Secrets of the Mirage

[Loren]

Still haven't messed with the front swaybar.

I did up the front tire pressures to 38 cold. Need to remember to rotate the tires before the next event.

Otherwise, the car did very well at the last autocross, and the only thing I changed at the event (other than tire pressure) was to check shock settings. I was already full soft in the front, and I went ahead and dialed it up to full stiff in the rear. Still not a hint of oversteer, and I think the inside rear is still staying on the ground.

Wheel hop is still there. This time, I was definitely able to reduce it by just thinking really hard about how much throttle I'm applying. If I don't overpower the front tires, it doesn't hop as much. Hard to think of "overpowering" anything with only 74 hp, but it is what it is.

Skidpad testing at the 12/2 Classic might yield some good information. I also want to try to get some on-car video of the front and rear tires to see what's actually happening on course.

[Loren]

I've been thinking about this wheel hop thing a lot lately. And trying to "feel" what's going on in certain situations.

I've been re-reading a lot of stuff about FWD suspension setup to be sure I'm not overlooking something, and it doesn't seem that I am.

What's happening is clearly the inside front wheel losing traction. Two questions: Why? and What to do about it?

Why is it losing traction? Insufficient droop travel on the shocks. I'm going to test this tomorrow a little bit by just jacking the car up and seeing how much I can lift one side before the front tire starts lifting. How much body roll occurs before things go bad? The front shocks (even at full soft) are resisting a fair bit of compression on the loaded side, leaving body roll to happen on the unloaded side as lift. I'm betting there's not more than 1-1.5" of travel.

This all adds up. I'm not getting the typical FWD rear wheel lift, either. Why? Because before it reaches that level of body roll, it tries to lift the inside front suspension more than it is able to... and that is effectively holding it down. Driving things like a sweeping turn, I feel like I'm almost lifting that inside tire... because I AM. The strut is bottomed out and pulling up on that tire.

What to do about it? The easy things are shock adjustment, tire pressure adjustment and swaybar adjustment. I've softened the front shocks as much as I can. That will let the unloaded front tire droop as quickly as it can to try to stay in contact with the pavement... until it runs out of travel... that's all we can do with shock adjustment. (and rear shocks are at full stiff to try to slow the actual body roll, which helped) Tire pressure... nothing we can do there is going to help with that wheel lifting. Swaybar... softer would be the logical choice, but in reality... it's still not going to help because the strut is simply running out of travel. Stiffer bar... well, that could potentially help keep the suspension from topping out, but it would effectively do the same thing that the strut is doing now... pull that wheel up. None of these things are going to help.

The simplest solution (assuming the desire to stay on stock springs at stock ride height), is to drop the lower spring perch (effectively making the front of the car lower) to reduce compression travel and increase extension travel. And then put a spacer between the upper spring perch and the body of the car. (to bring the ride height back up)

I'm thinking something like a 1.5" spacer, very similar to how a bolt-on wheel spacer works. Make the spacer tall enough to absorb the studs on the upper spring perch, and bolt it together. Then put studs on the spacer and bolt that to the car. I need to see if I've got enough space in the strut tower to accomodate this idea. It's going to require that upper perch to rotate just a little bit because the plane of the existing spring perch studs needs to be clear of the studs on the spacer.

I think that will solve, or at least REALLY improve the wheel hop problem, though. Fun side effect: I bet it will allow more body roll, and the outside rear tire will then lift.

Hmmm... plastic spacer, or aluminum spacer?

[Jamie]

If you mount the springs lower on the car, what's that do to the front roll center? A back of the envelope sketch makes me wonder if it'll go below pavement level...more body roll.

AL spacers - plastic will eventually disintegrate.

[Loren]

The spring relative to the suspension will be closer to where the shock is designed for it to be. As far as the shock and spring stack is concerned, it's a "body lift".

Spacers only need to last until July.

Man, life would have been a lot easier if they just made a set of Koni shocks for this car!

[BrantR]

I'm learning so much about suspension following this thread. Thanks for being so detailed in your findings!

[Loren]

Happy to share, but I'll be honest... I document stuff because *I* forget things.

It's also good to get different perspectives and ideas sometimes, as well as encouragement.

[Jamie]

The spring relative to the suspension will be closer to where the shock is designed for it to be. As far as the shock and spring stack is concerned, it's a "body lift".

Right -- you'll increase the distance between the roll center and the CG, which will increase roll.

[Loren]

Getting over my own head here, but with the suspension mounting points being the same, I'm not sure the roll center is changing. If I was raising the net body height higher than it is now, I'd be changing the CG. But the goal is same stock height, so I shouldn't be changing CG.

Regardless, this particular band-aid is what I need to do to make these shocks work with this car on these springs at this ride height. The wheel hop is really hindering the car, and likely to break things. Gotta fix that.

Will use aluminum for the spacers because I have to thread studs into them.

[Loren]

3 minutes of experimentation for an answer. I knew this before I even installed the suspension. But, I never actually measured it on the car.

The photo below is enough body roll to completely lift the inside front tire to where it can be easily rotated by hand. It's less than 1.5" of extension. And the outside isn't compressing at all. Right about 1.4 degrees of body roll. I think something closer to 3-4 degrees is more typical for a streetable "stock" autocross car. And if it's less than that, it should be limited by bump stops, not by shock extension travel... especially in the front.

I guess, in theory, going full stiff with my front struts would slow body roll motion and maybe help. But, if there's enough rebound damping, it would just duplicate the existing problem... possibly causing the inside tire to unload even quicker.

When playing with the jack, there was obvious wheel movement at just 1" of body movement, and the tire was completely unloaded by 1.5". Ugh. Time to start designing those spacers.

[Loren]

Plan #1: Custom water-jet cut aluminum spacers. Perfect shape to match the existing upper mounts, hole in the middle to clear the shaft and allow for shock adjustment. Would require a lot of additional drilling and tapping to fit. And would cost over $120 just for the cut aluminum. Pros: Lightweight and very cool looking. Cons: Expense and a fair bit of work... and working with aluminum isn't my favorite.

Plan #2: This came together pretty quickly. My goal here was to try to use "off-the-shelf" materials that require little or no additional machining to get what I need. The space between the two mounting nuts is almost exactly 3.5". The hole in the middle on the car is 3". Neither of those measurements need to be exact. A "3-inch" Schedule 40 steel pipe has an OD of 3.5" and an ID of 3.0x". That's wall thickness of .22"... plenty enough for a suspension spacer. OnlineMetals will cut that pipe to 1.5" lengths (+/- .125... close enough, and I can trim If necessary) for $1.94 each. After shipping (with a $5 shipping discount code), the total cost is $15.49.

The only required fabrication will be to weld tabs on the top and bottom to bolt to. I have some scrap around that I can do that with. Add a couple bolts and nuts... the materials will be down to around $25. If I do all the fab myself, that's it. And, this is "coarse" enough of a job, that I can do it with my stick welder. So, there ya go. $25 and a few minutes (okay, probably a couple hours) welding and grinding.

Not the lightest solution, but I'll take a couple pounds of added weight if it keeps the front tires planted!

If you can't visualize it, it's something like this:
https://www.amazon.com/dp/B01GQFE2MY

Only my struts only have two mounting studs. So, instead of doing a mounting ring, I'm just welding tabs on each side.

[Rpwolf]

iirc, your coilovers are single adjustable. so increasing the front rebound will also increase front compression. maybe the rebound curve is more aggressive than the compression it you could get by? doubt it, especially if you already have wheel hop problems.

btw, is the wheel hop problem worse turning right or left?

[Loren]

iirc, your coilovers are single adjustable

For $600? Um, yeah. Single adjustable.

So increasing the front rebound will also increase front compression.

Unless they are like Konis that only adjust rebound. But, I suspect that the adjustment changes both. (and shock snobs will say "OMG, they're not linear, not repeatable, they're complete junk"... they're probably right, but they're STILL 100x better than the stock shocks, and they meet my current needs)

maybe the rebound curve is more aggressive than the compression it you could get by? doubt it, especially if you already have wheel hop problems.

Wheel hop is due to the lack of shock extension travel which is because I have the shocks set up to be probably 2" taller than they were designed for. (plus, I'm running a longer spring than they were designed for) As it sits, any amount of body roll instantly unloads the inside front tire. Any significant G-force nearly pulls it off of the ground.

btw, is the wheel hop problem worse turning right or left?

I don't think so.

[Loren]

Plan #2: This came together pretty quickly. My goal here was to try to use "off-the-shelf" materials that require little or no additional machining to get what I need. The space between the two mounting nuts is almost exactly 3.5". The hole in the middle on the car is 3". Neither of those measurements need to be exact. A "3-inch" Schedule 40 steel pipe has an OD of 3.5" and an ID of 3.0x". That's wall thickness of .22"... plenty enough for a suspension spacer. OnlineMetals will cut that pipe to 1.5" lengths (+/- .125... close enough, and I can trim If necessary) for $1.94 each. After shipping (with a $5 shipping discount code), the total cost is $15.49.

Started working on this today. Almost done with basic fabrication of the spacers. Used the 1.5" lengths of pipe, some scrap 1.5 x 0.125 flat steel for the tops, and some scrap 1 x 2 x 0.1875 box channel (cut in half to make .5 x 2 C-channel).

As long as I don't have clearance problems with the larger studs on the coilover top mounts (I test fit with the stock upper mounts that I have on the shelf), it should work well.

[Loren]

Baaaah!

Started trying to fix yesterday's mistakes today. Namely:

1. Those studs are the wrong size. I had extra shock shaft nuts, and I mistakenly ran with that thread size. The mounting studs/nuts are smaller.

2. See that bead of weld next to that bolt hole? Yeah, I shoudn't have put that there. (it's also welded on the bottom, would have been fine) It's interfering with the mounting nut.

3. Doesn't matter, anyway, because while I thought there was enough clearance to install nuts there... there isn't. I need to trim 1/8" or so off of the pipe, or cut a slot in it, or something. Can't quite put things together.

So... ultimately decided that I didn't have time to do all of that rework today and aborted. But, with a little help from Philip, I did at least get the front struts out and LOOKED at things a bit closer. I have a better idea of exactly what needs to happen. I'll spend a few more hours on it next week, and hopefully get the spacers completed, painted, and installed.

Meanwhile, we slapped the struts back in at maximum camber. Which looks like it might be a little over 3 degrees. Good thing I'm running M0 tomorrow. We left it alone and reset the toe. Will be interesting to see what 3 degrees of camber feels like.

[Loren]

Yesterday. When I started realizing that I was going to have to cut up a bunch of my work on those stupid spacers and redo them, I was thinking seriously for a moment that maybe I should just forget trying to stay within stock rules and just go to Mod 4.

Today. I made that decision in exactly 4 seconds. That would be the 4 seconds between me saying "son. of. a. bitch" and "alright" in this video:

https://www.youtube.com/watch?v=JnGeqmrf3zM

3 degrees of camber gave the car a lot of front grip. Lowering the car by about 3" will get the suspension back into the range it was designed to work in. And maybe keep both of the front wheels on the ground.

[Loren]

Geeky math approximations from some still frames in that video...

Normal body roll angle at full tilt (no pun intended) is 2.1 degrees. Why is that more than the 1.4 degrees that I measured as "wheel lift angle" in the garage? Because in the dynamic environment (aka "real world"), the loaded side is going to COMPRESS while the unloaded side is extending. So, that's reasonable. Points to significant compresson damping, which is what those shocks feel like.

Body roll at the "whoa" moment was 6.2 degrees. Holy crap!

Over the 56" front track width, the "normal" roll angle equates to 2.05" of height difference from side to side at the point where the inside from tire is just about to lift. The extreme roll angle of 6.2 degrees equates to 6.08". That means that I probably saw about four inches of air under the inside front tire.

Yeah, let's not do that again.

[Jamie]

3 degrees of camber gave the car a lot of front grip.

So beyond what it did to the car, you already put it in M4...by your own ruleset, anything exceeding 2 degrees of camber is modified.

[Loren]

True, technically it wasn't "S5" yesterday. Maaaaybe it wouldn't be as inclined to tip itself with only 2 degrees of camber. (but, even with 2 degrees, with the RE-71's, it's never exactly felt "pushy") What I'm feeling is that as I get more comfortable with the car, I'm pushing it a lot harder. Yesterday, I was faced for the first time with the very real feeling that "yes, I could put this car on its roof if I'm not careful". That's fine for a $500 Festiva. But, I don't want to crash my $7,000 car.

Plus... I'm feeling like the car could actually be a lot faster playing by mod class rules. Even with the same tires.

It's something I wanted to put off until the end of this season. But, it just makes more sense to go ahead and do it now.

[Loren]

Looking at ride height today. The underside of the car is pretty flat. Not much in the way of protrusions. Looks like the lowest part is right about 7". So, if I lower the car 3", I've still got 4" of ground clearance. That's not bad for a lowered car. The coilover kit would let me go even lower than that. I've got the shock length maxed out... could probably get 4" lower just with that, and more with the lower spring perch. I could totally slam this car! But, that's not what I want to do.

Asthetically, I kinda want to get right about 2-2.5" lower. I'm not a fan of "tucked" wheels. The intersecting arcs of tire and wheel arch just look goofy to me. I actually LIKE a little bit of wheel gap. 2" would totally rock, as far as that goes.

The question is, is there significant BENEFIT to going lower than that. I'm gonna do some research on that.

[Loren]

Interesting tidbit:

One of the things you'll find all over the place about lowering a car is "keep the lower control arms level with the ground". This has to do with keeping the range of movement and associated camber angles in their best range of motion. If you lower the car too much, the control arm is angled upward on the outer end. Further suspension compression from there causes that control are to get significantly "shorter", compromising the camber curve.

Of course, this mostly applies to double-wishbone suspension, which my FWD car does not have. I have MacPherson struts in the front and twist beam in the rear. But, I do have a lower control arm... and to some extent, the "level" logic still applies.

What's interesting is that I just looked at the car, and it LOOKS like the control arm is slightly angled downward, as you'd expect. (center of motion is somewhere near level, keeping the camber "curve", in as much as a McStrut has one, fairly consistent) BUT... if you look closely, you'll see that the actual pivot point of the ball joint is much higher than the line that makes up the bulk of the control arm. If you compare that pivot point to the control arm pivot bolt... they're almost perfectly level! I have been saying for a while now that they did a pretty good job of engineering this car. This is an indication of that.

So, at least in THAT regard, lowering the car is compromising its design. I'll be taking the angle of the control arm away from optimal by lowering the car. It's a McPherson strut, which has little in the way of camber curve, anyway. So, it's not going to make a huge difference. And the benefit of lower CG height, roll center height, and stiffer springs will outweigh this particular compromise.

On a similar note, the steering tie-rod angle is presently slightly angled DOWN at the end, so when I lower the car, that will be more level. There will be significantly less body movement with the new suspension, so this probably won't be an issue. But, IF I started with that tie-rod level, and then had A LOT of compression travel, it could create a "bump steer" problem by effectively making that tie-rod shorter under full compression.

Previous measurements indicate that the stock front travel is 5.5". The coilover travel is 4.5". Haven't experienced the bad side of "not enough shock extension", and knowing that I have bump stops on the compression side... I plan to set things up with a little more extension than compression. This should help the suspension keep that inside tire on the ground. so, maybe 2.5" of extension travel, and 2" of compression. Which will be restricted by the bump stop to more like 1.5" of compression. That translates to "not a lot of change in angle of the loaded control arm OR tie-rod.

I think I'm okay in this regard. There shouldn't be any problems with these angles unless I went STUPID low, which I have no intention of doing.