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The I-15 bounce!

jeeper

jeeper

I live my life 1 dumpster at a time
Location
So Jo, Ut
I’ve been doing some reading on forums. The frame stiffening seems to help a lot of guys solve the problem. However, there is also an argument that the frame is engineered to flex certain ways to relief stress on other parts of the truck, and that stiffing the frame will put stress were it shouldn’t be. Any thoughts on that?
I can tell you that I have had this truck pretty flexed out, and the frame is for sure doing a LOT of the flexing... but then I don’t know if that’s a good or bad thing.
 
glockman

glockman

I hate Jeep trucks
Location
Pleasant Grove
I think adding your flat bed surely limited the amount of flex in the frame. One of the most informative threads I read was hypothesizing that the reason a flat bed doesn't fix the hop is that the top of the frame already has braces. It's the bottom that flexes away from each other and causes the hop. That would make sense why putting a single crossmember on the bottom tying the bottom of both sides together made a big difference in some of the trucks. I don't think that single cross member would limit the overall lateral twisting of the frame, just the torsional bounce when the bottom of each frame rail moves away from the other rail.
I'm going to fab up a cross member out of 2 pieces of 1 1/4" 0.125 wall square tubing welded together. This should be as strong as a channel iron brace but weight less and I already have the material.
 
jeeper

jeeper

I live my life 1 dumpster at a time
Location
So Jo, Ut
Those were very interesting. Did I read it right that the most deflection he was able to get was 0.033 inches? That’s not a while lot.. but it sure feels like a lot when it is bouncing!
A couple braces across the bottom would be much easier than some elaborate x-brace system that a lot of guys were building.
 
glockman

glockman

I hate Jeep trucks
Location
Pleasant Grove
So I finally got around to building and installing a crossmember. I used 1"x3" tube with 1/8" wall thickness. It has a dip of 2 inches to clear my exhaust. I attached it to the frame with 5/16 grade 8 bolts, 2 per side in new holes I drilled. I placed it as far back as I could. I took the truck from PG to Provo which isn't as bad as north of the point but there is usually some bounce in a few spots.
With this crossmember installed it was 70% resolved. It would hop once and stop. Previously that hop would start a feedback loop that would shake the entire truck. It's not totally fixed but it's significantly better and for a couple hours work and $30.
20181217_213948.jpg
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Hickey

Hickey

Burn-barrel enthusiast
Supporting Member
Location
Grantsville, Utah
rholbrook said:
Hickey, how does the semi feel? I can only imagine.
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It's not as bad as the long bed F-350 I had, plus I can shift the weight around quite a bit using the 5th wheel slider and the trailer tandem slides. I get most of my vibration from the shitty recap tires they've been buying us lately.
 
Herzog

Herzog

somewhat damaged
Admin
Location
Wydaho
glockman said:
So I finally got around to building and installing a crossmember. I used 1"x3" tube with 1/8" wall thickness. It has a dip of 2 inches to clear my exhaust. I attached it to the frame with 5/16 grade 8 bolts, 2 per side in new holes I drilled. I placed it as far back as I could. I took the truck from PG to Provo which isn't as bad as north of the point but there is usually some bounce in a few spots.
With this crossmember installed it was 70% resolved. It would hop once and stop. Previously that hop would start a feedback loop that would shake the entire truck. It's not totally fixed but it's significantly better and for a couple hours work and $30.
View attachment 117073
View attachment 117075
Click to expand...

I'm kinda blown away that a simple crossmember would have that big of an effect. Not doubting at all, just impressed!
 
I Lean

I Lean

Mbryson's hairdresser
Vendor
Location
Utah
Herzog said:
I'm kinda blown away that a simple crossmember would have that big of an effect. Not doubting at all, just impressed!
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My thoughts exactly!

Did you sleeve the 1x3 tube where the bolts go through, to keep it from crushing? I'm not 100% sure what forces that Xmember will see, so I'm just curious. I also have very little motivation to work on my F350, but that's a pretty simple upgrade. :D
 
glockman

glockman

I hate Jeep trucks
Location
Pleasant Grove
I Lean said:
My thoughts exactly!

Did you sleeve the 1x3 tube where the bolts go through, to keep it from crushing? I'm not 100% sure what forces that Xmember will see, so I'm just curious. I also have very little motivation to work on my F350, but that's a pretty simple upgrade. :D
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I didn't sleeve them. Thought about it after I welded the ends on.

Its quite surprising that it works. However, I've had several crew cab short bed super duty's and they don't hop like the long beds. The only difference is 1.5' of frame in front of the rear axle. Putting this cross member in ties that portion of frame together.
What's even stranger, there is a cross member on top of the frame right where I out this one.
 
XJEEPER

XJEEPER

Well-Known Member
Location
Highland Springs
I just stumbled across this thread......after having my kidneys pounded by my brother's 09 Dodge 2500 CC with 8.5' flatbed during a round-trip drive from Layton to Draper last weekend.
His 2012 2500 Megacab shortbed with 6" of lift and 35s didn't exhibit the bounce.
 
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glockman

glockman

I hate Jeep trucks
Location
Pleasant Grove
After I missed it and I Lean mentioned it I decided to sleeve the bolts. Got it painted and back on the truck.
Took it from PG up I15 to 90th south. There is still some shake but it's significantly better than it was. I'm pretty happy with the results for the investment. More miles will tell. I am pretty sure it's placebo but the entire chassis seems better. Bumps don't seem to bounce the truck like they did. Here is a better shot of the finished product.
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DToy

DToy

Registered User
Location
Lehi
I was recently in the market for a new truck and noticed the same thing. I ended up with a 2015 F350 CC SB and it rides 100x smoother than all of the longbeds I drove. My 2004 Dodge CC LB was bad too, driving the newer Ford LB's brought those memories back.
 
jeeper

jeeper

I live my life 1 dumpster at a time
Location
So Jo, Ut
Replacing the rear shocks on my truck made a HUGE difference in my bounce. By no means is it gone, but it was very tolerable to Layton and back the other day. Going to swap the fronts now to see what it does.
 
glockman

glockman

I hate Jeep trucks
Location
Pleasant Grove
Gravy said:
Chad is it worth building a crossmember?
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I have taken 3-4 trips into the dark land (SLC) and I have gotten "some" bounce on each trip but I'll stick with my initial estimate of it being 70% reduced. It happens less often and the bounce is less severe.

If I get in the worst lane in the worst sections with just me in the truck it gets the hop as bad as it used to on just about any concrete with 4 people in the truck. For the couple hours and $50 I spent, I'd do it again.

Side benefit, these frames are twisty. I used to be able to look in the mirror and see the bed twist when pulling off a dirt road say, out at Cherry Creek. That flex is significantly less now.
 
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