[hellafast]

So as I was driving home tonight I was about to get on the freeway and saw a straight away with no cars so I said what the hell lets have some fun. I down shifted into 3rd and took off I was going about 80-85. As I was about to merge i hit a little bit of a dip in the road and I felt like I was flying almost lost control tbh it scared the crap out of me. I really don't know too much about suspensions, coils or stability. So my question what do I need to upgrade to feel more planted to the road? My car is a 335i e92 sport suspension stock pretty much everything my wheels are the 18in 189 style. For those of you that live in the bay area I was merging onto highway 17 south off of San Thomas Expressway.

[mob17]

There are quite a few things you can do which will transform your car, but it all depends on your needs and budget. If you can post these then a lot of members with more knowledge than me will be able to help you make the right choice.

A general explanation is that non-M suspension is designed around runflat tyres. The bushes are soft, arms have give etc. Hard tyres + soft suspension is not good!

The first thing you should do is change to non runflats. This will give you a lot more grip and also improve the ride.

A popular upgrade that will give you the planted feeling is to upgrade the front and rear control arms/links and rear subframe bushes all to M3 ones. These are all bolt on parts, but your car will need a full 4 wheel alignment straight after. The stock arms/links are designed for runflats. I guess most usual non-M BMW owners won't go anywhere near the limit of grip, so BMW can get away with it.

Genuine M3 arms/links are pricey, but most people get TRW ones, which are significantly cheaper. TRW manufacture the M3 arms and the TRW branded arms actually have the M logo scratched off, so quality wise they are exactly the same as genuine M3 ones. The rear subframe bushes are soft on the 335, and it is these that gave you that horrible feeling at the rear. Upgrading to M3 ones gives a great improvement to the feel of the car, but these bushes are quite expensive to install as the rear subframe has to come down.

Obviously you could also upgrade the standard shocks which are also not ideal. You can find many combinations of shocks/springs and coilovers. Your needs and budget will help you find the right ones.

[OmidHariri]

Lol bump and rebound stiffness but I think that's deep into the suspension modding

[hellafast]

Id say my budget is about 2k-3k.

[ashmostro]

How big was the dip? Part of "high performance" is the driver... perhaps you should have lifted, with the steering wheel at 12, just for that moment having seen and anticipated the potential loss of traction.

Serious comment- no coyness intended.

[hellafast]

Originally Posted by ashmostro

How big was the dip? Part of "high performance" is the driver... perhaps you should have lifted, with the steering wheel at 12, just for that moment having seen and anticipated the potential loss of traction.

Serious comment- no coyness intended.

I think mainly it just got me off guard, it was somewhat of a shock to me when I hit it.

[nanceman]

I'm guessing the "flying" moment occurred not during the dip itself but the sensation of continued over travel of the vehicle upward in the vertical direction?

I've encountered this same unnerving issue which was engineered into our vehicles for one simple reason, to keep the rear tires in contact with the ground at all times. This is done to prevent any and all over-steer which would be caused by a momentary loss of traction at the rear axle. In short, these cars stock have crap for rebound damping. The suspension compresses well, but then on what should be the resulting travel back to static the spring is not damped properly and the vehicle "floats." If the car were properly damped for rebound, the rear of the car would do as the front does and follow the elevation. This is also an issue on the BMW Performance Suspension... Got to love lawyers, law suits, the "nanny state" (as the Brits say) and their impact on the world.

WARNING: ENGINEERING CONTENT (although HIGHLY simplified)

Basically, rebound damping is a control of the amount of tension the system must be applying to the damper in order to stroke (i.e. the wheel wants to go one way while the vehicle wants to go the other way). In a car that is under-damped for rebound, the force required to achieve this stroke is very low so under essentially any condition the wheel will "sag" enough such that it is in contact with the road surface. In a car that is properly damped for rebound, this stroke will still occur, however to the extent such that the nominal energy built into the spring from the compression event is damped (i.e. removed) from the system, thereby allowing the system to return to the desired static state. The concern for BMW engineer's (as a result of lawyer's... can you tell I love them?) is the following scenario:

1. The vehicle is mid-corner (or any instance where centrifugal force is being applied, i.e. lateral force against the vehicle)

2. The vehicle's front and rear or only rear suspension is suddenly compressed (i.e. you hit an abrupt bump).

When this occurs, if the rebound damping is too high (or really if there is much at all) there can be a momentary loss of traction at the axle affected. This translates at the rear axle to greater vehicle rotation around the moment of inertia (in this case, the front wheels). All of this boils down to one effect, "oversteer."

For the front axle on the other hand, it is actually desirable, from a legal standpoint, for there to be proper rebound damping and that is why it does not feel as if the front of the vehicle "floats." Just as a properly damped rear axle could cause a momentary loss of traction, so too can this happen to the front axle. The major difference being that when you momentarily lose traction at the front axle you get everyone's favorite safety net, understeer.

Simply put, proper front axle rebound damping plus an under damped for rebound rear axle means that during an unsettling event the vehicle will always mechanically default to the front end having less traction and the rear end having more, ultimately resulting in understeer.

You may have noticed with loaners (if you have them in AWD like we do here in Michigan), that modern (i.e. E9X plus) BMW's with AWD are actually better damped than their RWD counterparts. This is because if the same event occurs but to an AWD vehicle with a 50-50 torque split to each axle and the driver continues to apply power throughout the event (i.e. temporary loss of traction) the vehicle will pull itself toward its intended position, whereas with a RWD vehicle engineer's have to be even more cautious because they must further ensure that an application of power to the rear wheels will not overcome this designed effect and result in oversteer after all. As a result, rebound damping on a RWD BMW (and really most modern RWD vehicles with fully defeatable traction and stability control) is effectively non-existent.

Ultimately, however, Newton's third law ("for every reaction there is an equal and opposite reaction") rectifies this with absolutely terrible for performance and sense of stability we all know as "float" and that is the bane of all drivers who actually know what they're doing.

One final point of note is that this is due solely to the vehicle damping and is NOT a result of the overly soft bushings. The bushings contribute towards myriad other issues when it comes to precision and feedback, but the fault of "float" lies squarely with the vehicle damping (as evidenced in practice by the numerous members here who have installed new M3 control arms, subframe bushings, etc. either with the stock suspension or the performance suspension only to find this issue is still very well there).

Hope this isn't too abstract and helps!

[nanceman]

Copy of the above, Chrome double posted for some reason.

[nanceman]

Originally Posted by ashmostro

How big was the dip? Part of "high performance" is the driver... perhaps you should have lifted, with the steering wheel at 12, just for that moment having seen and anticipated the potential loss of traction.

Serious comment- no coyness intended.

Lifting would have exacerbated the issue, as the moment of the vehicle in the event would further be causing the rotation of the vehicle about its front axle (i.e. you would be further loading the front tires and unloading the rear tires by lifting and, in fact, making the issue worse).

The best thing you can do in any scenario such as this is to maintain the dead load of the vehicle by holding the throttle steady and not further rotating the vehicle length-wise about one axle or the other causing more load to be on the rear axle or the front axle.

This is of course the safe approach, and I will certainly add the disclaimer that given sufficient practice, experience, and skill events can be better combated by adding a "weight shift" on top of steering input.

[CALWATERBOY]

Originally Posted by hellafast

Id say my budget is about 2k-3k.

Right. Your alignment is spec, yes?

OK - as your stock rear is fairly poor over bumps & in curves, install a Limited Slip Differential and M3 Rear Subframe Bushings.

A good start! You notice a pronounced improvement....now think about Koni Yellows + Eibach Sport linear springs.

M3 suspension arms eliminate slop from rubber bushings; are lighter + stiffer.

Camber plates have delightful effect up front.

Any of the above are real improvements - total is a very diff, grossly improved ride. Smoother than stock too.

M3 sways stabilize that puppy - must have LSD first.

[ashmostro]

Originally Posted by nanceman

Lifting would have exacerbated the issue, as the moment of the vehicle in the event would further be causing the rotation of the vehicle about its front axle (i.e. you would be further loading the front tires and unloading the rear tires by lifting and, in fact, making the issue worse).

The best thing you can do in any scenario such as this is to maintain the dead load of the vehicle by holding the throttle steady and not further rotating the vehicle length-wise about one axle or the other causing more load to be on the rear axle or the front axle.

This is of course the safe approach, and I will certainly add the disclaimer that given sufficient practice, experience, and skill events can be better combated by adding a "weight shift" on top of steering input.

I didn't say get off the throttle completely. Lifting slight before the event occurred to stabilize suspension loading and forces on the tires would have helped. Look into the path you are heading- be it a straightaway or a turn. If you have poor visibility, don't go nuts with the throttle, particularly while merging. This is just good sense on public, poor quality roads.

[nanceman]

Originally Posted by ashmostro

I didn't say get off the throttle completely. Lifting slight before the event occurred to stabilize suspension loading and forces on the tires would have helped. Look into the path you are heading- be it a straightaway or a turn. If you have poor visibility, don't go nuts with the throttle, particularly while merging. This is just good sense on public, poor quality roads.

I should have guessed that was the implication, given I've seen you also head to the road course. More a comment for the benefit of those who would go toward an extreme and by lifting they would slam the throttle shut.

It is definitely good sense on public roads, I always do a recon run before I even push to what is safe on public roads (7-8/10ths). Always good practice to ensure no dust/leaves have accumulated, no slick areas, etc.

[Jase 320d]

Yup ditch the runflats! Thats the first port of call! Can't wait to bin mine!


The M models come without runflats so what does that tell you straight away

[ashmostro]

Fair point indeed

[lafuerza]

In all the research I have done before heavily modding my suspension to get rid of that bounce in the rear at a minimum get better shocks and to really feel connected in the rear replace the subframe bushing. Start with shocks and go from there. I would be nervous around dips at any speeds. Now very tight and confidence inspiring. It will come down to budget and what you be want out of the car from there.

[HP Autosport]

'More planted on the road' is a very subjective question. As others have said, when you go over a large crest and catch air or not, you always want to slightly lift and even out the steering wheel to maintain control of the vehicle.

In terms of suspension, you can always upgrade your shocks and springs or go with a full coil over setup, depending on what you plan on using the car for. You can go with shocks/springs + good tires for under 3k.

-Mike