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Handling problems

6K views 49 replies 19 participants last post by  39 master 
#1 ·
Need a handling guru to find my problem. Background: built a frame for my '39 out of 3/16" wall 2" X 4" on a frame jig. all is square with the world. MII style front suspension with Detroit Eaton springs, tubular A arms and properly aligned. Rear is Ford 8.8 with air bags (50 PSI), triangle 4 bars and 7/8" sway bar. No sway bar on front. All goes well until about 60 MPH then the slightest outside force such as crosswind or center crown will upset the vehicle stability. The last test (above 60) resulted in a rocking motion that frankly scared the hell out of me. This car should be stable at and above this speed but I can "feel" something is wrong. Everything is new except the tires which have proper pressure. Anybody have an input? Going to disconnect the sway bar and see what happens tomorrow.
 
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#6 ·
a flat or usually home made chassis can end up with less strength than a factory frame. so the factory builds them in a way the makes them strong. You will never see a flat chassis or a square tube from a factory car so they can get buy with thinner metal. But thats more of design than anything else if its sound it should be fine.
Not true.. A channel frame is NOT stronger then a tubing chassis.. Not at all.. The factory use's channel because it is cheaper not stronger..:nono:

And the channel they use under the trucks is 1/8

You will never see a race car chassis built out of channel..:mwink:
 
#7 ·
Poor handling

The frame is STOUT! I also added a K member with two cross ties. There is NO twist and the frame was built on a jig, it's true is all direction so that's not my problem. Today I took the rear sway bar vertical links out and did a test drive. No difference except some body roll when I turned a sharp corner. Noticed that once at speeds about 50 MPH and accelerate things started to happen. The car would roll a bit and dart a bit. Gave me a really bad feeling that things could go from bad to crash at higher speeds. Measured the front tires at rest and then jacked the front off the ground and measured again. The toe in changed by half an inch!!! Is that normal or is that my problem? I think my first step is to put a sway bar on the front and go form there. Also going to take it back to the front end guy for his input.
 

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#9 ·
Measured the front tires at rest and then jacked the front off the ground and measured again. The toe in changed by half an inch!!! Is that normal or is that my problem? I think my first step is to put a sway bar on the front and go form there.
Geometry is out of whack. Relationship between upper and lower control arms and steering rack is bogus. Strongly suggest procuring the book "Tune To Win" by Carroll Smith. When I was wanting to build an IFS from scratch, I read author after author with no help, until I read Smith. He was the Team Chief for Ford Motor Company when they went to LeMans with the GT40 and whipped Ferrari. He has since passed on to the big race track in the sky, but he knew his stuff. In the back of the book are instructions of how to construct "paper dolls" with construction paper and stick pins, to test the geometry of a proposed suspension system before you spend any money on metal. I followed his instructions and built my own independent front suspension for a T project that worked really well. That is probably the fondest memory of all my hot rodding projects, an IFS from scratch that worked well. There aren't many guys on the planet that have done that.
 
#8 ·
My knee jerk to your problem is one that hcompton mentioned. Caster. Add more caster and see what happens. The sway bar missing in the front may be compounding the problem once the stability goes away.
Adding castor to my '56 wagon during the last alignment was an immense improvement and stopped the wandering over expansion joints. :)
Mark
 
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#12 ·
I would start back at square one. Square and center front and rear. Check wheel base. I don't trust those computerized alignment racks (or maybe it's the operator).
Problem could be in the rear as opposed to the front end. Never ran bags, but I think they might be suspect to the problem.
You never mentioned shocks on the rear unless I missed something. Surely you are running shocks. Are they mounted at a big angle or are they way inboard?
I would get rid of the rear sway bar, at least until you get the problem solved. (You already took it off-no difference). A sway bar is not really meant to stop a swaying problem. but it is to stiffen the spring on the outside of a turn. Of course this stops or reduces the sway in a turn. Get your swaying problem fixed first, then worry about sway bars. Sway bars will make your car loose or tight depending on a lot of other things, but not meant to keep it from swaying back and forth. There is another problem.
The thought just struck me that you car may have a low rear roll center with a high center of gravity and soft springs to go with that rear sway bar. This combo might could cause a sway? Just thinking.
Dirt circle track raced for a long time. Handling problems are interesting to me.
More info. I think we need good pics, front and rear.
 
#13 ·
If you really have a 1/2" toe change, then as Tech said, the geometry is off. That alone would make the car wander and dart all over road.

There are several things that can cause toe change, but the main thing to remember is that the lower control arms and the tie rods need to make the same arc. As a general rule the tie rod length from the center of the inner tie rod end to the center of the outer tie rod end and the lower control arm from center of the inner bushings to the center of the ball joint should be very close. The angle of the tie rod and the lower control arm should be equal or nearly so.

There are kits to lower the outer tie rod ends on the MII. This is a common place for bump steer in the MII, but that might not be your problem. The rack could be too high, low or out of position forward or backward.

When you crank in a lot of caster, the steering arms on the spindle are raised. This will sometimes increase the bump steer since it changes the angle of the tie rod in relation to the lower control arm. This is one reason for the bump steer kits on the market. Don't misunderstand, caster is good, but you have to make sure that changing one thing doesn't cause issues elsewhere.

As others have said, having a sway bar only on the rear is not common. That setup would most likely cause oversteer since it would increase the slip angle of the rear tires.

I personally would try to sort out the toe change before anything else.
 
#14 ·
Bumpsteer

as tech inspector says the geometry is wrong, pivot locations, rack location angles, lengths. A good friend is the retired head of Ford Racing and he spent a lot of Sundays on Fords computers redesigning the suspension for his 427 cobra, and in 2009 won 3 first and one second at the Optima. a lot of his suspension info is on the Club Cobra Forum
There is a lot of good suspension info on youtubes. a good primer is from Brent Vandervort, owner of Fatman Fabrication.here is a link to his 4 part presentation
https://www.youtube.com/watch?v=MpMgwbn6ctU
 
#17 ·
You can see the different mounting on these two pictures.. He has the top.. You set the caster by rotating the top a-arm in the slots.. This will move the top ball joint behind the bottom which in turn gives it more caster..



 

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#18 ·
Along with the other comments I would introduce yet another possible scenario, something called roll oversteer. If the chassis pictured ---with flat lower a-arm inner pivots and the rr four-bar with angles that may as seen from the side slant down slightly--- is sitting at any rake at all, geometry is going to cause what I might term as a reverse-skateboard effect, when the car leans in a corner the tendency will be for the suspension to steer slightly in the same direction you're steering, which will cause it to lean more and steer more as the problem compounds, net result being the type instability you're talking about. Lifted 4x4's with link-type suspensions ('90s Grand Cherokees) get that and can become so bad as to be nearly undrivable on the street, to mention an extreme example.

Along with giving it some caster, fixing the bump steer (!), maybe adding some anti-dive to the upper control arm...I would look into that. Without re-fabbing anything the band-aid solution for such a problem is to give it enough sway bar, front-and-rear in a balanced combo, to increase roll stiffness enough so it doesn't get as much of a chance to get into that condition. Stiffer shocks would also help.
 
#21 ·
Handling

This is a standard street rod aftermarket Mustang II so there is no reason to try and reinvent it. Don't believe I reported bump steer, which it does not have. When I accelerate and decelerate is when things happen and it appears to be related to loading and unloading the rear suspension. Still am going to move forward and add the front sway bar, grease and check the rear 4 link for smooth operation. Tires are also a bit of concern because they were made in 9/09 have lots of sidewall (205/75-15R). Maybe, just maybe it will be a combination of sway bar, binding 4 link and tires. Stay tuned.
 
#24 ·
The problem with the alignment shop's (here any ways) The have no clue when you pull in with a street rod with a mustang II base front end on a street rod.. Another thing is.. They have no idea how it was installed how off it may be ...

He had a lot of good things told to him here.. And still chasing his tail on this.. At this point.. I hope he figures it out.. He will soon or later see the answer was posted here..;)

As far as the sway bar goes.. I don't fell that is your problem.. Very easy to test.. Drop the back one out to see if things change.. I never use one to start with.. Good luck..;)
 
#26 ·
Instead of all the trips to the alignment shop while you are doing caster testing, you can use a level or plumb bob and string to set your caster. Drop a bob or plumb up a level in front of the A-arms. Measure back to the upper and lower ball joints. Yes I know we are working in inches now. A little trig would convert, but not necessary. But you will have a reference point to adjust from and be able to return to.
 
#36 ·
that is one heck of great idea for somebody with access to a lathe. create a second nut for the top with room for a guide string from the center..

and a second bottom nut with a plate on it. so you can see where the plumb bob is in relation

even better would be an pointed device that could center between the ball joint studs.. again with some kind of pointing device. the car would have to be level.. and it would only work with short /long arm suspensions.. or MII spindles.

i can envision it.. but i have no access to do any proto types.. somebody run with this will you..,
 
#27 ·
I didn't see any note of a panhard bar or other rear end locator.
Get rid of the rear "sway" bar and install a front one. There are bolt on ones available.
If you are using a M ll front end Fatman has the alignment specs. Call him.
The lower controll arms must be parallel to the ground on a M ll front end.
You need some toe in at ride height. If it is changing radically as the suspension moves you have a set up problem. The alignment shop can steer you in the right direction.
If there is looseness in the rack and pinion, you need to replace it.
Make sure the rod ends have the correct taper and are in good shape.
I hope there are new ball joints all around.
 
#28 ·
OK update. Front sway bar in (not easy, reroute brake lines), reset the pinion angle and made sure everything was tight. Problem is still there but I can better define it now. Going along about 45-50 MPH and accelerate (not jump on it) and there will be a slight body roll which seems to be coming from the back. Let off the gas and get the same "twitch". The rear shocks are Monroe 555003 and wondering, as mentioned earlier, if the shocks are not up to the job. May need to find a stiffer shock. Specs are: extended 14.75", compressed 9.625" eyes top and bottom .625". Anyone know of a shock that falls into that range?
 
#29 ·
Sounds similar to what my son's MCSS would do after a weld broke on one of the lower link bars. They were South Side Machine lowers that had to be welded over the factory arms. Have you checked the rear CA's for damage?

Russ
 
#30 ·
Can you put up a picture of your rear suspension? It looks like you have a triangulated 4 bar but I can't zoom in on you existing pictures.

I had the a similar problem but my shocks are much farther inboard giving poor rear roll stiffness. The solution was QA 1 single adjustable shocks. However they were too much of a good thing in that they are too stiffly valved. Your car may be a bit heavier than mine so this should not be an issue. I'd suggest you jack the car up, place on Jack stands, disconnect the shocks, and Jack the rear end up through its normal range. Do one side then the other then both. Look for binding or strange movements. Make a notebook record.

Several of the cruise guys have cars similar to yours with QA 1 and they don't have the " twitch" problem. They all ride rather firm but not extra hard like mine. I'm more limited in cornering by carb flooding than traction.

I have a pair of special rear anti roll bar links that I conjured up a couple years ago. I wrote them off as an engineering mistake...day dream gone bad. Haha
I'm going to re test them today since I've changed things around again. I'll post results and maybe a picture.

Most of this involves fine tuning....a tweek here a tweek there. Remember that the factory may have a whole team and a fleet of cars to work with to fix just this type of problem. We are at best just one engineer in a back yard garage with a tape measure and notebook.
 
#31 ·
Handling problem

S10xGN that's what I thought, something broken or a weld broke on the 4 links but I had each one off while resetting the pinion and didn't find any problems. Today I tried different pressures in the bags but the problem stayed the same. I was going along at 55 MPH and hit a dip in the road and the car just floated over it, no bump steer and any other handling issue. Even when I hit a bridge mis-match there is no bump steer. Only time the "twitch" happens is accelerate and decelerate. New motor mounts and trans mount. As bentwing said next is to disconnect things and jack the rear end thru full range to see if something strange happens. What are CAs?
 
#33 ·
Handling

Yes, control arms...I was calling them rear black bar thingies! Took the shocks off and discovered they do not have a steel insert in the rubber bushing. Specs on the shocks are the eyes are .625 (5/8"). My mounting bolts are 5/8" shank and the rubber bushing hole is 5/8" so there was never a steel bushing in them. Of course they do not pivot well when bolted up so I'll see about a lube in those joints. Did take the car on a short spin without the shocks and too much going on to really tell if the shocks were binding.
 

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#34 ·
I think one of the problems here is that you don't understand some fundamental terms...

'Bump steer' is something you do have. Remember when you jacked up the front wheel and the toe-in changed? That is 'bump steer'. The wheel turns or steers because you hit a bump.

It will also do this when there's body-roll in cross-winds etc.

But I think your instability problem is also largely because there's not enough lateral location of the rear axle.

Shocks don't have anything to do with this. Springs or springing media don't have anything to do with this. Especially, freedom of movement in rubbers doesn't affect it. It's the amount of lateral movement which is possible with your angled links on the rear.

A Watts Link, a Panhard bar, a Mumford Link, sliding joints, they are all preferable because they don't allow the rear of the chassis to wander. And when it wanders your lack of sufficient caster and your bump steer come into play and make things terrible.
 
#35 ·
Did some road test today with a passenger for second observation and input. On our short trip everything was just a little off when accelerating or getting off the throttle (slight twitch). Did notice that at speed (55 MPH) the car wanted to follow a crack in the road. Latter deflated the bags from 55 lbs to about 40 lbs and accelerated there was a very notable twitch (more of a jump). When I got off the throttle the same thing happened. Inflated the bags and the problem just about went away. Now, inflated bags jack the rear a bit (not much between 40-55 lbs) which would change the caster by damn little and toward zero not 5 degrees. Increasing the pressure in the bags will also change the 4 link angle but by more because they are short. More pressure in the bags also stiffens the rear suspension and MAY prevent side "twitch" or thrust of the body/frame by the suspension. Next step is to add a diagonal track bar to the lower control arms and see if this will eliminate the twitch. Stay tuned...
 
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