so today i stiffened up the rear to help fighting the understeer i have been getting since the motor swap.

that seems to have worked well,

but as a unexpected sideeffect, it also greatly reduced my highspeed bumpsteer in the front ...

Andy

What did you do to stiffen the rear?

I'm sure this goes with out saying (asking...) but do you have the spacers in under the rack bolts?

Interesting.

ummmm

stiffening the rear of the car WONT CHANGE the bump steer of your car..

The geometry of all suspension components are the same....

Perhaps you should have shocks valved to suit your corner weights/spring weights/hp etc....

So now you have understeer, and the bump-steer isn't as noticeable.

The effect of "high speed" suspension dynamics are a little different, and I'm only mentioning it because you said "high speed bumpsteer" As your car approaches it's critical speed (point of suspension geometry limitations) it becomes harder and harder to control. Cars with a lot of over steer may veer off the road, and cars with understeer may just run into something.

Understeer and oversteer are the the amount of traction the front tires in relation to the rears and accounting for turn resistance under various situations. Less traction front=understeer.

Your front tires are no longer making enough traction as your car reaches critical speed to jolt the car under a bump steer situation.

yes, got bumpsteer kit (spacers) ... i hooked up the rear sway bar ...

could a stiffer back mean less body roll and hence less bumpsteer ???
Andy

uhmmmmm, i know i look like it, but i'm not THAT stupid ...

i know the geometry didn't change. but the bumpsteer is most certainly less than before the change. the understeer is gone, car is more balanced, high speed bumpsteer is almost gone.

all by simply hooking up the rear sway bar.

so, what could be happening here? the only thing i can think of right this second would be less body-roll ...

yes? no? what else?
Andy

Bumpsteer, for your daily commuter, is most easily detected when the car jolts sharply to the left or right when hitting a bump. Most people notice it best when driving in a straight line situation.

The advantage of assuming this is that your car is usually not experiencing body roll in a straight line situation.

thank you for explaining to me what bump steer feels like!

i had no idea that what i felt before and *thought* was bumpsteer was in fact actually bumpsteer!
wow!

now, in my world, bumpsteer is actually the worst at high speed corners/sweepers. if you hit a bump while cornering, the car is upset greatly. i know how that feels (thanks alfa!) and i don't like it.

now that the rear has been stiffened up, the same corner at the same speed going over the same bump feels MUCH better than before.
as in, much less jumping around of the frontend. much less jolting on the steering wheel.


so, anybody out there got any educated guesses as to why my car feels different now than it did last night?
Andy

Your front inside wheel is lifting. Trying leaning in when you turn...



Yeah. That's why I stated my process for assumption. I had some doubt that you were the "douchebag everyday driver."

Hey Andy

I noticed you said you added the rear swaybar. That would definately make you turn in worse as it causes your car to twist more and tires lift. I had a read bar on my car for a long time, and once I removed it my turn in was much better. Maybe that is why it feels like your bump steer isn't as good.

turn in is good. better now actually that the understeer is gone. also, my front alignment is set to be a bit twitchy which greatly helps with turn in.

anyways, i got a full rollcage, so chassis twist shouldn't be too much of a problem.

and just for the record, i *do* know how to get rid of bumpsteer.
that was not my question ...
Andy

This blurb pretty well explains bump steer.
It can either kick the steering wheel against the direction you are trying to turn or
pull it towards the direction you are trying to steer.

Bumpsteer Basics and Small Solutions
What is Bumpsteer? Is it a mysterious term that racers use to excuse their ill handling racecar? The technical explanation is a change in toe caused by suspension travel or an unwanted steering input caused by such a toe change. If you already understand that explanation you probably have fixed any bumpsteer you may have had. If not, imagine driving through a parking lot. The right side of the axle has a speed bump and you try to go around. You miss, the right front tire hits the speed bump and your car steers itself to the right even though the steering wheel did not move. Now imagine you are going to turn a corner and you choose to go through the corner at a very high speed. As you approach the corner you turn the steering wheel in and the car begins to roll. As you have the steering wheel set in position to carry you through the turn, the car continues as the suspension compresses on the loaded side of the car. The front wheel turns in more than you had originally intended, causing the car to turn more sharply. You correct the steering wheel in the opposite direction pulling the car out of its roll and you miss your intended path through the turn. Both of these examples illustrate bumpsteer and are undesirable when maximizing the handling performance of your Mustang. Two things happened when the vehicle crossed the speed bump or when the vehicle entered the turn. First, there was suspension travel, the wheel, spindle, a-arm and other components moved upward as it crossed over the speed bump or when the vehicle began to roll into the turn. Second, this suspension travel caused the wheel to change direction even though the driver did not turn the steering wheel. This change in wheel direction made the vehicle turn in a direction not intended by the driver.

When the suspension travels up and down, the wheel moves up and down in an arc or half circle around a pivot point. This pivot point is not a physical location or suspension part but is an imaginary point on the chassis, similar to a vehicle's center of gravity. The distance from the pivot point to the wheel and spindle is equal at all points in the wheel's range of vertical movement. The tie rod and steering rack must be positioned so that as the wheel moves up and down, the tie rod follows an arc which is parallel to the arc followed by the spindle. If the steering rack or tie rod is not positioned correctly, the distance between the steering rack and wheel may differ from the distance between the pivot point to the wheel at different suspension heights. When this occurs the steering rack will push or pull the spindle as the suspension moves up or down thereby turning the wheel and causing a change in toe angle.

The direction the front wheels are pointed is called toe angle. When both wheels are pointed parallel there is 0 degrees toe. When the front of the wheels are pointed inward, there would be positive degree toe or what is called toe in. When the front of the wheels are pointed outward there is negative degree toe or what is called toe out.

again:

i KNOW what bumpsteer is. i KNOW how it feels. i KNOW how to fix it.


anybody want to comment on my actual question?
Andy

I'm thinking the point that JP is trying to make is: You may not have changed the bump steer at all! You changed the symptoms you were feeling that you believe were bump steer, which may or may not have actually been the real thing. Kickback through the steering wheel is not always bump steer, though we do tend to assume.

Then again, less body roll == less suspension deflection under load, which when you have bump steer going on == less bump steer. As was implied by the example given above.

--DD

try this, The steering had so much load attempting to steer you in the path you chose that it couldn't bound and rebound in a bump steer action. With the car flatter and both front wheels are working to put the car in the path you chose there is an issue with the steering angle. Have the alignment checked and have the car balanced.

And if you think I have a clue about what I wrote I have ocean front property for sale in MN

yes. bumpsteer obviously affects the suspension. at least in my car it get's unsettled. i'm thinking a stiffer suspension setup should result in less suspension upsetting caused by the bumpsteer. that's my theory anyways. and that's why i asked for other opinions on the subject.

anybody who thinks my car does not bumpsteer is welcome to take it for a spin and feel for themselves.

that is, until i dropped the car the 2" it needs to go down. that is about all i need to get rid of the bumpsteer.
confused yet?

i currently got "stock" rideheight but the bumpsteer kit installed. hence, the geometry is off and i get large amounts of bumpsteer ...
Andy

Yeahhhhhh... I don't think you car bumpsteers..... I am packing my bags right now

in the example the author gives two examples of how to experience bump steer.

I think you may be experiencing the "body roll" bump steer scenario.

with the rear roll bar attached you now have less body roll and therefore less body roll bump steer.

there's my $.02. keep the change. =)

Andy that was mean! I asked if you had the bump steer spacers installed, I assumed you lowered your car to match. When cornering with out the rear bar your front would load really heavy and scrub. This probably lowered the front the 2" hence eliminating the bump steer. Now less roll so less loading and less lowering.

BTW thanks, I'll be facing the same issue in the spring.

My guess is that its something to do with the rear toe. Having reduced wheel travel during cornering you have stopped it doing something. I'd look at the back. I think about 2mm toe in each side for stability.

What a smartass...

Slight toe out? Ive heard from my AX buddy that helps a lot and eases sharp consecutive turns, but eats up the tires

I posted the blurb just in case you actually listened to a couple of folks on this thread
(not Alpha) and with it could analyze WFT is going on with your car.
Instead you get jacked up & tell us that you know everything..great...

OK, here is my answer to your question.

Your addition of the rack spacers will change the front bump steer dimension/ratio.

Your addition/use of the rear AR bar doesn't change your front bump steer one damn bit....just as Tim says. It may lessen the *effect* of what bumpsteer dimensions/ratio that is there, but it is still the same.

The rear bar will change the rear bump effect by not allowing as much toe change....it's the same also however. It will also help you lift the inside rear tire off the ground at your next AX.

There are several things that will increase or diminish the front bump *effect* that upsets the car and/or driver....springs, shocks, scrub radius, yada......but you already know this, right?

Stuborn Krauts have been a part of my life ever since I can remember....I may even be one.

I think to really understand this I'll need to drive your 914. Bring it to the GGR autocross this Saturday and I'll take it for a spin.
I think the rear bar helps keep the car flatter when turning, so maybe it is changing some angles up front, but not the tie rod height