Technical Model T Roadster front end shake (spring reverb?)

Before I start, let me say I have spent a couple nights on the search looking for what I have going on with this car and I haven't found my answer yet. I have a 1920 T Roadster with a 47 axle and bones, MG lever action shocks, panhard bar, 40 spindles and brakes, spring of an unknown nature but it is 2" wide and I have it mounted over the axle. The car has a shake from 62-68 mph but peaks at 65 and gets better if you go slower or faster. At speeds of 70-100 mph it's smooth as glass. I have had no less than 7 different wheel and tire combos and 4 shock combos and it is always the same with the speed of the shake going up or down slightly, like with one combo it was the worst at 62, another at 60, etc. Thee shake is up and down and alternating, like left up, right down, then right up, left down as if it is just pivoting on the spring perch. I have tried more spring, less spring, stiffer shocks, softer shocks all with the same result. So before I remove the front end and chuck it in the dumpster I thought I would see if any of you fine fellows have any answers for me. The current wheel/tire combo is wires and 600-15 bias ply tires that have been road force balanced and have minimal runout >.125" one wheel balanced with .5 ounce and the other with .75 ounce weights. I have also had 4, 6, and 7 degrees of caster just for fun. Someone was telling me about spring reverberation but couldn't remember much about it other than if the spring begins to vibrate the shocks cant overcome it. I forgit to mention if I am on a butter smooth road it wont do it, but if while on said butter smooth road I run over a big crack, stick or cigarette butt it will start to shake. Help!

 

First off your shocks are to close to the center of the axle to do much good. The axle end needs to be at least out at the spring perches. How much air pressure in the tires? That big a tire with your weight try 18 PSI to start. Try in the range of 15 to 25 to see what works the best. and are the shocks new? If not any free travel before damping starts with your inboard axle connection spells trouble.

 

Hi John, it sounds like a harmonic to me. I take it this has been going on before you bent the kingpin? Have you checked all your ball joints and whats the toe in like ? JW

 

I was wondering about the shocks being too close to the center, I have had 3 different sets of tube shocks on the car hooked from the wishbones to a F1 frame bracket with the same result. The front tires seem to like around 24# if it's too much higher it shakes worse and much lower it feels mushy. The shocks are not new, I'm not sure if they have been rebuilt or not but they are pretty tight. The car has been sitting for a few days so I will go unhook one and see if there is any free travel and report back. Hey JW! The shake has been there ever since I began driving it (5 years or so) and I forgot to mention all the steering rods, tie rods and king pins are new also. Toe in is .125", but I have tried toe in set at .1875" and .250" and it's the same.

 

ball joints? really?

 

Tie rod and drag link ends are ball joints. Do you feel it as a shimmy or something else, through the wheel or the whole body? Also have you checked your hubs are running true? JW

 

I've dealt with this on my RPU. It will scare the shit outta ya.

I swear they would bounce six inches off the road.

I think it is the combination of many factors, light car, I-beam axle, poor shocks, shock placement, soft spring, tires, tire pressure, etc.

When I was running dirt trackers up front, it would happen quite a bit if the air pressure was above 20 psi.

I have since changed to firestone deluxe champions, and have not had it happen since.

Rich

 

It's just the front axle pivoting on the spring perch, the windshield vibrates and the steering wheel shakes. I've checked run out on the hubs and even balanced the drums on our crankshaft balancer. I have yet to find the leaf I removed, I was thinking I might put that spring back in then wrap the spring pack just for shits.

 

If all the various frontend joints are nice and new and tight etc you should be able to eliminate that being a probable cause. Wheel alignment would show up as tire wear issues before that would be a cause. Alignment (toe) would also cause handling changes such as understeer etc. Also - I drove my hot rod around the block (road test) with NO front shocks in place and no shake or wobble - just a lot of "after-bounciness" (my own tech-term there) so shocks are not the culprit I'd say.
This is a vibration/wobble deal so it would most likely be from something moving - ie spinning etc. I would be looking at the wheels - front AND rear. Obviously balance but also run-out. Bent rims or tires not beaded properly can be a major cause. Have you tried other wheels on the front? Check wheel bearings are not on the loose side too as anything loose that can amplify even a small amount shake from imbalance etc. Also check your caster angle as if a bump starts the wobble off then that is a likely cause. Around 7deg is midrange.
Not all I-beam axled hot rods wobble but yours does - so something has to be different to those others that don't wobble.

 

Does it keep happening if you turn into a corner and maintain that same speed? also have you checked the kingpin inclination and camber for scrub point? Just shooting ideas out there. JW

 

I've done the dance on the wheels and tires. In my original description of the issue I went into more detail but I'm sure it's not the wheels, tires or bearings. I just re packed the bearings 2 weeks ago, and all 7 or so sets of wheels and tires did the same thing. The tie rods and ends are new as are the drag link and ends. Caster is currently 4 degrees, it was 7 and did the same thing.

 

Ok John lets try the spring as every thing else seems ok. If you were to get some oil between the leaves and see if the character of the problem changes. JW

 

I had te same problem and it was too much toe in combined with to much tire pressure. Went to .125 toe in and 28 lbs tire pressure.

 

I'm running .125" toe in and 24# in the tires, but I have tried the toe in several different spots and lots of different air pressure settings. I can't find the leaf I removed from the spring pack, if I find it I'm going to put it back in and see if it changes anything. It had the shimmy before I removed the spring, but I have changed many other things since then.

 

How about steering box? How is it for being tight

 

These issues can be xtremely frustrating. This may be a bit of a flyer but, have you considered the possibility that this might be an engine or driveline balance/vibration issue? At the speed the vibration occurs, what happens if you slip it into neutral to unload the driveline or change gears to change engine RPM? Just thinking out loud and trying to help.

 

Johnny wishbone, measure the wheelbase on each side of the car. A difference can cause the front axle to 'hunt' and shimmy


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Words have meanings. Without meaning, words are useless. Tie rod ends, ball joints and draglink ends are 3 distinctly different things. Is it ok with you if I call a table, a chair?

 

Shotrod, the steering box is a reversed Corvair, I jsut went through it and it is tight and smooth. Anothercarguy, I have overdrive, and it does it in 3rd, 4th and neutral. I even went as far as to shut the engine off going down a big hill to eliminate it from the equation and to see if I could hear anything funny. It still shook, but I didn't hear anything that gave me any clues. Manyolcars, that is actually something I haven't checked yet! I have checked the runout on the rear axle to make sure the rear axle wasn't bent, but I didn't think to measure wheelbase on both sides. Stay tuned......

 

Hey John, found anything ??? JW

 

Toe in. Your shocks are mounted fine. Lots of T Buckets use the same shocks in the same placement. I think you'll find it's toe in. Are you measuring it correctly? Tell us how you're doing it.

I've had what you described and it's always when I'm adjusting my toe in.

 

Hi, I had a bad shake that tore the support rods off the radiator before my buddy put a wrench on the front motor mounts and tightened them about 5 turns. I will be using cotter pins from now on, Duh! Smoothed it right out. check your engine mounts. happy motoring!

 

If it is "death wobble" I recommend a V.W. dampener. I have one on my roadster. If it is bounce and If you have bias ply tires find someone to shave them.

 

When Measuring toe in, I put the car on jack stands and use my other 2 short jack stands to set behind each front tire so that the top of the stand is right about the center of the tire. I make paint marks on the tires and rotate them to the rear and line them up with my stands so the height is equal then measure. Then rotate my paint marks to the front, move short stands to the front and repeat the process. I can get my measurements to repeat so I believe it to be accurate. I keep going back to the amount of flex in my front axle, thinking it flexes way more than it should. I can get my left front tire 12.5" off the floor and the other three tires are still firmly planted. It may go farther, but I ran out of jack...

 

A Duece, My engine mounts are tight, I had just changed out the rubber biscuits right before I made this post. Thanks for taking a guess though, you never know....

Steel Rebel, It is a bounce and I do have bias ply tires (new ones). I was going to see about having the tires shaved, but my runout is only around .125" - .140" which doesn't seem like much to me, but I guess it cant hurt to make it better. I need to get back on it but I'm mad at it right now and I keep having to go to funerals (4 in a week all unrelated) and have not had the time.

 

Okay, this is what I do. I raise the front end enough that the tires are off the ground. An inch or two is all I need. Make sure tires are pointed straight ahead. Then, I mark the middle of each tire on the front side and the rear side. Measure the distance between the front side and the rear side. The difference is toe out or toe in. Adjust your tie rod accordingly. My car likes 1/8" toe in. I've done it on my car and another car that has a solid front axle. It works. All you need is a grease pencil, a tape measure, calculator or pen and paper and a floor jack.

But you have to make sure everything on the frontend is tight and as it should be.

 

I can't believe it but the wheelbase is exactly the same on both sides. Maybe off 1/32" or so, but I would have never guessed it to be that close. Dammit! I thought we were on to something.

 

That is more or less what I do, but I use the same mark on the tire for front and rear and just rotate the wheel to move the paint mark just in case the tire tread isn't perfect I'm using the same point on both tires and it takes tire and wheel imperfections out of the equation. I have it set at 1/8" toe in right now, but I have tried it all over the place to see if it changes anything and 1/8" in is where it drives best.

 

Johnny

Sorry for your losses. When you get yourself back together do yourself a favor and find someone in your area that still shaves tires and have it done. Shouldn't cost you more than $30 or $40 each and you will get that out of the way.

Gary

 

OK, please check the bushings in the spring eyes and the perch eyes, something looks odd about them to me. Also always....did I say ALWAYS, roll the car forward and THEN measure your toe-in. If it needs adjusting, readjust, roll the car backwards a few feet, then roll FORWARD again a few feet, remeasure. I have never heard of checking toe on a car while up on a jack, thats a new one on me.

 

The bushings in the spring are all fairly new, they look odd I guess because it's a goofy setup using the 2" wide spring and the stock shackles that are about twice as long as what is usually on this style front end. I have also set toe in by making paint marks and rolling the car back and forth, and I always put the car on the ground and double check after I tighten everything just to be sure. If I didn't mention before, this is not a new problem, I've been chasing it for 5 years now and I have had at least 7 different sets of wheels and tires and 4 different shock setups. I wonder if the length of the shackles has anything to do with it?