Handling issue at speeds over 75mph...

[fastx19]

So, had the rotary powered X at a hillclimb this weekend and ran into a problem with the handling.

At lower speeds, 70mph and slower, there is really no problem. Anything over 75 and the steering becomes very twitchy. Driving straight down a road which is not perfectly smooth the car will tend to dart from one side to the other. You have to hold the wheel very, very tight and not react to the car wanting to visit the left or right side of the road. Now, for autocross, this is no big deal and the speeds are rarely this fast. However, on a hillclimb, this is a big problem. As the speed increased, the problem got worse. 85mph is getting scarry.

Let me give you the specs and then I will tell you what I think is wrong. Then we can all see how wrong I am !

The car has 1/16" toe in on both the front and rear. Camber is about 2 degrees on the front and just a bit less on the rear. I can't remember off hand what the caster is, but should be very close to stock.

Tires are 20x9.5x13 Hoosier R35 slicks on 13x8" Kodiak rims. Tire pressure is 27psi rear and 25psi front Springs are 450lbs on the front and 550lbs on the rear. Shocks are Koni double adjustable with an adjustment at the top of the shock and one on the bottom. (Rebound and Compression, I believe in that order.)

The car weighs in at 1500lbs and is currently 49% front and 50% rear.

Now, I when I did the suspension, I set them up like an autocross, where the struts are pretty much full power all the way around.

I have the struts now set to full soft for Rebound but forgot about the compression setting on them :wall: Based on this, I am thinking having one aspect of the shock set to full soft and the other set to full hard, this would cause all types of funny things to happen at a higher speed. If

What are your thoughts?

By the way, we found out with a radar gun that 8,000 rpm in 2nd gear is 85mph... So, I think we have a three speed car with two extra gears... I can't imaging trying to go from 3rd to 4th in that thing. Hell, 85 down a single lane road in the woods is like warp speed in the sci fi movies!

Thanks for your comments on this.

Eric

 

[Matt Brannon - Midwest Bayless]

Some thoughts

You've got a pretty good head scratcher here. Is this a new condition that the car didn't exhibit previously, or is this the first time at high speeds and you're noticing this?

With 1/16 toe in, it should be relatively straight line at higher speeds. If you had said 1/16 toe out, one might expect a little wandering at high speeds. I am regularly high-speed at over 100mph and to be honest there's not a lot of drama. And this is with 1/8 toe OUT in the front. That said, bad bump steer can really cause problems, especially in turns.

You mention "over rough roads". Sounds to me like the car is bump-steering. If the car is slammed and the angle of the tie rods are level, or worse yet, inclining toward the outboard ends, then bump steer can be a real problem and cause what you're noticing here, as it causes one side of the car to toe out when the suspension is compressed, dragging the car to that side for an instance. This can cause all kinds of fun with bumps in the middle of turns. It will jerk the steering wheel and cause understeer or oversteer depending on what side of the car the bump is on and which direction the turn is.

One other thing to check is to make sure your right-side steering rack bushing hasn't fallen apart and is causing excessive toe changes on the right side. Just jack that corner up and grab the wheel at 9:00 / 3:00 and rock back and forth. If you hear or feel a clunking, then grab the end of the rack with wheels turned full left, and see if you can wiggle the rack gear inside of the R&P assembly by pushing/pulling up or down on it. If you can, that means the bushing is bad.

If everything is tight and no components are slopping around, and you've strung the car and are certain that its aligned correctly and not crabbing, then bump-steer is your likely culprit.

-M

 

[Mark Plaia]

Also....

You mentioned that the struts are set up full hard on compression and full soft on rebound?

That would give a spring/damper relationship that is very stiff to compress, but unloads almost instantly. Seems like you would want the compression soft, to let the spring work, and the rebound stiff to keep the spring from unloading quickly.

Stiffer rear springs than front are going to make it more sensitive to oversteer.

Matt's comment on bump steer is very appropriate. Has the steering rack been raised? When we built the Dallara racer, we followed the PBS recommendation for raising the steering rack, but we did not raise the pivot points of the suspension. It gave improved (though not perfect) tie rod geometry. I still have drawings of the raising blocks that I designed.

Ciao,

 

[Matt Brannon - Midwest Bayless]

Bump steer

If you find out that bump steer is the culprit, you have a couple of solutions. As Mark said, the best solution is to raise the steering rack mounting points by using aluminum adapters.

A poor-mans approach is to fabricate new tie rod ends with 1/2" spherical bearing rod ends, then drill the taper seat tie rod holes in the carrier arms out to 1/2" and install the new tie rods underneath the carrier arm as opposed to on top of the arm as the original layout used. You can use shim washers to increase the drop as needed. Be sure to use Grade 8 hardware.

-M

 

[fastx19]

Great ideas!

Ok, the steering rack has been moved up, per PBS and everyone else. The tie rods are level when the car is on the ground at ride hight. Total suspension movement is less than 1"

The ends are not spherical yet, as I did not think it would be a big deal, but do you suppose? hmm...

I am going to adjust the compression and rebound and see what the deal is and report back.

If you think of anything else, let me know. I am going to do some testing this weekend if I can figure out what is going on with the transmission... That is another story...

Ah, got to love racing

Eric

 

[duderduderini]

Control arm bushing

Hi
I once had much front end weirdness from my rx7 which no one could find. In the end I bought adjustable lower control arms and found the cause of my problem... the urethane control arm bushings had flogged out (on the original control arms) and i had infinitely adjustable alignment.. Have you checked all the bushes on the front and rear? They tell me rear worn ball joints can lead to dodgy handling as well
Good luck.. I know how frustrating this is
Nick

 

[mx5rush]

The car weighs in at 1500lbs and is currently 49% front and 50% rear.

Easy fix. c: I think you've stripped out too much weight. You're missing 1%. Add 15.15 pounds to get 100% and it'll be fine! :grin:

The only handling problems I've ever had were fixed with new tires... except for once when I broke a sway bar link, and that was easy to find.

 

[karhawk]

Is something like this what you are talking about?

It seems like an elegant solution when your tie rod ends get worn

 

[Steve Hoelscher]

Suggestions

All of the above is certainly on point. I think you have the shocks set backwards. Reduce the compression and set the rebound stiffer. That should help.

It does sound like either a bump steer issue or a bushing issue. However, if it only appears above 75 MPH I think I know the problem. Aero.

I have had some pretty fast X19s and have noted that as speeds increased, the aero problems show up. First, is the radiator venting. If you haven't vented the radiator through the hood, you should. The airflow through the radiator is forced out underneath the front axle. This high pressure area creates pronounced front end lift at speed. A problem compounded by the car's light (1500 lbs) weight. Excessive compression dampening will worsen this problem.

The second aero issue is the low pressure area created behind the rear window. If the car still has the glass in it then the rear end will get noticeably light at speed because of the airflow off the roof creates a big low pressure area over the rear axle. The resulting lift makes the rear light and compounds the darty behavior.

 

[Black-Tooth]

Eric... this is an idea from...

... WAY OUT in LEFT FIELD...

As I have no real experience in this arena that you play in...

I'm taking this from a FWD issue I had on a Honda where I "mistakenly" adjusted the toe-in to 1/8" overall... and what it required was a toe-OUT of 1/8". (In the slightest bit of rain it was all over the place!)

I'm wondering how this might impact the RWD IRS on an X if it was adjusted to a 1/16" or 1/8" toe-out and if it might solve the issue.

It would be a rather simple and quick experiment...

Let me know what ya think or if ya try it... and keep up the great work!

 

[fastx19]

Thanks!

I am going to try a couple of items and test each one to see what the effects are.

1. Fix struts
2. Change aero issues. The rad is vented into the trunk, but not out the hood.
3. Replace steering control arms with heim joints instead
4. I will even stick a splitter on the front just for fun.

I will let you know how this goes. Thanks again everyone, I really appreciate it.

Eric

 

[kmead]

... WAY OUT in LEFT FIELD...

As I have no real experience in this arena that you play in...

I'm taking this from a FWD issue I had on a Honda where I "mistakenly" adjusted the toe-in to 1/8" overall... and what it required was a toe-OUT of 1/8". (In the slightest bit of rain it was all over the place!)

I'm wondering how this might impact the RWD IRS on an X if it was adjusted to a 1/16" or 1/8" toe-out and if it might solve the issue.

It would be a rather simple and quick experiment...

Let me know what ya think or if ya try it... and keep up the great work!

Its unlikely this would work to ones advantage.

As with all things mechanical, any assemblage of parts involves a build up of errors in each of the parts which are referred to as tolerances. There is a nominal size for each piece and an understanding that each piece will vary from that nominal dimension by some amount, the tolerance for error if you will. This means each part in the steering mechanism has some unintended movement which needs to be accounted for in how the car will eventually be aligned.

Added to this tolerance chain of all of these parts together is added the dynamic nature of an automobile front suspension.

On a front wheel drive car the wheels are pulling forward (driven by the engine) and trying to turn towards each other, taking the slop and tolerance out of all of the rack joints and tie rod ends. Thus the driving force ends up bringing the wheels to close to no toe in under way, bringing the car straight down the road. Thus why they are statically set up with toe out.

On a rear wheel drive car the force being applied to the front wheels is the drag of the tires on the road which is trying to pull the wheels apart (outward), again taking the tolerances out to bring the front wheels to approximately a no toe condition. Thus rear drive vehicles tend to be set up toed in.

In the X's case the rear suspension is tuned to ensure understeer near the limit so it is set up statically with toe in despite it being effectively a front wheel drive system in the middle of the car. You need the rear wheels toed in to ensure the rear tires are steering into the corner (the outside tire having the greater grip and motive force) so the static toe in on an X works for the total dynamics of the car.

I hope that is clear and makes sense. There are other factors and choices in suspension design that change the above but this is generally the case.

 

[Rupunzell]

I would NOT use the rod end pictured for ANY suspension part. Yes, it might not fail, but that is a metal bearing surface rod end that is not very high quality.

If anyone is going to convert their suspension bearings from ball joints or rubber bushings to spherical bearings/rod ends.. only use high quality ones that are fiber reinforced Teflon lined, alloy steel housing and ball or 400 series stainless steel ball in a alloy steel housing... Perferably the bearing housing will be staked to reduce slop and increase bearing ball pre-load.

Cheap rod ends or spherical bearings WILL make little if any improvement over the stock bushings..

It is preferable to put internal threads on the rod with a external threaded rod end, this will lower the stress risers involved. Proper external threaded rod ends will have rolled threads and cutting threads internal is OK, cutting external threads is not perfered due to the inherent nature of threads being a stress riser and all the things that can go wrong with cutting threads.

Bernice

It seems like an elegant solution when your tie rod ends get worn

 

[Rupunzell]

I am going to try a couple of items and test each one to see what the effects are.

1. Fix struts
* Possible to set the struts to equal compression and rebound, this should be a better starting point.

2. Change aero issues. The rad is vented into the trunk, but not out the hood.
*This is commonly done on the X, we did this on the LeMons X racer.

3. Replace steering control arms with heim joints instead.
*If you do this, use high quality rod ends and AN bolt of the proper grip lenght. As delivered Grade 8, 5 or similar part threaded bolts will NOT do. There should be no threads in the loaded area of the joint. I have a Very strong dis-like for using SAE or similar bolts in shear loaded applications, if an SAE or similar bolt is used, it must be modified remove the threads from the shear loaded area.

*The bolt holes should be reamed fitted to a few thousands of an inch.. a sloppy fit here will cause problems, including joint failure.

*Use a "safety" washer on the open end of the spherical rod end. This will help prevent a total joint failure and it is likely required by most race organizations.

*Measure the bump steer over the expected amount of suspension travel, before adjusting.. Bump Steer can be adjusted by adding or removing height between the rod end and steering arm. Zero bump steer is NOT always ideal, it depends on a host of factors including driver perference. ie: Toe out in bump can help reduce corner entry understeer..

*Was this X chassis lowered as outlined in the PBS book?

4. I will even stick a splitter on the front just for fun.
*Done properly, this will help.

I will let you know how this goes. Thanks again everyone, I really appreciate it.

*Bernice

 

[Black-Tooth]

Yep... and more...

Karl...

"In the X's case the rear suspension is tuned to ensure understeer near the limit so it is set up statically with toe in despite it being effectively a front wheel drive system in the middle of the car. You need the rear wheels toed in to ensure the rear tires are steering into the corner (the outside tire having the greater grip and motive force) so the static toe in on an X works for the total dynamics of the car."

Exactly what I was thinking... and I too have generally aligned the rears as you stated. I was really shooting from the hip with my idea... but in any case, given the UNIQUENESS of Eric's problem... it wouldn't hurt for him to at least check his rear alignment. I noticed that none of this was on his list of things to do...... Since its a possible FREE and simple fix, I think it would still be worth investigating before I jumped into doing all the other stuff!

 

[kmead]

Yep good point.

 

[budgetzagato]

Alignment specs?

The stock front spec is about 5/16" toe-in... would this help?

Alignment is more important the wider the tires... I have heard this affliction referred to as "road happy"... my 850 Spider was like this with 205/60R13s on it. Stock tire/wheels made it better, if boring..