818 bump steer

[Dave 53]

A bit of front tow out makes for a great feeling "turn-in" which is desirable on a track, autocross or if one simply likes the feel of it (which I do). The downside of too much toe out is straight line instability. I have 0.11 total front toe out. Turn in feels great and the compromise in straight line instability is minimal on hundreds of freeway miles to the track and at 125 mph on the track straights. If I trailered my car to the track, I probably would have a bit more front toe out.

I assume the factory alignment on my 2023 Toyota Corolla is toe in which is appropriate for a mass produced daily driver. If I ever enter the Silver State Classic, I think I'd go for a bit of toe in.

When you get your alignment (and on corner weight scales), make sure to be sitting in the car and maybe half a tank of gas. I make up almost 10% of the weight on the suspension, so it makes a difference.

[J R Jones]

Clarification needed...

It seems if the rear has zero toe, that lateral arms are by definition, equal length.

My understanding was that rear bump steer was caused by the inboard lateral link mounting points and the outboard lateral arm mounting points not being parallel to each other. Which is adjusted by the upper trailing arm. In other words, the lateral links have a bit of a twist.

I found without enough castor, my car was a dancing bucking bronco under acceleration. Dailing in as much castor as possible calmed it right down. If you intentionally reduce castor, I'd suggest you hold on and do some hard accelerations on the test drive.

I'm going to be doing some spring swapping soon and when I have the coil overs out, I'm going to do your laser on the stud bump steer test front and rear.

David, absolutely correct, I once knew better and checked my suspension then forgot the detail you mentioned. So the lateral links should be parallel in yaw (azimuth) and pitch.
I am not a fan of that design so I am changing it, ultimately to control arms.

BTW when I replaced the 2002 steel stamped front lower control arms with 2006 alloy LCA I found the front bushing mounts did not align with the rear Z bracket mounts. I had to elongate the front bracket holes to swing the Z brackets out about 0.6 inch. PIA.
jim

[RPGs818SNA]

What an elegant solution to bump steer Jim! I especially like your laser on a stud solution to measuring steering deflection. It’s simple, understandable, repeatable, and sufficiently precise. I also appreciate your dispelling the idea that lowering the steering arm end is substantially different from raising the rack. I think your solution results in a stronger mechanism too. Your rack supports are a work of art. It doesn’t appear that they would cause you any radiator mounting issues.

I have a question about your process. I understand using various block sizes between the tie rod end and the spindle to determine what size minimizes bump steer as evidenced by the laser plot. It sounds like you ended up with a 1.8” optimum block. You then reported “Rack spaced on mounts 0.65 in and shifted 0.75 forward” and appear to have about the same tiny amount of steering variation through the wheel travel range. How did you get from a 1.8” block size to your rack adjustments? Are they vertical and horizontal rack displacements?

Thanks for sharing this excellent bump steer solution.
RPG

[Dave 53]

I just checked my bump steer front and rear. My method isn't as sexy as using a laser, but it provided some rough data. I measured the angle of the chassis with a phone app and shimmed the ruler so it was perfectly perpendicular to the chassis.

Rear has an extremely small amount of compression toe in. Perfect.

I have the Baer kit with all of the spacers. Front still has toe in under compression. Nothing that I notice while driving. I didn't take any measurements or do any math to quantify. If I add any more spacers to the Baer kit, the nylock part of the bolt won't be grabbing any threads. Is it safe to add spacers and just count on thread lock? (I'm thinking, yes).

Or if Jim goes into production...

Under compression bump left down.jpg

Relaxed bump left up.jpg

Measuring setup bump measure.jpg

[J R Jones]

What an elegant solution to bump steer Jim! I especially like your laser on a stud solution to measuring steering deflection. ItÂ’s simple, understandable, repeatable, and sufficiently precise. I also appreciate your dispelling the idea that lowering the steering arm end is substantially different from raising the rack. I think your solution results in a stronger mechanism too. Your rack supports are a work of art. It doesnÂ’t appear that they would cause you any radiator mounting issues.

I have a question about your process. I understand using various block sizes between the tie rod end and the spindle to determine what size minimizes bump steer as evidenced by the laser plot. It sounds like you ended up with a 1.8” optimum block. You then reported “Rack spaced on mounts 0.65 in and shifted 0.75 forward” and appear to have about the same tiny amount of steering variation through the wheel travel range. How did you get from a 1.8” block size to your rack adjustments? Are they vertical and horizontal rack displacements?

Thanks for sharing this excellent bump steer solution.
RPG

RPG, Thank you. First a note on measurement resolution.
The axis of toe rotation is a line through the ball joint centerlines (duh). My laser target was a convenient 25in plus distance. IMO toe is a function of tire radius, or 25% of circumference. That means my graph represents twice the actual toe change.
I speculate that the 2002 donor WRX did not have much bump steer but I never had it with the (2014) unfinished kit I bought. I could not measure the donor for reference.
I theorized that FFR copied the control arm mount geometry, but did not copy the rack mount geometry. The rack mount is the B/S root cause, not the steering arms.

My review of forum data showed two dimensions for the 818 tie rod B/S spacer, 1.8in (46mm) and 2.65in. I measured those spacers with my laser as reference, not a target. I wanted consistent method/metrics. That had no bearing on my rack reposition.
In my professional life I did advanced development including proof of concept vehicles and one-off running prototypes. I worked mostly without a net (support) but with an aggressive timeline so in most cases it was empirical design and validation.

In this case I removed the rack from the FFR mounts and put it on a motorcycle chassis lift. I jacked it up 2 inches and flipped the FFR B/S curve.
I dropped the rack to 1 inch and the curve was pretty straight.
I made shims and used clamps to hold the rack on the FFR mounts. The 0.650 wood shim at the FFR mount angle has a vertical hypotonus of about 1.2 in. That curve was pretty flat and 0.70 and 0.60 shims made the curve worse. I built the aluminum blocks at 0.675 thick plus appropriate stands for securing the Subaru isolator clamps. That dimension was not as good as the 0.65 wood and I fine tuned the blocks to that dimension. (measure twice, cut once)
Where the blocks attach to the FFR mounts was another opportunity tor tuning. I tested in 0.62 increments, three up and three down. In the end the Subaru clamp mount boss centerlines are about 0.75 forward of the FFR mount boss centerlines. The steering column joints have to compensate, but it fits.
jim

[RPGs818SNA]

Thanks so much for explaining your design approach, Jim. I suspect your empirical design, validation, and implementation approach is the fastest way around most problems.

I spent some time measuring my standard FFR rack mount, which is a chore at this stage of assembly with the body, radiator, battery, etc.in place. I drew that up and overlaid it with what I believe to be an approximation of your design.

Bump steer adapter rev 1.jpg

Given the dramatic reduction in measured bump steer, I suspect your design would reduce experienced bump steer to between acceptable and negligible. Getting out that last little bit of curve would require lengthening the rack and shortening the tie rods, no small task.

I imagine the rack would need to be raised another inch for race configurations using the upper wishbone mounts, but a revised bracket should be simple to implement. There are some variations in rack design verses donor year and model, but again easy to compensate for.

Overall, I think you have introduced a simple and effective alternative to the Baer solution. Well done.
RPG

[J R Jones]

RPG, your suggested rack position will interfere with the 818 frame structure.
Your through bolts contribute to that and ignore the stagger between frame fasteners and clamp fasteners.
My blocks have (4) threaded bosses, the upper clamp bosses are off-set forward ~0.75 inches.
Also the left (rectangular) clamp needs a bit of edge removed from the corner flange to clear the left diagonal frame tube.

I am not concerned about using the higher UCA mounts. That position may require a bit more vertical shimming; the UCA has less impact on bump steer than the LCA.
I do not know how repositioning the UCA impacts the steering arm B/S spacer length.
jim

[RPGs818SNA]

Sorry Jim, I knew something was wrong, but didn't understand the staggered holes. I corrected my drawing. I'm sure it's still imperfect, but hopefully explains the concept of your design for the benefit of others.
RPG

[lance corsi]

The uca does affect bump steer. As the lca rises, it describes an arc larger than the arc followed by the uca. If the uca causes camber increase, and the lca does not follow(larger radius) the distance from the steering knuckle to the steering rack changes, thus causing bump steer. The trick is to position all three elements such that the distance between the steering rack and the knuckle remains constant throughout the travel.

[J R Jones]

Qualifying the control arm influence on the knuckle camber and B/S, the (vertical) distance LCA ball joint to tie rod ball is 2.5 in. The distance from the UCA ball joint to the tie rod ball is 13.5 in. 5.4 to 1.
Rosanna Danna: "It just goes to show ya, It's always something".
Extreme castor will lift the steering arm, altering the relationship to the steering rack.
My geometry has been established prior to alignment, so I will measure it again, post alignment.
Lance, have you evaluated bump steer on your Corvette suspension/steering?
jim

[lance corsi]

Yes as well as I could, using my autocad. I must however correct my earlier post regarding bump steer. Since the uca and lca are different lengths, the resultant effect is a modified arc that the knuckle travels in. My main objective was to try and match that arc to the radius of the tie rod. It’s a compromise at best. I was primarily concerned with the numbers mainly through the usable suspension travel, not the full articulation, which is more than my suspension travel. It took a lot of fiddling to minimize b/s and was even more challenging to reproduce on the car since the “v” of the frame was what I had to accommodate for my attachment points. My biggest downside was the wheel offset used on the original corvette wheels. I couldn’t buy wheels with the required back spacing other than corvette wheels(inexpensive wheels) to eliminate scrub radius, so I will eventually need to buy wheels with more offset in the future. Probably some 3-piece wheel$. Oh my! Mo money! I tried the corvette wheels, but much too wide. Also GM mounts the uca at an angle to introduce anti-dive, which I felt unnecessary with such a light car, so I mounted my uca level to minimize the confusion of increasing caster, etc.

[lance corsi]

Fwiw, I settled on a 5 degree caster angle. And just for giggles, I measured my wheelbase and just by luck it measures within .100” of perfect square. Anyone else measure their wheelbase?

[J R Jones]

Fwiw, I settled on a 5 degree caster angle. And just for giggles, I measured my wheelbase and just by luck it measures within .100” of perfect square. Anyone else measure their wheelbase?

Symetry is a trickier evaluation with IFS and IRS, considering all things adjustible. My chassis suspension mounts are spot on, but by F to R wheel hubs differ 0.25, again without adjustment.

I bought a (repaired) C4 Corvette as a donor that had a RF accident history. As It came apart I also found the floor pan broken under the driver seat.
The RF frame horn was accordianed behind the front crossmember resulting in ~0.75 short wheel base on the right. A buddy was a professor at the local tech school body shop. I let him use the car for school demonstration purposes but their straightening left it ~0.375 short on the right. I ended up building a new frame anyway as my Avanti project required a 102 inch wheelbase.
jim