1987 Jeep J20 wheelbase change/weight distribution - Pirate4x4.Com : 4x4 and Off-Road Forum
 
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Old 01-12-2020, 09:14 PM   #1 (permalink)
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1987 Jeep J20 wheelbase change/weight distribution

I have a '87 J20 long bed (only way the J20 came). 131" wheelbase. It currently has some mild suspension work done, a modified L29 454 big block/4L80E and borg warner 4470. I'm probably going to build a 2wd front axle for it out of 3" DOM .500 wall and toss in a 2wd 4L80E. Will eventually sit on 35's, I've got 33's on it at the moment.

I want to possibly cut the frame and shorten it. A short bed J truck has a 119" wheelbase and I would source a shortbed to toss on it. This is fairly easy to do on these trucks.

My truck weighs 5,200 lbs, 55% front/45% rear

How would moving my rear axle forward 12" affect weight distribution? I know it will move more weight over the rear tires. Is there a way to calculate? Someone on here previously gave me a way to calculate moving weight out past the rear axle and it was spot on. I'm thinking there may be a way to figure this out here. If I can't source a shortbed within reason I can flat bed it too.

I hope to run the thing (with a 200 shot of nitrous) during hot rod drag week and the big sky 200 offroad race in the same year/season. So I'm trying to improve total weight (to less than 5K) and improve weight distribution.
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Old 01-17-2020, 03:11 PM   #2 (permalink)
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What class are you looking to run? If it's the same Big Sky 200 we run there are restrictions on changing your wheelbase. If you made it match the short bed truck with a 119" wheelbase you would be good.

Either way durability is way more important then weight distribution.
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Old 01-18-2020, 02:14 PM   #3 (permalink)
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Yea I recently read through the stock mod rules. I'd like to most likely run stock mod. My mild all iron 454 seems like it would still allow me to stay in stock mod the way I read it.

And yea I was thinking even though the J20 could not come as a short bed, 119" was still an OEM wheelbase and would be accepted. Rules say +/- 4" from OEM wheelbase. I might even push the front axle forward another inch and move the rear axle further forward for a 115" wheelbase. Still fitting in the +/- 4", losing weight and improving weight distribution.

The rest of it, like durability sort of goes without saying.
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Old 01-18-2020, 11:22 PM   #4 (permalink)
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Yeah as long as it looks like a short box J truck you'll be fine. Inch on the front won't be any issue.

What engine mods to you have?

We are currently reworking the rules and hope to have them re-posted soon. Mostly clarifying the sections but we have some possible tweaks on the engine and suspension side.
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Old 01-19-2020, 11:43 PM   #5 (permalink)
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Engine is an all iron L29 "Vortec" 454. Only performance mods are a custom camshaft with the associated springs, and roller rockers because I needed adjustable valvetrain for the new cam (which isn't even a .500" lift cam), and headers. Which appears to fit within the class rules right now?

Will 295/75R16 tires meet the 33" tall requirement for stock mod, even though they're like 33.4"? Rules right now say as measured, does tech actually measure the tire?

Last edited by The_Punisher; 01-19-2020 at 11:48 PM.
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Old 01-20-2020, 12:22 PM   #6 (permalink)
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Should be alright, although typical roller rockers are used in higher performance stuff. Cams and headers are allowed but should be mild and not require addition internal mods.

On tires - yeah those should be fine, some folks are running a sidewall marked 35 but the tires run small and measure 33".
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