Yeah, I keep thinking about switching to 17s because of this.... I'd love to borrow someones and drive it around before committing though haha

 

Certainly a good idea to try out before committing - it would suck to find out it didn't work, even though, IMO, there's a high probability that it would help.

Also, all tires are not created equal - some have stiffer constructions than others of the same size - it would be good to research ride quality ratings to assess what you want to buy.

 

When the FD was engineered over 20 years ago, they had a wheel diameter and tire size in mind when doing suspension design. You are a +2 in wheel size with a larger contact patch and significantly more unsprung weight than they would have designed for. Some manufacturers, like Porsche, will even issue a maximum wheel diameter recommended for each model based on suspension design. Ever lift a stock FD wheel? They are LIGHT, a little under 16 lbs. Even a very lightweight 18" wheel will be in the 18 lbs range, and most are 22-25 lbs. Your tire with the 18" size will be heavier as well. I suspect if you dropped back to either OEM 16" or even a lightweight 17" that you would feel a significant improvement in steering response, nimble responsiveness, noise, and ride quality.

 

^^
This is true.
The Yamaguchi FD book states that there was a last minute on the production line recall to add a reinforcement to the boxed section in the lower sill in front of the rear wheel- added to increase chassis rigidity because the car was slapping going over freeway expansion joints (you can actually see the cut/weld where they added it).

But still, the rest of us aren't complaining of whacks going over normal road features in our FD with 18s.

Either the OP likes to drive with windows down and is more sensitive to sounds than the rest of us, its the tires he has chosen or there is something up with the car/part he has added.

 

Was this ever resolved ? I have ohlins DFV as well and new toelinks trailing arms and new pillow ***** with a poly diff bushing and I know exactly what op is describing. Everything is great . But when ever you go over little cracks in the road or little gaps/tiny pumps / lane markers , the rears seems like there is no traveling almost like the rear justs drop into the hole and bounce back up making the slap thud noise. Very annoying
Makes the rear sound like a rattle can and things are about to all let loose . Oh. And I'm on 16inch stockers with brand new tires. So I know it's not a wheel tire size issue. Could be messed up swaybar links ?

 

What locations are the poly bushings you put in the rear?

 

As of now just the diff is poly

 

are the toe links and trailing arms stock?

any aftermarket end links?

 

Toe link and trailing is aftermarket stock original end links. Could the bushing on them are busted ?

 

Aftermarket suspension componants that use pillow ***** tend to clunk

 

It's the poly diff bushings.

I think you'll have much better luck with the OEM bushings in the diff carrier.

M2 style links and arms don't make noise going over bumps but could make some noise under heavy braking or cornering if they become worn/sloppy.

 

It might be binding if it wasn't lubricated. However, the differential does not rotate that much on the bushings

 

But the diff/axles are like 60lbs suspended by just the two bushings to the subframe.

When the body of the car drops into a dip the inertia of the differential could cause clunking in the harder diff bushings or even pull the subframe in a way that causes a clunk in the subframe to unibody interfaces.

Mazda was getting a slapping in the rear of the FD over expansion joints from just too much flex in the unibody and had to add the box section in the rocker below the door that left that short vertical seam at the last moment.

If you want OEM NHV you must use OEM components.

 

OR not BUT I think that could be the problem.

The diff is 80lbs and those bushings may as well be metal because there is no give and the sleeves are metal

That said overall it's worth using stock bushings just to get rid of the tire noise, the jolts, etc....etc....

Bottomline: those bushings are a nightmare on a street driven FD.

 

I have stock diff bushings! It's not the diff bushings

 

I'm from Sacramento as well and have this same exact issue. I have Ohlins DFV suspension, all new powerflex bushings everywhere, new pillowballs, new inner and outer tie rods, new front sway bar links, and stock wheel sizes. Been slowly tackling different areas to see if the noise would decrease but so far it hasn't. Will probably try wheel bearings next to see if that fixes anything.

 

check your rear sway bar bushings and endlinks

 

May sound silly but is the spare tightened down well?

Jpr

 

Yes, and even the NVH with good condition OEM suspension and mounts is not up to 2017 level level of refinement. So if you daily drive a BMW from the last 10 years you're not going to get that on an FD.

 

Wheel bearings are unlikely to help. If they were loose enough to cause your noises, they would have long ago caused other issues.
At the risk of stating the obvious and being redundant, but knowing a bit about NVH and handling, the list of harshness suspects are, mostly in decreasing order:
o Tires - pressure, construction, tread wear. High pressure means more harshness. Some tires have hard sidewalls for quick response - those are usually harsh. Low profile tires are harsher than ones with taller sidewalls. Worn tires transmit more noise, etc.
o Suspension bushings - softer is generally quieter. Aftermarket ones are by design stiffer for more crisp handling.
o Shocks - too-high damping levels increase impacts and road noise. Softer will ride better, but can degrade response and handling.

 

Op did you fix it ?

 

This is the closest i can replicate it while the car the still, by pushing in the trunk

still trying to get a gopro mounted under there to get a full video during speed and to replicate the slap/thuds