Need explanation

[jhlasserre]

Hello,

I am having trouble discerning the following:

LWB-long wheel base-right?

What years feature does this refer to? Early 911's? I have no idea also don't understand how people convert their car to a long wheel base; I don't understand how thats possible, please explain.

SWB-short wheel base? Newer 911's?

Long hood <'74?

Short hood>'74?

Also what the heck is Bumpsteer?

Thanks for the help and enlightenment.

[Flieger]

longhood = pre 1974

short hood = 1974 on up

SWB = short wheel base = pre 1969 model year

LWB = long wheelbase = 1969 on up until the 996 came out (special racing models excepted such as GT1 and 935 which could have varied a bit).

The wheelbase change was made without modifying the bodywork or engine position and improves weight distribution and stability.

Bump steer is the change in toe angle that occurs due to the wheel moving up or down. This occurs because the tie rod and suspension links do not follow the same arcs.

[James Brown]

your good on the long hood, swb vs lwb is hard to do as you have to lengthen the car behind the doors.

Bump Steer

A. Bump Steer Definition

Bump Steer is when your wheels steer themselves without input from the steering wheel. The undesirable steering is caused by bumps in the track interacting with improper length or angle of your suspension and steering linkages.

Most car builders design their cars so that the effects of bump steer are minimal. However, you must still take care to bolt on your suspension carefully so as not to create unwanted bump steer. Make sure that you are always using the correct components for a particular car. Bump steer must be designed into the car and cannot be adjusted out if improper parts are used or if pivot points are moved without considering bump steer design principles.

In order to accomplish zero bump the tie rod must fall between an imaginary line that runs from the upper ball joint through the lower ball joint and an imaginary line that runs through the upper a-arm pivot and the lower control arm pivot. In addition, the centerline of the tie rod must intersect with the instant center created by the upper a-arm and the lower control arm (See diagram below).

The instant center is an imaginary point that is created by drawing a line from the upper a-arm ball joint through the a-arm pivot where it is intersected by an imaginary line that extends from the lower ball joint through the inner control arm pivot. Where the two imaginary lines intersect is the instant center.

Sounds complicated? Really it is very simple. To achieve zero bump the front end must be designed correctly. The tie rod must travel on the same arc as the suspension when the car goes through travel. Simply matching lengths and arcs to prevent any unwanted steering of the front tires.

To exaggerate, if the tie rod were only 10" long and the suspension were 20" long then when the suspension traveled the tie rod angle would shorten much quicker than the suspension arc. In this scenario the tie rod would shorten much quicker through travel than the suspension and the car would toe in drastically over bumps. The shorter arc of the tie rod would pull on the spindle and toe it in through travel.

I. Bump Simplified

When designing a car, if the centerline of the outer tie rod lines up with the centerline of the lower ball joint, and the inter tie rod lines up with the lower pivot point then the length and angle of the tie rod and suspension will be the same resulting in zero bump. Most car builders design their cars in this fashion.

[Flieger]

To go from SWB to LWB you need to cut out the old torsion tube + weld in a LWB one. You then bolt up LWB trailing arms and LWB spring plates and are good to go. The rear hubs and bearings and such are different, however.

[jhlasserre]

I got it! Thank you for taking the time to explain bump steer and there is no way in hell that I'm changing out the torsion tube on the car. James thanks for going into detail, now I am clear.

Best, Josh