Maybe it's obvious, the solution, I mean. Here's the symptom: The car runs, drives, sounds completely normal except when traveling between 38-41 mph. The whole car shudders and shakes like I have severe flat-spotting on the tires or one or more are out of round. The steering wheel does not shake like out of balance tires. Doesn't matter what gear I'm in, how fast I'm accelerating or just cruising. I hadn't driven it for a couple of weeks and it seemed to be worse after a long sit. It never goes away, but does moderate after being driven several miles. I'm thinking tires, or maybe suspension bushings? Shocks/struts? The mystery for me is why it only occurs at a certain speed only. It's like there must be some frequency harmonics only at that speed? I have Continental Extreme Contact DWS tires with fairly recent date codes (they were on the car when I bought it). Any ideas on where to start looking for causes? Anybody else experience this symptom? Thanks!

 

Flat spots. Just like wheel balance harmonics, flat spots hace a resonant frequency that will make its presence known at certain speed range.

To get rid of them you need to get tires good and warm. But its not so warm out now, making that more difficult.

Flat spots can either go away, mostly go away, or never go away. Depends on tire, storafe temps, storage time, load. Won't know which of the three until you try to get rid of them.

Even if they go away completely, they'll be back after winter storage (if winter is a thing where you are). Usually winter storage flat spots go away after a few driving cycles.

To prevent them either fit cheap storage wheels (or even wheels made of plywood), and lay real wheels on their sides, or store with car on stands (but don't store with suspension dangling! Support under control arms so suspension at normal ride height).

I don't bother with any of that, and just get rid of the flat spots with driving.

 

If it goes away at ~70mph it's likely axle cup vibration.

 

Good point. Only reason I dismissed that was he said it gets better as things warm up. But I suppose that could be true of axle cups (grease loosens up) just as much as tire flat spots

 

I store the car in my sister-in-law’s underground parking garage at a constant 55° all winter long. I inflate the tires to 45psi and it sits on them all winter long. I bring the tires back to 32psi when I get it out in the spring. Wouldn’t the flat spots get out of sync every time I went around a corner? Last month I drove the car from Minneapolis to Road America and back. Even got it on the track for 4 laps of spirited driving! Flat spots still there. What are axle cups?

 

The flat spots out of sync will still find a resonance, just like out of balance wheels, which also go out of sync to each other, manage to always vibe at same speed range.

Axles are paet of the cv joints for the left and right drive shafts, that connect diff to each hub. The inner cv consists of a tripod shaft, each leg.of the shaft has a bearing that rides inside a cup much like a wheel. As suspension moves up and down, these bearing wheels ride in and out of the cup, and each bearing spilts the load of transferring power from the diff to the wheel.

As these wheels ride in and out of the cup, they wear groves, or pits. Eventually these pits transfer vibes under acceleration which is often most predominant at certain speeds, but always under power, never at coasting.

This effect is more immediately introduced if you lower your suspension. Then immediately these bearing wheels are riding in the most pitted area.

There are several fixes:

Swap axle cups left to right (messy, bit effective. Only cost is labor and grease. You need to clean and regrease to do ot right)

Add axle spacers. Split style spacers make this an easy, and not too expensive a fix

Replace axle cups (harvest from used axles, or axle cups still available new from Honda. Not cheap though)

You can actually combine above fixes to extend axle cup life 4x. First fix, swap cups left right. When issue reoccurs, add spacers. When issue reoccurs, swap cups again, leaving spacers in place.

Lifespan 1, original
Lifespan 2, swapped cups
Lifespan 3, spacers
Lifespan 4, swapped cups again with spacers

This works because it keeps exposing these bearing wheels to fresh surfaces that haven't been pitted inside cups.

The definitive diagnosis for axle cup issue is if vibes go away when you let off the gas.

 

My car just turned over 58,000 miles. Is it likely that the axle cups have pitted in such a short period of time?

 

Not enough to cause this issue. Especially if you didn't recently lower the suspension.

 

Stock suspension throughout. Pretty sure it's all original as well. I thought I'd better recheck the tire dates. Although the tread depth is almost like new, the manufacturing date codes are as follows: both front tires are 0311, right rear is 1616, left rear is 2717. The tires are getting old, especially the front ones. I should probably just buy a new set of tires next spring when I take it out of storage. Let the current ones flat-spot over this winter.

 

This probably isn't caused by flat spotting.

Maybe one of your tires have a bulge or slipped belt?

Did someone rotate the tires recently? Did they put a rear on the front by mistake? Rear hubs are smaller....so a rear wheel up front will cause a lot of issues, and potentially some damage to the wheels/hubs.