ti pat

Hello guys I would to know why honda's car are sensitive of scrub radius?
I have several cars, sometimes with a lot of positive scrub radius but they was never so sensitive than my honda on bumpier roads, the car wants to pull or follow ruts in the pavement with some positive scrub radius.

Does it come from the steering or from other think?

Thanks a lot guys.

beanbag

Ask John lear. He's usually in the suspension forum

Johnny Mac

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by ti pat &raquo;</TD></TR><TR><TD CLASS="quote">Hello guys I would to know why honda's car are sensitive of scrub radius?
I have several cars, sometimes with a lot of positive scrub radius but they was never so sensitive than my honda on bumpier roads, the car wants to pull or follow ruts in the pavement with some positive scrub radius.

Does it come from the steering or from other think?

Thanks a lot guys. </TD></TR></TABLE>

First of all, for those who don't know what scrub radius is, it is the distance measured at the ground level of the ground piece point of the projection of the kingpin inclination angle (as projected through the upper and lower ball joints) and the center of the tire contact patch. Thus, scrub radius defines the moment arm of the acceleration/deceleration resultant force acting at the contact patch that tend to rotate the wheel about the steering axis. A positive scrub radius is where the steering axis projection point (at the ground) is located more inboard - that is it is located closer to the lateral center of the car. With positive SR, a positive acceleration (i.e. speeding up) would cause a clockwise torque about the steering axis for the left tire. Similarly, for the same situation for the right tire, a counterclockwise torque would be generation. Now, if both driven tires are equally loaded and contribute an equal amount of tractive force, then the moments about the steering axis for each tire would cancel and there wouldn't be a net torque acting through the steering. However, if one tire or the other has a different accelerative load thanks to rough pavement or different coefficients of friction, then one side or the other will contribute more torque into the steering system than the other. This will be felt as a steering kickback to the driver.


FWD cars are usually set up with negative scrub radius because it is self-centering, especially on rough roads or when one tire has more grip than the other. How does this work? Well, lets say that one tire has more traction thant the other for whatever reason. The car will tend to yaw in response to the unequal loading. More traction on the left front tire will tend to rotate the car clockwise. At the same time, the negative scrub radius combined with the unequal tire traction will tend to counter this yawing by inducing a counterclockwise steering torque, thus helping to cancel the yawing.

So you see, by using positive scrub radius, the steering torques due to uneven driven tire acceleration on FWD cars makes the situation worse by inducing a steering torque in the same direction as the yawing moment acting about the cars center of mass.

JuanTushag

will a positive scrub radius add to unstability, or ,,,lessen stability. ( I am refering to unstable in a positive way ie, a fighter plane is unstable compared to a cessna ). Basically im asking if that is why FWD cars are considered to turn better with a wider front track?

ti pat

Good explanation Jhonny Mac. But why my honda is more sensitive than my others FWD? Something according to pinion /rack ratio?

L8APX

Johnny Mac you must be a double-brained rocket scientist...

I like to think I'm a pretty smart guy, but nearly all your posts leave me feeling dumb and inert...

Where do you learn all this stuff???

unrealwrc

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by L8APX &raquo;</TD></TR><TR><TD CLASS="quote">
Where do you learn all this stuff???</TD></TR></TABLE>

If you have to ask you really do not care. Go to college or read a book, Take the concept of scrub radius and wiki it or google search. You can do this with almost any theorey or tech nowadays and get indepth info.

Johnny Mac

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by JuanTushag &raquo;</TD></TR><TR><TD CLASS="quote">will a positive scrub radius add to unstability, or ,,,lessen stability. ( I am refering to unstable in a positive way ie, a fighter plane is unstable compared to a cessna ). Basically im asking if that is why FWD cars are considered to turn better with a wider front track?</TD></TR></TABLE>

Yes, on FWD the positive scrub radius tends to force (through the torque induced through the steering axis) the steering in the same direction as the yawing due to the unequal tire traction. Thus, the driver needs to counter this with steering input.

The yawing is due to a torque balance on th car's center of mass due to the traction differential between the two driven wheels. The torque through the steering axis is due to a torque analysis through the steering axis. Positive scrub radius will will cause an opposite torque than will negative scrub radius. So what you want is for the steering axis torque to counter the yawing torque to help self-center the car.

Well, fighter planes are unstable to increase the absolute responsiveness of the plane in the event of a dogfight. Stable aircraft like stable cars tend to be very numb and unresponsive. That's one of the reasons why FWD run with a loose setup verses a stable understeering setup. Another reason of course is to save the front tires from going off - but thats another story.

Johnny Mac

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by ti pat &raquo;</TD></TR><TR><TD CLASS="quote">Good explanation Jhonny Mac. But why my honda is more sensitive than my others FWD? Something according to pinion /rack ratio? </TD></TR></TABLE>

Two possible reasons: One the torque through the steering axis feeds a force through the steering rack that will depend on the dimensions of the steering arm and the rack and pinion ratio. Shorter steering arms equal larger force fed into steering effort (i.e. Net Torque = Force*moment arm). Secondly, there may be other forces present depending on the geometry of the tie rod ends. This effect is known as roll-steer. This effect will also require driver input at the steering wheel.

So the short answer, is that your Honda has, based on suspension geometry, steering geometry, ect. a more pronouned effect than the other car you're referencing.

turboteener

Something is wrong with your car. My Civic has never had the wandering problems you are describing. You need to check your alignment, steering linkage, tie rods, and suspensions components.

ti pat

No problem with my civic. With +45 offset wheels I don't have nothing, but when increase Scrub radius with +30 offset wheels I have this feeling on road.

Thanks Johnny thanks, I'm thinking on same thing for pinion rack ratio and steering arm lengt. But I don't know roll steer, I will make a search, maybe I use ather words in french.

thanks a lot

ti pat

Ok I make a little search, and roll is "bump steer" but during roll.

I give same name for the two ones. Now, i know there is two names.

Very thanks johnny mac.