-12) died from an ATV accident when we lived in TX about 3 years ago. It happened on flat, paved, neighborhood streets when turning a corner. He was not wearing a helmet and he was out of control (in more ways than on the ATV).

IMO, the root cause was his parents. They did not enforce safe behaviors on the ATV. So my advice is - establish the rules and punishments up front with your son. Then, be unrelentless enforcing his behaviors, up to and including selling the ATV.

Good luck,
Latexman

 

[ivymike]

yeah, the neighborhood kids, their parents, and the other people in the neighborhood are all fairly cavalier when it comes to road safety here in Tx. I've seen a woman accelerate rapidly toward a kid's toy in the road, in a residential neighborhood, as the kid ran for it, because she wanted to get past the toy before the kid got there vs. having to wait for the kid to get out of the street. Kid's caregiver yelled at her, and she rolled her window down to yell a profanity-laced tirade about kids being in the road.

 

[MintJulep]

Since you're so much heavier than the intended load maybe you turned the geometry "inside out" so to speak.

I can't imagine that the designers would design steering geometry as you describe.

 

[ivymike]

nah, it's that way even if I just push down with one hand.

 

[Compositepro]

Check the tire pressure, too. I've noticed that my ATV does what you describe when riding on pavement with low front tire pressure.

 

[waross]

The tie rods behind the center and the rear facing pitman are designed to give good geometry on turns. On a turn, the inner wheel always turns in more than the outer wheel.
However, investigate the possibility of lowering the pitman to make the steering links more horizontal.
Take a look at possible failure modes. You don't want to have a high center situation push the handlebars into your son after making modifications.

Bill
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"Why not the best?"
Jimmy Carter

 

[ivymike]

I can see that the effect you're describing is an inherent outcome of the roll-steer behavior I was describing above...I'm not sure whether tighter cornering capability and/or reduced tire wear (the goal of this geometry, right?) is worth it on a 70cc

nearly impossible to lower the pitman arm w/r/t the rest of the vehicle, due to interference with steering shaft support.

 

[ivymike]

okay, after watching the little guy zig-zag down the street a couple nights ago, I decided to try out the steering knuckle swap. The parts fit together fine with the tie rods in front of the steering shaft. The vehicle was much much more stable when driving straight... unfortunately the geometry was WAY off when making turns. After a few degrees of handlebar turning, one wheel would stop turning altogether and the other would turn in steeply. Made sharp turns very interesting. I swapped them back.

I did notice when I put everything back to the original setup that I could increase steering resistance (friction) by tightening the steering kingpins (bolts).. with extra friction the vehicle feels more stable at speed, but it still has a noticable swerving behavior when you let off the throttle at high speed.

 

[LionelHutz]

It sounds like you will need to move the inner connection point down or the outer connection point up. I'm guessing that the steering is a shaft with a tab welded on that moves the tie rods. Similarily, the knuckle is a welded piece with a tab as well. Could you bend or replace the inner tab so it's lower or the outer tab so it's higher? Also, if the tie rods are on the top of the inner tab then swap them to the bottom and if they are on the bottom of the knuckle tab then swap them to the top. If you can move the points far enough you can reach the point where the bump-steer characteristics reverse.

 

[ivymike]

had looked at that, it's a bit tight in there. thinking about whether I can do a drop-arm setup on the steering tab.