Steering rack spacers when running larger rims?

[sacoffee]

I have awful bump steer on my 88' with 17" rims. They are turbo twist rims with spacers, is this due to scrub radius and going back to a 17" Fuch type rim (sans spacers) will solve it or do I need to add steering rack spacers to get it aligned back to level?

Stupid question?

[Flieger]

Lots of variables going on here- camber affects sai (since its a strut) which affects scrub radius as well as steering weight. Caster effects it. Larger diameter wheel doesn't matter. What matters is tire diameter. Taller tires amplify the caster and sai effects. More offset affects scrub radius and kickback. If you get taller tires but keep the car the same height then you can get bump steer...

We need more info/pictures.

[sacoffee]

Yeah complicated �� what pics would you want?

FYI I am running 225 45 17's up front.

[javadog]

For starters, how about one from the side that would show the suspension height in the front? If it's too low, you'll get bump steer.

JR

[Peter Bull]

Your front wheels should have more or less the same outer diameter as the stock wheels. So in that case wheel size should not affect bump steer.

Lowering the car, on the other hand, will result in bump steer. Some of that can be taken care of with steering rack spacers, but you can't lift the rack by much more than about 15 mm before things start to bind and rub.

/Peter

[Trackrash]

I have awful bump steer on my 88' with 17" rims.

Are you sure what you are experiencing is bumpsteer?
Dartyness and instability can be caused by wide wheels, lots of offset (spacers), or camber and caster settings.

My car is like Dr. Jekyll / Mr. Hyde depending on what tire I have in front. With my 205/55-16s in front it drives like a dream on the street. When I put my 225/50-16s on it is scary darty on the street. Both are on 7 x 16 Fuchs.

[Rob T.]

I also don't think your problem is bump steer.
The position of the suspension arm to the tie rod does not
change by mounting larger wheels.
And as far as i know this is the reason of bump steer...

[sacoffee]

Here is a side view pic

The handling is fine all the way around expect when going over bumps, the steering wheel jerks violently.

[911pcars]

Define bump steer. You might be describing something else.

S

[wayner]

While considering bump steer, you might also want to look up a condition called "tramlining."

http://en.wikipedia.org/wiki/Tramlining

Aside from other factors already mentioned, here are two effects to consider:

1) Is the steering affected by the outside of the tire dragging the tire off centre (bump hitting more to the outside of the left tire causing the car to steer left suddenly? (The initial impact wouldn't have to be very far off centre to have an affect).

OR

2) is it bump steer where the geometry of the suspension runs out of range as it moves up, thus causing the tie rod to pull the wheel in a different direction from straight ahead?

I'm not sure how you would isolate the two to figure out which one it is, aside from having a suspension guru add a bump steer kit with exactly the right geometry for your specific car.

My car does the same thing but we probably have different ride heights so a generic bump steer kit might help us both but would not be ideal since it doesn't take into account our cars different geometry.

[sacoffee]

Originally Posted by 911pcars

Define bump steer. You might be describing something else.

S

Here's a description from the internet


"Bump steer is not a very common condition, but it can be a very unnerving thing to experience if you're unfortunate enough to encounter it. It is usually most noticeable when crossing a major dip or bump at speed, as when crossing a railroad crossing or passing through an intersection with a lot of road crown. As the suspension bounces over the bump, it suddenly feels as if someone tugged on the steering wheel. The car twitches or jerks to one side as if it has a mind of its own."

[911pcars]

Originally Posted by sacoffee

Here's a description from the internet

"Bump steer is not a very common condition, but it can be a very unnerving thing to experience if you're unfortunate enough to encounter it. It is usually most noticeable when crossing a major dip or bump at speed, as when crossing a railroad crossing or passing through an intersection with a lot of road crown. As the suspension bounces over the bump, it suddenly feels as if someone tugged on the steering wheel. The car twitches or jerks to one side as if it has a mind of its own."

That's fine. Various internet descriptions, some okay, some just adequate, some incorrect.

Accentuated when the ride height is adjusted higher or lower than the factory spec, thus various strategies to minimize this effect on a lowered (usually) 911. Excess bumpsteer compromises the designed suspension geometry which results in excessive toe-in or toe-out during suspension movement (jounce and rebound). The result is a loss of steering stability (tires point in conflicting directions). You do not want the tires doing this, especially while cornering at speed.

Sherwood

[Rob T.]

here is a very good description what bump steer is

[javadog]

Originally Posted by sacoffee

Here is a side view pic...

...which shows that your car is below factory height. If you do suffer from bump steer, your lowered height will make it worse.

Originally Posted by sacoffee

The handling is fine all the way around expect when going over bumps, the steering wheel jerks violently.

If you are talking about a bump that only one wheel experiences, that's somewhat normal for a 911.

I have no way of knowing what effective wheel offset you have (don't know the offset of those wheels, or what spacers you have installed) but that can exacerbate things if it varies much from factory specs in the front.

Personally, I think the car would drive better if you lost the wheels and tires you have and put on a set of 16 inch Fuchs.
JR

[sacoffee]

Originally Posted by javadog

...which shows that your car is below factory height. If you do suffer from bump steer, your lowered height will make it worse.

So I should install spacers?

BTW offset is ET 15

[Walt Fricke]

When my track/street SC got "darty" on me on the street, at the suggestion of the tire dealer I used I put on new front tires. Top of the line. The old ones were farther down the food chain, and well worn (but not so well worn that a thrifty guy like me knew he had to change them out). World of difference. I would never have believed it.

Since this change which produced bad handling involves new tires, though, maybe that's not part of the problem.

[sacoffee]

The problems been there with old and new tires

[sacoffee]

I installed the spacer kit today, noticeable difference! Easier to steer at Los speeds and violent jerking of steering wheel less violent.