Measuring bumpsteer and building a gauge

[sgernon]

After looking at the bumpsteer gauges sold in the market I think I'm going to build my own. I was thinking about possibly making 2 changes to the standard way bumpsteer is measured, let me know your thoughts on if these changes are smart or not. I've never measured this before so I have no real world experience, I'm only thinking about other things I've done in the past with other projects.

1. Rather than using 2 indicators on a flat plate like the Longacre tool I was thinking to make a frame in a Y shape like the quick drawing I made below. Also rather than bolting it on I was thinking to use magnets to connect it to the brake rotor itself. Using the Y shape I would put the indicator at the center and as the spindle turned in or out due to bumpsteer I could see the changes on it without having to calculate anything. Am I missing something here?

2. Rather than remove the springs to check can I simply wrap a chain around the frame and under a floor jack (or bolt to floor jack) to adjust the A-arm travel when measuring? I've used this method in the past for removing springs from cars with no drivetrain installed. Would this method be reliable for measurements? More importantly would anything think there would be a safety concern?

Quick top and side view

63103

[David Pozzi]

The width being measured needs to be the same as the tire diameter, or if a narrower span is measured, you will need to multiply all readings by a factor.
I put a flat plate where the wheel bolts on but use a tripod base with a frame rising up that is piano hinged to the base. I use one 1" travel dial indicator & on the opposite side use a threaded bolt exactly .5" shorter than the dial indicator when fully extended. Readings start with indicator compressed .5" & this squares the hinge to the plate. This is very important or readings will be off.
I can post a photo if you are interested.

To take readings by pulling down, you need a framework under the car, & turntables to reduce friction. Most turntables have graduations but are not accurate enough at all to work.

[sgernon]

Please post a pic if you have a chance. I'm following some of what you are saying but not all of it. I didn't realize the readings needed to be the same as the tire diameter but I was thinking there needed to be some specifc distance needed and that I would have to use some form of multiple. It's been a long time since I used my math skills but I was thinking since I can position the the indicator out the Y leg the distance needed to offset for the delta between the brake rotor and the wheel. Since I have ~24" tires and 13" brakes I was thinking to set the guage at a 12 " width and using a something near a 2X multiplier.


I don't think it will be possible for me to pull the car down in my garage. I dont think I have the space for a frame and attachments to pull it down safely. With my aproach of pushing the A-arm up do you think there would be an issue with the jack walking and corrupting the readings or is there something else I am missing?

[exwestracer]

Please post a pic if you have a chance. I'm following some of what you are saying but not all of it. I didn't realize the readings needed to be the same as the tire diameter but I was thinking there needed to be some specifc distance needed and that I would have to use some form of multiple. It's been a long time since I used my math skills but I was thinking since I can position the the indicator out the Y leg the distance needed to offset for the delta between the brake rotor and the wheel. Since I have ~24" tires and 13" brakes I was thinking to set the guage at a 12 " width and using a something near a 2X multiplier.


I don't think it will be possible for me to pull the car down in my garage. I dont think I have the space for a frame and attachments to pull it down safely. With my aproach of pushing the A-arm up do you think there would be an issue with the jack walking and corrupting the readings or is there something else I am missing?

In your case, you would need to divide the tire diameter by the distance from the balljoint (SAI) line out to the indicator and use that as your multiplier. Measuring sideways as you are, rotor diameter has no influence on the readings.

[sgernon]

First off thanks to both of you for responding. As stated in the first post I have never done measured this but am trying to learn.

Why would I need to measure from the balljoint (SAI) for the multiplier? From google I found out SAI is the angle between the verticle at the center of the tire patch and the line between the ball joints. I thought in measuring for bumpsteer we were simply looking to understand the steering angle change as spindle travelled through the arc during suspension compression and rebound. This makes me think you need to realign the tie rod angle so it's arc more closely match the SAI. While I think I understand how this is related to SAI, I'm confused how it would create the multiplier for measuring it differently . Based on your response and the fact that I know you definately know your stuff I have a feeling I'm thinking about this all wrong. Am I just thinking it's a guess and check process while you are referring to how I can measure the actual spacing needed on the tie rod? Am I even close with anything I am writing?

I also just realized I labelled the drawing wrong - Side view first and top view second. Does this change anything?

[exwestracer]

Bumpsteer is actually the deflection of the spindle angle as the suspension travels up and down. When using a typical bumpsteer gauge, you are measuring actual toe change, although not necessarily the same amount as the tire sees (reason for the multiplier). The method you proposed above is measuring spindle deflection, although in thousandths of an inch instead of degrees. The length of "lever" you are measuring this on is from the ball joint line (steering axis) out to the indicator. To figure out actual toe change with your tires, you have to relate the length of this "lever" to the tire diameter.

Hope that made sense...