Power steering build, MR2 Electric Pump Options

[drive-ability]

I'm now working on a power steering system for my V8 car. I'm using a Golf P/S unit and a Toyota MR2 electric pump. There will be some pictures soon, I'm building the bracket to mount the power rack to the 914 cross member.
I'll doc the build up in this thread.. (IMG:style_emoticons/default/driving.gif)

[byndbad914]

1" longer is still a pretty good amount, which means that it might be hard to get the bump out of it, but I think it is more doable at 1" than, of course, 4" like I thought before (IMG:style_emoticons/default/smile.gif)

charlie - I don't know about a stock 914 but I think it is rather low - the rack sits just above the lower arm pivot point and pivots at the same width and lines up decently with the upper mount locations, but when I built my tube chassis car I relocated the rack up, etc and have measured my bump - I have 0.005" on the first 1" of compression and an additional 0.007" on the second 1", so 0.012" overall for the max compression I expect on a race car, which is essentially what one would call "zero bump".

So, back to drive's issue, if the rack were in stock location then I can guarantee the 1" difference per side in tie rod length would have a strong effect for creating bump steer, like 1/4" in the first 2" of motion I would hazard to guess, and a street car should expect 3" of compression travel at minimum so the bump could be significant. Just being a bit off can cause 1/8" per side.

However, the rack is relocated a bit, so I dunno what effect that would have - it might be even worse. I suspect that an aggressive rack relocation and/or tie rod offset might fix it over a small range of motion but it will never be a "zero" bump setup because within just a couple inches of motion the difference in pivot locations will never want to zero out over 3" of travel (or more). But if it were an 1/8" even on 2" of compression, it isn't optimal but on a street car it probably wouldn't be that noticeable for most people (I would notice it cuz I am used to recognizing it). On a racer with really sticky and wide front tires, it would be very noticeable.

[drive-ability]

1" longer is still a pretty good amount, which means that it might be hard to get the bump out of it, but I think it is more doable at 1" than, of course, 4" like I thought before (IMG:style_emoticons/default/smile.gif)

charlie - I don't know about a stock 914 but I think it is rather low - the rack sits just above the lower arm pivot point and pivots at the same width and lines up decently with the upper mount locations, but when I built my tube chassis car I relocated the rack up, etc and have measured my bump - I have 0.005" on the first 1" of compression and an additional 0.007" on the second 1", so 0.012" overall for the max compression I expect on a race car, which is essentially what one would call "zero bump".

So, back to drive's issue, if the rack were in stock location then I can guarantee the 1" difference per side in tie rod length would have a strong effect for creating bump steer, like 1/4" in the first 2" of motion I would hazard to guess, and a street car should expect 3" of compression travel at minimum so the bump could be significant. Just being a bit off can cause 1/8" per side.

However, the rack is relocated a bit, so I dunno what effect that would have - it might be even worse. I suspect that an aggressive rack relocation and/or tie rod offset might fix it over a small range of motion but it will never be a "zero" bump setup because within just a couple inches of motion the difference in pivot locations will never want to zero out over 3" of travel (or more). But if it were an 1/8" even on 2" of compression, it isn't optimal but on a street car it probably wouldn't be that noticeable for most people (I would notice it cuz I am used to recognizing it). On a racer with really sticky and wide front tires, it would be very noticeable.

I normally learn as I go and it seems nothing has changed (IMG:style_emoticons/default/dry.gif) I'll just have to start with evaluating the basic P/S operation. If it works well I'll relocate the rack, why should this be any different than many of my projects..
I did take some measurements using a laser, moving the wheel up and down. It looked like I had some toe in but not much maybe 1/8 inch over 6 inches or so of movement. I did mount the rack higher, closer to the body and forward a bit as well. This was a crude test / measurement at best.
Nobody said this would be easy (IMG:style_emoticons/default/hissyfit.gif)

Here is the set up I used. The laser is level and the suspension is hanging down as I jack up the control arm the laser line should twist if the toe changes. I can't see it move much. I did it a few times and didn't see any real difference, less than the thickness of the laser line for sure. I see less than I mentioned above, looks like less than 1/16 or so.
What I did see is a lot of change in caster, honestly I don't know if thats normal or a big deal.

(IMG:http://members.cox.net/altjamy/Power%20steering%20Laser%20shot.jpg)

[byndbad914]

I normally learn as I go and it seems nothing has changed (IMG:style_emoticons/default/dry.gif) I'll just have to start with evaluating the basic P/S operation. If it works well I'll relocate the rack, why should this be any different than many of my projects..
I did take some measurements using a laser, moving the wheel up and down. It looked like I had some toe in but not much maybe 1/8 inch over 6 inches or so of movement. I did mount the rack higher, closer to the body and forward a bit as well. This was a crude test / measurement at best.
Nobody said this would be easy (IMG:style_emoticons/default/hissyfit.gif)

Here is the set up I used. The laser is level and the suspension is hanging down as I jack up the control arm the laser line should twist if the toe changes. I can't see it move much. I did it a few times and didn't see any real difference, less than the thickness of the laser line for sure. I see less than I mentioned above, looks like less than 1/16 or so.
What I did see is a lot of change in caster, honestly I don't know if thats normal or a big deal.

I believe I figured out your setup from the pic... depending how far away you take your measurement, the more toe you will see... so that can be all over the place.

BTW my first track day with the tube chassis setup when the front arm broke in half that Kanna Motorsports sold me, I called my friend and said "nobody said it would be easy, they just never said it would be so damn hard!" so, yeah, the projects never seem to end and the more you do the more you find you need to fix (IMG:style_emoticons/default/dry.gif)

Here is how a typical bump steer setup works - you mount a flat plate where the wheel mounts and the flat plate can be marked at your tire diameter (1/2 the diameter forward, half back) or the dial gauges set at a width of the tire diam. You would set up dial indicators front and rear at those points (or some now have a roller at one end and a single dial indicator) and you would be measuring the toe change at the tire's diameter.

A way you could more or less do this using your laser and a nice flat piece of plywood is

1. cut the piece of plywood to be say 8" tall (enough to fit a 130mm bolt pattern at 5.119" diameter) and to a length of around 24" overall (so 12" each way from center) which more or less mimics your tire's diameter (close 'nuf).

2. drill a bolt circle, at the center of course, using one of your spacers for a template.

3. mount the piece of plywood and get it level to ground.

4. set your laser up to run parallel to the board (you would be perpendicular to the board with your current setup), with the steering of course centered like you are driving forward in a straight line. Get the laser parallel by taking a measurement front and rear (should be the same value if the laser is parallel to the board). So if the plane of the laser is 4" away, you should measure 4" front and back... whatever the value, write it down.

5. jack the hub up ~ 1" and measure now from the front edge of the board to the laser and see what the difference is. the diff between the new value and the value you wrote down in step 4 is your bump at 1" PER SIDE. If the value got bigger, you toe'd in and smaller, then obviously toe'd out.

6. Take measurements every inch for 3" up and 3" down and you will get a bunch of different values that will be your toe curve.

This is way crude, but IMO if you can't measure the diff with a tape measure with 1/16" hash marks (.062") then I wouldn't worry about it. If you can see a measurable difference with the tape then that can't be considered "zero" bump.

Measuring perpendicular like you did will include noise due to scrub radius, etc.

You are on the right path to checking it tho'!!

BTW, as for caster, it isn't a big deal, but because the lower arm moves the bottom of the strut up and down in a truly vertical plane and the strut is actually angled back and hard mounted at the top, you should indeed get a slight increase in caster thru compression and less thru rebound.