Car Shakes at 50 mph, but not always

[expat]

Hmmmm.

I have to say, just out of interest, when I had the tires installed, I went over the border into the U.S. Olympic peninsular WA I camped for the weekend (in order to get my duty free allowance) and had my Tires installed at Discount Tire in Kent WA.

After the instal traffic was building up as it does on I5 in the late afternoon, in fact I did not get up to any reasonable speed until I turned off I5 to back to the Olympic peninsular.
Where was I when I fist felt the shake?

On the bridge deck of Tacoma Narrows Bridge!!
https://archive.org/details/SF121

Now, having said all that, I'm not convinced that the shake is wind related. I can only say it doesn't 'feel' right.

For the record: the car is not heavy, under 1400lbs, I rebuilt it myself, the springs are Stock as used in the UK (a little harder than stock North America)

 

[Trav]

Could the guy have damaged something lifting the car?

 

[Gokhan]

Are there are any covers missing or broken underneath the car or in the wheel wells? As I said, that could create wind turbulence.

 

[CapriRacer]

Originally Posted By: Gokhan
It's not anything in the drivetrain because then changing the tire pressure would have some effect on it.....


I completely disagree with that statement.

The tire acts like a spring-mass-damper system attached to another spring-mass-damper system. Changing the pressure should have a MAJOR affect on the way the suspension reacts.

OTOH, tire pressure should have had minimal affect of the drivetrain and related resonances. I see that expat disconnected a brace to the engine because it caused more vibration. I think that is another clue!

Originally Posted By: Gokhan
........It's not a loose or broken part in the suspension because then you would see it at all speeds and it would greatly depend on the pavement conditions......


I disagree with that statement as well.

Spring-mass-damper systems have a resonant frequency and that's why wheel end vibrations tend to show up in the 50 mph to 70 mph range - the resonant frequency range for these sorts of things.

The fact that it shows up as early as 40 mph, tends to say it isn't the something in the suspension, but it could be a oscillation not caused in the suspension itself.

For example, vehicles with too much caster tend to develop an oscillation (shimmy) that will appear at speeds other than the 50 to 70 mph range. In Jeeps, it's called a "death rattle".

So I tend to think there might be a worn ball joint that is shifting position. In one position the caster is OK, and in the other it is too much.

 

[expat]

I have not looked at the car yet (too early here on the West Coast)

The front suspension consists of upper and lower 'A' Arms attached to a turret that is bolted to the verhicle frame.

At the upper 'A' Arm the verticle link and spindle attaches via a conventional Ball joint.
The lower 'A' Arm locates via a Bronze Trunnion that is internally threaded to accept the bottom end of the verhicle link.
All components were new 22k ago.
Coil over Shocks and Springs attach between lower Trunion and the Turret.

The engine mounts also attach to the suspension turret immediately behind where the upper 'A' Arm attaches. So a suspension oscilation could transfer directly to the engine mounts.

Caster is set by the designe of the welded turret, slight adjustment is made by shims where the turret mounts to the frame

http://www.triumph-spitfire.nl/images/chassisimages/DSC05646s.JPG

When I say the oscillation starts at 40, that might be if I was REALLY looking for it.
I would say it is not normally noticeable unti 50 mph.

 

[Gokhan]

Ball joints should be easy to check -- there is either zero freeplay or not.

P175/70R13 tires have about 914 rev per mile (could be higher or lower depending on tire). So, at 55 miles per hour, this is 50270 rev per 3600 seconds or 14 oscillations per second. I think you said something like 4 oscillations per second. So, it sounds like your resonance frequency is quite lower than the frequency of the tires. That's why I think it's not drivetrain related.

If you have a very windy day, do the head wind vs. tail wind test I mentioned.

 

[expat]

I'm not ruling out the wind thing, but have been going over the front end just to see what I can see.

By and large, every thin looks very good and tight:

Upper Ball joints, lower trunnions, suspension Bushings, Tie rods ends (inner and outer) Sway bar mounts and end links, steering rack mounts, engine mounts, Suspension turret mounts (torqued to spec) Brakes not binding and pads wearing evenly.

The only thing I found is worn inner wheel bearings.
Not BAD and I could only feel 'play' on one wheel but they were singing (I think I would have heard this if it were not an open car)

I have removed the hubs and removed the bearings (I need the #'s to buy the replacements. I learned long ago not to ask for Triumph parts by application at the parts counter

On close inspection they are showing 'Copper' on the bearing surfaces, they are shot!

I can't believe this is the sole cause of my problems, but it's a start.
The more time I spend on the front end the more chance I'll see something.

Probably won't get the new bearing until Monday, but that's OK.

 

[expat]

It's been awhile, I ended up having to mail order the Bearings from Oregon (I decided to get new inner's and Outer's and seals on both sides) Then, I busy with work.

I finally got the car back together again today, everything checked, every steering and Suspension bolt torqued to spec, Tires rotated Diagonally (Yet again) And Guess what....The Shake has gone

I have made several trips on the 'Test Road' the last included a 105 mph blast, and it's a smooth as silk.

But I honestly can't say that any one thing was the obvious culprit for the shake.

Or why it should suddenly occur after installing the new tires.
I guess this might be forever a mystery.

Thank you everyone for your help and input

 

[Gokhan]

Perhaps they damaged a wheel stud when they installed tires and they replaced it and they damaged the bearing in the process? Just a wild guess.

 

[expat]

No, I was watching the whole time during the Tire change.

 

[Gokhan]

Originally Posted By: expat
No, I was watching the whole time during the Tire change.

Why would I even think otherwise. That's what I do too if I take my car to a shop.

 

[CapriRacer]

It just might be that the bearing had a set position and so long as load was on it, it stayed in that position. But when the tires were installed, not only did the vehicle go up on a lift, but the tension on the hub changed and allowed the bearing to shift slightly - and what you were feeling was the bearing shifting ever so slightly, where before, it was wedged in position.