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Discussion Starter #1
I *think* I have an idea what the issue is, but looking to hear other opinions.

First off suspension mods

Mach 1 springs (ride height is pretty close to stock, maybe a tiny bit lower)
Tokico Blues (30K miles on them, but I also installed them 15 years ago)
03-04 Cobra steering rack, MM shaft, all new tie rods
Brand new M-3075-A control arms
94-95 spindles
Stock Caster/camber plates.
OEM sway bars
235/45/17

Out back
MM LCA's. NEW OEM uppers, new axle bushing, new quads. No issues with the torque boxes. 255/45/17 out back

Over the winter I did the Cobra steering rack install, with the new tie rods and MM shaft. All bushings/balljoints/control arms have less than 1K miles on them. There is no play in the steering. A tiny bit of movement of the steering wheel does translate down to the front wheels.

I purchased a caster/camber level, and some toe plates, and did a self alignment. I am not running a bumpsteer kit, and have stock C/C plates. I set it to -0.5* camber, and set the toe to 0. Initially I wanted to confirm the car did not pull to either side first at speed before adding some toe-in. I haven't been back to readjust the toe yet. Did not adjust caster because there is no adjustment on stock plates.

The car drives great from 25-45MPH. Steering is appropriately heavy and had a great feel to it. Problem is when I go faster. At speeds of 60-80MPH, the steering feels very light, and I get a lot of feedback from the road and it tends to follow every little groove or rut. On new pavement, the car tracks perfectly straight, but the steering is feather light.
My thought it I need to add some toe in. It's zero toe now, so at higher speeds, the rubber bushings deflect and I get a slight toe out condition, which causes the steering to feel a bit light. Anyone else's opinions?

I am planning on some MM C/C plates, and new shocks/struts soon.

Most of my driving is backroads up to 45MPH, so I haven't been too concerned and why I haven't dragged out the toe plates again. However last night I went out on the highway for 10 mins, and decided I need to adjust something here.
 

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I run 0 toe on my mach1 street/HPDE car and it's just fine. I do have CC plates and poly in the rack and CAs.
 

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Rolling resistance of the tires is a constant. It does not change with vehicle speed, so it won't deflect the FCA bushings more at 80mph than at 25mph.

The faster the car goes, the more air gets under the car lifting it. This will reduce the steering effort as speed increases. With less than 2 degrees of caster and zero toe, it is entirely possible that the lift is making this much change in steering feel. With c/c plates you should be able to achieve 3.5 to 4 degrees of caster. With a little bit of added toe in, this will make the car much more stable.

Another thing to check is the engine rpm that you are checking the steering effort at. Fox Mustang PS pumps have a rising flow rate with rpm. They are just not a good design. This means that the steering assist is going to rise some with rpm. I would double check that you are at the same engine rpm in both high and low speed cases.
 

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Discussion Starter #4
Thanks Jack,

I was thinking added caster with the C/C plates would help, but never thought to think of lift as a possible reason. I don't think engine RPM is an issue in this case. I have underdrive pulleys on, and tend to drive around at 3500RPM when cruising the streets, and was in 5th gear at 2500RPM on the highway.

I'm probably going to change up my suspension setup shortly. Unsure if my particular spring/damper setup is partially to blame, but the car does feel a bit floaty in turns. I'm torn between H&R sports and current tokico blues, or perhaps H&R race with Koni SA's or another damper. Thoughts?

After revisiting springs/damper and installing C/C plates, i'll adjust alignment to your recommendations and give it a shot
 

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Rolling resistance of the tires is a constant. It does not change with vehicle speed, so it won't deflect the FCA bushings more at 80mph than at 25mph.
Kind of picky, but rolling resistance actually does increase very slightly with speed. About 25% higher at 100 mph than it is at 50 mph according to one reference. If nothing else, you're distorting a greater volume of tire tread/sidewall (circumference) per unit time.

I do agree that aero is by far the most likely cause, though those Tokicos can't be helping either directly or indirectly (by allowing slightly greater upward variations in ride height)..


Norm
 

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Discussion Starter #6
though those Tokicos can't be helping either directly or indirectly (by allowing slightly greater upward variations in ride height)..
I suspected this as well. Previously, I had Ford Racing C springs on the car with the Tok's and stability was much better. However, I changed springs to get some ride height back and that may be when this problem first started to rear it's head. I'm sure my spring/damper combo is not the best.

I certainly don't mind throwing some money at this to correct, but unsure of what spring/damper to choose. I'm stuck between H&R sports and race, but need to select a proper damper to match those as well.
 

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One I track I've frequented has a 0.8 mile straight. Speeds of 145mph are easy to reach. It's amazing how light the steering feels at those speeds with stock aero.
 

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Discussion Starter #8
So got a chance to take the car for another ride last night. Definitely something I need to address.

I'm starting to second guess if the steering does in fact get lighter as speed increases. I tried to test at a few different speeds, and it was inconclusive. I even varied RPM with no change at all. I got back when the sun was going down, so didn't have time to double check my alignment specs.

How i'll describe it now? "Floaty". For instance, if I shake my wheel side to side at speed, there is no instant response. The car seems to roll slightly, settle, and then respond. I'm starting to suspect the dampers...as they are at least 15, possible 20 years old and the car did sit for 10 years. When I ran them years ago with FMS C springs, the car felt like it handled on rails...and rode like a truck.

Let me ask a question. If the dampers were failing, how would the car react? They pass the bounce test, but I remember this car driving as if it was on rails when I first installed them. Now it feels "soft?

For refresher I have OEM Mach 1 springs. I believe the H&R race springs I was looking at are only slightly firmer than the Mach 1 stuff. Not sure that justifies a change.
 
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