Just a lil quickie of a question..

Miller220 · Oct 7, 2009

Hello fellow stangnet......ers. My question, although about my mustang....is probably more of a RWD cars type question in general, ANYWAYS....so I've had my car for a few months now, LOVE it.....but anyone who lives in the metro detroit area knows how beautiful our RACE grade roads are Anyways, more to the point. When I go over bumps in my car at any sort of speed, my car jumps around a lil. Is this normal? I'm asking, because it isnt a HUGE jump, but it'll cut to the left 6-8ish Inches, or to the right. It also does this at 35+ mph. Any ideas? The car is an 02 GT, all it has on it is an exhaust and some window tint. Thanks for any help you guys might be able to offer

98COBRA281 · Oct 7, 2009

sounds like bump steer, is the car lowered?

trinity_gt · Oct 7, 2009

I notice mine will react surprisingly to certain things, such as one rear tire falling into a recessed pothole. That will certainly generate a bit of a jump. 6-8"? I don't know that it jumps that far but it's definitely noticeable.

Welcome to the world of live axles combined with 4-link rear suspensions. You have to understand that the rear suspension is an exercise in compromise. With four links holding the rear in place, there's a doubling up of duties: the upper links not only control axle wind-up but they play a huge role in keeping the axle centered under the car. The lower arms obviously work with the uppers to form triangulation for lateral control but they also obviously locate the rear longitudinally.

There's an old saying that goes something like "Jack of all trades, master of none." Because each link has multiple duties, they're not that good at any one of them. Because the axle is, for all intents, a rigid body and because the upper arms are shorter and angled very differently than the lowers and, as the axle moves up and down, travel through very different arcs, there has to be a good deal of compliance built into the bushings in order to provide the "give" to let all these conflicting arms do their thing without binding up. This compliance allows a lot of rear axle movement. In fact, there's so much compliance in the front bushings of the LCAs that Ford actually had to put a shock absorber in to dampen fore/aft axle movement (the so-called "quad shocks.")

All that movement allows all sorts of toe-changes at the back, especially when the hit is asymmetric: one rear falling into a pothole will result in the axle pulling back on that side a bit (bushing compliance), causing the rear of the car to steer to that side momentarily. In addition, the rigid live axle means that when one wheel hits a bump and the wheel reacts, the other wheel will be affected because of the solid connection between the two.

Ford did better with the S197, with two LCAs, a single UCA for torque reaction and a panhard bar for laterals. Ford did even better with the New Edge Cobra independent rear suspension stuffed into the same mounting points as the live axle.

Miller220 · Oct 7, 2009

Sadly, my car is still lifted, so I guess that rules out bump-steer. Am I to assume, from the post of biblical proportions, that the car jumping on bumps and potholes is just part of the engineering? If so, is there any aftermarket parts I can put on to get rid of it? If I lived in florida, I wouldn't think twice of it, but a lot of the roads I go on are worse then most dirt roads that I've driven in some southern states

trinity_gt · Oct 7, 2009

Miller220 said:
Sadly, my car is still lifted, so I guess that rules out bump-steer. Am I to assume, from the post of biblical proportions

I apologize if the engineering terminology was over your head, nOoB.

that the car jumping on bumps and potholes is just part of the engineering?

It's a consequence of Ford using Carter-era engineering for upwards of 25 years.

If so, is there any aftermarket parts I can put on to get rid of it?

Swap in a Cobra IRS. Some Cobra owners covet the stick axle you've got for straight-line (drag) racing.

Or you could visit Maximum Motorsports and scope out their Rear Grip packages. For example:

Rear Grip Package, 1999-04, HD, Standard [MMRG-5] - $575.90 : Maximum Motorsports Performance Mustang Suspension Store, Buy Now!

You're welcome.

wmburns · Oct 7, 2009

suspension binding?

Wow. I had/have a 1996, 2003, and a 2000 Mustang GT and never noticed it "jumping" 6-8 inches going over a bump.

When driven like a normal car, IMO a Mustang doesn't really drive any different than any other car.

Are you sure that the suspension is not binding? Bad shocks? If one shock is totally shot and the other is working, this could cause some strange driving issues.

Like the other posters have stated, the Mustang is not the most advanced platform for extream driving. However, there are many of us out that have logged hundreds of thousands of miles street driving just fine.

stangGT97 · Oct 8, 2009

trinity_gt said:
I notice mine will react surprisingly to certain things, such as one rear tire falling into a recessed pothole. That will certainly generate a bit of a jump. 6-8"? I don't know that it jumps that far but it's definitely noticeable.

Welcome to the world of live axles combined with 4-link rear suspensions. You have to understand that the rear suspension is an exercise in compromise. With four links holding the rear in place, there's a doubling up of duties: the upper links not only control axle wind-up but they play a huge role in keeping the axle centered under the car. The lower arms obviously work with the uppers to form triangulation for lateral control but they also obviously locate the rear longitudinally.

There's an old saying that goes something like "Jack of all trades, master of none." Because each link has multiple duties, they're not that good at any one of them. Because the axle is, for all intents, a rigid body and because the upper arms are shorter and angled very differently than the lowers and, as the axle moves up and down, travel through very different arcs, there has to be a good deal of compliance built into the bushings in order to provide the "give" to let all these conflicting arms do their thing without binding up. This compliance allows a lot of rear axle movement. In fact, there's so much compliance in the front bushings of the LCAs that Ford actually had to put a shock absorber in to dampen fore/aft axle movement (the so-called "quad shocks.")

All that movement allows all sorts of toe-changes at the back, especially when the hit is asymmetric: one rear falling into a pothole will result in the axle pulling back on that side a bit (bushing compliance), causing the rear of the car to steer to that side momentarily. In addition, the rigid live axle means that when one wheel hits a bump and the wheel reacts, the other wheel will be affected because of the solid connection between the two.

Ford did better with the S197, with two LCAs, a single UCA for torque reaction and a panhard bar for laterals. Ford did even better with the New Edge Cobra independent rear suspension stuffed into the same mounting points as the live axle.

Wow, excellent information! I think I learn something new/useful every time you post in a thread.

trinity_gt · Oct 8, 2009

wmburns said:
Wow. I had/have a 1996, 2003, and a 2000 Mustang GT and never noticed it "jumping" 6-8 inches going over a bump.

When driven like a normal car, IMO a Mustang doesn't really drive any different than any other car.

I agree the 6-8" thing sounds excessive but if you've ever had a chance to drive different cars you'd know that the 79-04 Mustang just does not deal with road imperfections all that well compared to other cars. It's not alarming on its own but compared to more modern hardware, driven back to back, I find myself saying "Wow. The Mustang is actually pretty bad..."

Of course it's tough as nails, pretty reliable (except perhaps too many stories of upper control arm brackets pulling away from the floor/chassis and/or cracking...), inexpensive and gives decent NVH characteristics. But it's just not very good at keeping the back of the car from dancing around on rough surfaces.

Kilgore Trout · Oct 8, 2009

trinity_gt said:
I notice mine will react surprisingly to certain things, such as one rear tire falling into a recessed pothole. That will certainly generate a bit of a jump. 6-8"? I don't know that it jumps that far but it's definitely noticeable.

Welcome to the world of live axles combined with 4-link rear suspensions. You have to understand that the rear suspension is an exercise in compromise. With four links holding the rear in place, there's a doubling up of duties: the upper links not only control axle wind-up but they play a huge role in keeping the axle centered under the car. The lower arms obviously work with the uppers to form triangulation for lateral control but they also obviously locate the rear longitudinally.

There's an old saying that goes something like "Jack of all trades, master of none." Because each link has multiple duties, they're not that good at any one of them. Because the axle is, for all intents, a rigid body and because the upper arms are shorter and angled very differently than the lowers and, as the axle moves up and down, travel through very different arcs, there has to be a good deal of compliance built into the bushings in order to provide the "give" to let all these conflicting arms do their thing without binding up. This compliance allows a lot of rear axle movement. In fact, there's so much compliance in the front bushings of the LCAs that Ford actually had to put a shock absorber in to dampen fore/aft axle movement (the so-called "quad shocks.")

All that movement allows all sorts of toe-changes at the back, especially when the hit is asymmetric: one rear falling into a pothole will result in the axle pulling back on that side a bit (bushing compliance), causing the rear of the car to steer to that side momentarily. In addition, the rigid live axle means that when one wheel hits a bump and the wheel reacts, the other wheel will be affected because of the solid connection between the two.

Ford did better with the S197, with two LCAs, a single UCA for torque reaction and a panhard bar for laterals. Ford did even better with the New Edge Cobra independent rear suspension stuffed into the same mounting points as the live axle.

Awesome tech!

I would submit however that, at least in the case of my car, the IRS does have some issues in stock form. Specifically I cannot do burnouts without massive wheel hop.

trinity_gt · Oct 8, 2009

Kilgore Trout said:
Awesome tech!

I would submit however that, at least in the case of my car, the IRS does have some issues in stock form. Specifically I cannot do burnouts without massive wheel hop.

Oh absolutely. And even though axle tramp can generally be addressed by stiffening up the mounting points (at the expensive of impact harshness, noise, vibration etc), I don't think the IRS is as reliable, overall, as the stick axle for serious drag racing type stuff. If I were to drag race a Cobra, I'd source a live axle for it. If I were to street-drive it or road-course it I'd definitely stay IRS.

2002SN95MUSTANG · Oct 8, 2009

I have had the same problem before. I was going down a road that has some bumps in it, kinda like a whoops section on a dirt bike course and they were in the middle of a very slight left hand turn. I mean very slight, like the top of the steering wheel is at 11:30 instead of 12:00. And the back end came around enough to almost make me go in the other lane. Haven't had that problem since I put in my shocks and springs along with poly spring isolators and suspension bushings.

GDawg · Oct 9, 2009

I would have your alignment checked, specifically your caster. Many people only really think about camber and toe but if your caster is too low, your car will not track as well as it should. Just a suggestion...

GettinThere · Oct 9, 2009

Miller220 said:
...the car jumping on bumps and potholes is just part of the engineering?

Yes, especially on curves.

Miller220 · Oct 9, 2009

Thanks everyone who replied again. The post was biblical in nature due to the size/wealth of info and I think I'm going to start lurking Trinity gt's threads more <.< regardless....I retract my initial 6-8" ALL the time, I'll usually only get that while I'm accelerating on a back road on a really big bump. On the highways, I'll only go about 2-4" on normal to medium-large bumps. I was just curious is all, because my 07 cobalt that I've driven since...07 usually had a pretty smooth ride, and its just part of the fun of having a new car I suppose...getting used to its quirks. Thanks for the help guys

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