1986 - 8500 GVW - 460 - 4X4 - Manual Trans- Reg.Cab - Twin Beam Leaf Spring front suspension

I have had my 86 rig for 11 years and love the old boy except for one thing. Rough ride on the concrete freeways/interstates here around Seattle /Tacoma. I don't drive it a lot on the I-5 freeway but every time I do I come home and rethink my plan to keep maintaining this old truck.

The issue seems to me to be that the spring rates are just too heavy for that situation and an empty truck (it does have the weight of a large canopy on it).

At issue is the concrete is laid down in sections about 10 feet long. Where each section meets the next (seam) there is a minor vertical displacement (from all the years of wear and ground movement) that is just enough to cause the truck to drop maybe a half inch as each axle passes over that seam. This is enough to really upset the handling and cause the truck to start to pitch fore and aft. This resonates into a bucking so severe you would think the truck is going to shake itself apart. Mind you on a smooth asphalt road surface you never experience this.

Have others on the forum experienced similar issues with the 80-86 series 8500 gvw? It's a real downer.

I have reduced my tire pressure to 40 psi. I run 33X16X12.5s. I may keep reducing pressure until it absorbs more of the shock, but that would likely cause excessive tire wear and be no good when I did get a load on the truck.

I think the spring rates are so stiff that the shocks never get a chance to dampen the bumps in the pavement. It seems 60-65 mph is just right to set up a resonance that results in the severe bucking. I had this with the previous set of shocks also.

Is the solution lower spring rates? Or is this old suspension design going to behave this way regardless of spring rates?

 

My buddys '72 F-250 crew cab also does this, so don't think this is only with the 80-86 era trucks. Trucks just drive like trucks.

 

The only fix is to change wheel base!

This is an old common known problem on I5 with certain wheel bases...

IIRC The sections are 120", wheel bases close to that cause bucking.

-Enjoy
fh : )_~

 

Festus,

Are you saying that a regular cab F150 would do the same thing on I-5? My situation can ruin a trip up to Seattle from where I live in Pierce County (about 50 miles). Sunday I took a light load of furniture up to my daughter and all that shaking caused some extra damage to the wood finishes on an antique furniture set. Just an example of why I hate that so much!

Tom

 

Any vehicle that has a WB of around 120 inches will buck on I5... (I believe that's the dim, I haven't been on I5 in 15 years)

Of course the cozier the ride the less it's felt.

You will find that issue on any concrete roadway, however the section lengths dictate the wheel bases that it affects.

If your gonna run concrete roads keep your shocks up!

I5 also has seriously bad joints.

-Enjoy
fh : )_~

 

If thats the case then there essentially is no fix except to avoid I5 and other concrete highways all together. I5 stinks though Tacoma and parts between Tacoma and Seattle that are not resurfaced with asphalt.

Thanks for reading and commenting!