I’ve run both the Dobinson’s 4” flexi setup and the 3.5” 144/145 VT and they are both excellent. I agree with most of what people have said above about the VT coils without a lot of weight because dual rate coils are designed so they ride in the transition zone between the dead coils and base spring rate.
In your case, I’d listen to
@crikeymike about the Flexi going 3” up front and 4” in the rear. I can’t say I noticed a ton of difference between Flexi/VT on the front end and I have no extra weight up there, but the VT coil controlling the big ass rear is better. The primary point is that given the weight distribution has changed with the engine swap and that should be the focus, not “soft” because, again, dual rate coils take care of this complaint at the design level unless you put so much weight on them that they are just riding full time on the base rate. You can look at the dead wind coils on mine and see that I’m not compressing heavily through the transition rate.
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I carry a 38” spare, tools, spare axles/high lift in my cargo area, just a rear tube bumper and welded flipped hitch tucked up in the frame rails. So not light but none of the usual heavy bumper/carrier stuff. I got 4.5” out of the 4” Flexi, 5.5” out of the 145 VT coils. The front tends to be just a bit higher on both setups since I just have a tube stinger up there.
Here’s the best comparison shots I have between the two. First set is the 3.5” VT and the second is the 4” Flexi. That’s a 5.5” vs. 4.5” lift on 38’s. If you have kept the rear very light, that 4” Flexi coil will be excellent back there and it’s longer than the VT, which is a consideration if you are planning long travel back there as in my setup I am just unseating the coil at full droop and that’s with a rear shock just under 12” of travel.
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