Truck shakes highway speed

[Jaxson Stene]

Hey I have a 1984 gmc 1500 with a 5.3 swapped into it. It has started to have a shake when going around 98 to 109 kph. It’s also not consistent just here and there, have had friends drive beside me and my tires aren’t bouncing only can see the bed shake while the cab isn’t. Not like an old truck shake feels more like a bad shock kind of shake. I have freshly balanced tires, also shook with my old pair. Shocks are good new u joints new wheel bearing all around, also has new leaf springs “old ones had cracks”. New transmission mount. Just looking for some insight anything helps thanks!!

 

[YakkoWarner]

A bent or slightly out of true wheel can have that effect (as learned from old british cars with wire wheels that are never perfectly straight). At different speeds you'll get different shakes - when I had an early 90's compact wagon I could immediately tell one of the wheels was not right after getting a full set of tires - turned out they used the spare on one of the rear corners and when I took it back to them they wouldn't belive me it was out of true until they out it back on the machine. Marked that rim as BENT and only used it for the spare.

This may not be whats happening in your case but since you just went through everything else its an unusual but not impossible place to check.

 

[1976dully]

Do you feel it in the steering wheel when it happens? Does that have a carrier bearing?

 

[TotalyHucked]

If it's inconsistent, it's likely wheel and tire related. I fought it and fought it and fought it on mine. 5.3/4L60 swapped originally, now 5.3/T56. Played with shimming the trans up and down, played with angle shims for the rear axle. It comes and goes, same speed, sometimes it'll do it and sometimes it won't. What I've narrowed it down to is "phasing" of the rear tires. I figure both wheels are just ever so slightly not balanced and sometimes they're opposite of each other and it smooths out. Then other times they're aligned and the shake gets worse.

They're just cheapo cast wheels. I've had 2 sets of tires on them. My first set, I had rebalanced 3 times at 2 different shops. The 3rd time (second at this same shop) I asked them to dismount and spin the tires 90* on the wheels and rebalance. They got MUCH better after that, very rarely would I have a vibration. Then when those tires were worn out, I got new ones put on by that same shop...vibration is back. Haven't driven it enough to warrant taking it back again.

 

[chevdude]

Wouldn't the shop guy notice a wheel runout when doing the balancing? How about driveshaft imbalance?

 

[Ricko1966]

Did you check your drive line angles and U joint phasing? Did this truck drive fine before the swap. I'm assuming this is 2wd. Is it 1 piece drive shaft or 2 piece driveshaft?

 

[edgephoto]

Did the shop measure tire runout(Road Force)?

Just because they are balanced does not mean your tires won't cause a vibration. I see this a ton in my day job.