Bounce slow motion video

[miro]

Like I asked before has this truck had frame work done to it?

[Rob]

This problem is very likely what could be considered normal "wear and tear" given the age and duty service of the truck/tractor.

The best place to start now that we are convinced there may not be an unbalanced condition existing is to start measuring for height, length, and square. Assuming there is no obvious frame damage such as crush, bend, roll, or sway, this is the next logical step. Simple tools are required such as a drywall square, scribe tool, tram gauge, (or ruler), decent length or chain, porto power, (or large tree) will get the job started.

Rob

[David]

What if you check the frame for square (diagonal measurements between crossmembers). Then make a hard turn in one direction. Stop at the end of the turn and check same measurements. Then do the same thing in the opposite direction.

[Rob]

What if you check the frame for square (diagonal measurements between crossmembers). Then make a hard turn in one direction. Stop at the end of the turn and check same measurements. Then do the same thing in the opposite direction.

You could do that but are now introducing "twist" into the frame members and it's difficult to isolate anything loose that way. This is due to steering axis inclination built into the steer axle and spindles themselves. Ladder style frames do twist and move by characteristic. It is best to be on a known flat surface when taking measurements, with the suspension unloaded or in a neutral phase. Rob

[leversole]

I am in Houston for a few days...will dive back into that when I get back...

Frame hs been shortened...

[miro]

If the frame has been shortened could you take some pictures of the frame from the back of cab to the fifth wheel and post them when you get back?

[leversole]

Absolutely!

[b61fred]

My b61 did the same thing, mine started at 42 mph and ended about 48 mph. I found out what it was by mistake though. The trunion stand bushings were worn out and the tandem would wiggle a little bit left to right. They should give a little in a turn. It would start an ostilation at those speeds and would shake pretty good then get less and less. I would imagine that it would come back at a higher speed but my truck was not capable of reaching that speed.

[leversole]

If the frame has been shortened could you take some pictures of the frame from the back of cab to the fifth wheel and post them when you get back?

Here are a couple of the pictures...only the inside frame was shortened...dont know if they bought new rails or what, but the outside rails are NOT spliced...also, cant figure why there are two cuts on the inner rails...

[220cummins]

Here are a couple of the pictures...only the inside frame was shortened...dont know if they bought new rails or what, but the outside rails are NOT spliced...also, cant figure why there are two cuts on the inner rails...

Every hole in the frames should be nut and bolted together with fine thread grade 8 fasteners. Especially where the splice was made. There is the flex in the frame rails as rob mentioned.

[Rob]

Every hole in the frames should be nut and bolted together with fine thread grade 8 fasteners. Especially where the splice was made. There is the flex in the frame rails as rob mentioned.

Yes, that many holes in line with one another is bad news. Close them as suggested and tighten each fastener you can get to before proceeding further with isolation.

Rob

[miro]

Also I don't see any cross members could you take a picture from the rear of the truck showing the cross members. Also are the bolts body bound bolts, this would help with strength? The old rivets worked quite well but it is hard to replace them in the field.

Edited December 18, 2011 by miro

[leversole]

Also I don't see any cross members could you take a picture from the rear of the truck showing the cross members. Also are the bolts body bound bolts, this would help with strength? The old rivets worked quite well but it is hard to replace them in the field.

The only cross members are the one at the back of the cab and the one at the rear, these plus the trunnion mount is all there is...I will start tightening and filling holes...

Thanks

[miro]

There is your problem, not knowing how much space from the back of the cab cross member to the trunnion, you should have one cross member right in front of the of the front drives, and have no more than two feet between cross members. Your frame is flexing in this area causing the wobble. Mack used to have a couple of different cross members that you can put in with out pulling the bogies. Maybe someone that works at a Mack dealer can help you source them. I remember that the were not expensive and not hard to put in. Mike

[Rob]

There is your problem, not knowing how much space from the back of the cab cross member to the trunnion, you should have one cross member right in front of the of the front drives, and have no more than two feet between cross members. Your frame is flexing in this area causing the wobble. Mack used to have a couple of different cross members that you can put in with out pulling the bogies. Maybe someone that works at a Mack dealer can help you source them. I remember that the were not expensive and not hard to put in. Mike

My thoughts exactly. I've made the crossmembers from 1/4" wall tubing and bolted webs through the rails, and the new crossmember. Get everything tight.

Rob

[220cummins]

There is your problem, not knowing how much space from the back of the cab cross member to the trunnion, you should have one cross member right in front of the of the front drives, and have no more than two feet between cross members. Your frame is flexing in this area causing the wobble. Mack used to have a couple of different cross members that you can put in with out pulling the bogies. Maybe someone that works at a Mack dealer can help you source them. I remember that the were not expensive and not hard to put in. Mike

exactly. I think mack still sells a three piece one that you don't have remove the the boogies just measure it out use existing or drill new holes.

[leversole]

I dont know if half my mind is trying to convince the other half that we have found the answer or not, but the feeling, like everyone said, was that it was coming from the rear because I felt it fully in my seat, which some poeple said was from the rears, but the video should the rears NOT moving, but it has always felt like it was coming from under the back of the cab...

My first step will be to fill in all of the empty holes and make sure everything is torqued properly...then I will sit down and figure out what to do about another crossmember...I got to get throught Christmas first though!

Thanks everyone for your input...

[miro]

Just to try and ease your mind, you have tried just about everything in the front and it has not helped, and you say you feel it in your bum, listen to your bum. Have a Merry Christmas everyone.

[leversole]

Well, today I filled all of the holes with grade 8 bolts and EVERYTHING is tight, I guarantee the frame rails are not moving...all it did was AMPLIFY the bounce, it is worse than it has ever been and now at a few mph larger range....I guess the little bit of frame looseness from the row of empty bolt holes was absorbing some of the bounce...I guess I am back at square one...Whenever the sun shines I will get the video back out and try to locate the source...

[shortstack]

Don't get frustrated! You'll get it figured out eventually. Trey

[Rob]

Well, today I filled all of the holes with grade 8 bolts and EVERYTHING is tight, I guarantee the frame rails are not moving...all it did was AMPLIFY the bounce, it is worse than it has ever been and now at a few mph larger range....I guess the little bit of frame looseness from the row of empty bolt holes was absorbing some of the bounce...I guess I am back at square one...Whenever the sun shines I will get the video back out and try to locate the source...

You are moving in the right direction now. Look at your trunion bushings, torque link bushings, spring boxes, all for looseness or non center. You are correct in that the rails were absorbing some of the oscillations started elsewhere in the truck. Spring pins are a definate culprit as are the bushings in the spring eyes.

Also something you may want to do regardess is to unload the spings by jacking the truck on the frame, and paint grease onto the extended spring leaves. If these things don't readily slide, or bind, they can cause all kinds of similar problems to what you have.

I'm thinking something is moving fore and aft causing your problem.

Rob

[David]

Try setting your camera at the back sighted down one of the frame rails. What was the distance between the crossmembers? I may have a three piece fou you. I'll look when I get back. How long are your drive shafts? Have you checked the carrier bearings?

[leversole]

Will check all of these suggestions out...will also get the inside dimensions of the frame rails...once I get the steer tires back in the front and replace the front seals I will air up the tires and should have the strongest bounce possible and the camera may reveal more clues...

Though I feel it in my seat, you can actually see the radiator and hood bouncing up and down what looks like several inches...I just dont see how an imblanced wheel (which has been spin balanced with the tire) can cause this much problem, though it does take a second or two to "build"...

Once again, thanks for all of the input...

[Rob]

Will check all of these suggestions out...will also get the inside dimensions of the frame rails...once I get the steer tires back in the front and replace the front seals I will air up the tires and should have the strongest bounce possible and the camera may reveal more clues...

Though I feel it in my seat, you can actually see the radiator and hood bouncing up and down what looks like several inches...I just dont see how an imblanced wheel (which has been spin balanced with the tire) can cause this much problem, though it does take a second or two to "build"...

Once again, thanks for all of the input...

You might want to carry a non contact heat thermometer gun and shoot each axle at the bearing area to check for abnormal heating during operation. Have you checked for proper torque on the rear drive axle bearings? I'm assuming the universal joints are not binding and have been lubricated. Also look at the carrier bearing if there is one.

Rob

[Retro Racer]

Some really great information offered here, but I'm going to throw a spanner in the works!

I'm not that familiar with the underside of Macks, mostly worked on trailers for 22 years where castor isn't so much of an issue... But looking at those spacer blocks between the steer axle/springs in the video has me wondering if you have speed induced castor hop?

Do Mack trucks usually use tapered spacers to set up castor or is it dialed in elsewhere in the suspension/bracketery?

I do remember Kenworths had tapered (weld on) spring mount seats on the drive axles to match up driveline angles and old Volvos had tapered spring seats between the front axle/spring pack to set up the desired castor, but those spacer blocks on your truck appear to be just that... Not tapered... Just an idea and I hope you solve this sooner rather than later!