Author Topic: Jeep racetruck ideas  (Read 3742 times)

87Warrior

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Jeep racetruck ideas
« on: February 27, 2014, 11:59:04 PM »
As of lately, I have been doing a whole lot of reading regarding vintage (80s) Jeep racing. I have also wanted to get more involved in the Autocross scene in the area once I sold the Miata. This type of research is proving to be somewhat difficult since I have never actually never given a second thought to, gasp, lowering a Jeep.

I am after something comparable to these fine machines:



Although I am simply in the idea stage of this possibility, I am trying to better understand how the suspension characteristics of a lowered vehicle compare to a lifted vehicle. I also want to better understand how these suspension systems should be built based on stock configuration. My target truck is the 1991 MJ I bought to build as a krawler on 37's. Luckily the only work I have been able to do to it was getting the 4.0 to fire and the floors replaced.

Front end:
Maintain the stock 5 link suspension. Use a shorter coil spring (approximately 2") to set the ride height. Run a stiff shock and thicker swaybar. Replace the stock control arms with home built arms of DOM and rod ends. I imagine using rod ends vs bushings would firm up the response. The stock steering linkage would be replaced with GM one ton tie rods in a, inverted T/crossover style. I believe the ridiculously flat nature of the draglink would eliminate that "dead spot" due to TRE roll so many people have with the inverted T steering on a lifted vehicle. The trackbar/panhard mount would need to be raised by at least an inch to make clearance for the front differential (yes, 4wd!)

Rear end:
Maintain the leaf spring suspension with medium/firm shock. Replace the stock MJ shackles with shorter XJ aftermarket units, resulting in 1" drop. Remove one overload leaf from the spring pack resulting in about a 1/2" drop. Weld heavy duty spring plates to the bottom of larger axle for another 1" drop. Retrofit a rear sway bar and links.

The drivetrain:
4.0L engine attached to a manually shifted AW4 automatic. Run a 242 transfer case (2Hi- 4Hi - AWD - N - 4Lo). The Dana 30 front axle will replaced with a low pinion variety to keep the driveshaft out of the motor mounts and would be fitted with CV axle shafts to replace the 'bucking bronco' effect u-joint axle shafts have on hard surfaces. The rear axle would be a Ford Dana 61 full floater. Disc brakes all around and a 4.10 gear ratio.

Problems I can see:
- The front axle has a 5x4.5" bolt pattern while the rear has a 8x6.5" bolt pattern. I could run spacers up front (meh) or run two of the same style wheel with different bolt pattern. This would cause some issues when it came time to rotate the tires, carrying a spare, or having a flat
- Minimum of a 16" wheel to clear rear brakes
- I don't know how to pick a tire
- Parts are heavy but should be indestructible on the street
- It is a 4wd Jeep, lowered, for street use
- I don't know what I am doing

What does AR say? Have I lost my marbles?  :lol:

MadFairlane

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Re: Jeep racetruck ideas
« Reply #1 on: February 28, 2014, 03:28:17 AM »
Would it not be better to dump the transfer case and make it just RWD?
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MiniVanMan

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Re: Jeep racetruck ideas
« Reply #2 on: February 28, 2014, 07:24:20 AM »
Lose the tcase and front axle, convert it all to rwd. Look up Team Petty Cash, they have been racing XJs at 24 hours of lemons and kicking ass with it.

87Warrior

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Re: Jeep racetruck ideas
« Reply #3 on: February 28, 2014, 10:21:06 AM »
I have thought about going with a 2wd front axle. I don't know if I would gain anything as the suspension mounts are identical between the 2wd and 4wd axles.  The only clearance issue I foresee is the the actual differential housing will be very close to the trackbar/panhard frame bracket. I don't see why building a short bracket would be any problem to gain some clearance.

Since I am very unfamiliar with road racing or the like, are there any disadvantages/advantages to RWD vs AWD? I figured with the extremely light ass end of the MJ, in comparison to thr XJ, a little pull from the front would be beneficial.
« Last Edit: February 28, 2014, 10:26:12 AM by 87Warrior »

Wheels84ss

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Re: Jeep racetruck ideas
« Reply #4 on: March 02, 2014, 08:22:22 PM »
  Ok there are a few things here I have to ask before we go deep into it... I am assuming the front is a Solid front axle and not a IFS. If this is the case we are really behind the 8 ball as far as a monster handling vehicle. Second is the 91 a 4wd or an awd setup. Meaning is it always 25% per wheel or does it float around. I autocross/road race with my wrx and can tell you depending on setup the awd can be an absolute monster done right. But it is also more weight up front and with a pick up wouldb have to look at the numbers to see how far out you would be.
You skeeted all over her birth control? Thats fucking cold dude :lol: +REP

87Warrior

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Re: Jeep racetruck ideas
« Reply #5 on: March 02, 2014, 09:28:43 PM »
The MJ always had a solid front axle, in either 2wd or 4wd models. Everything about the steering, suspension and brakes are the same between the 2wd and 4wd axles.

The transfer case I have been thinking about using is a 242. Here are some Specs I stole from the Novak website.
Quote
The 242 has a driver's side front output and centerline rear output. The NP242 features two ranges; a low range gear that is a respectable 2.72-to-1 low and a direct-drive in high range. It features three drive modes; 2wd, full-time 4wd and part-time 4wd, yielding:
  • two-wheel-drive, high (direct-drive)
  • four-wheel-drive, high (48/52 differential torque-biased-drive)
  • four-wheel-drive, high (locked, part-time)
  • neutral
  • four-wheel-drive, low (locked, reduction-drive)
http://www.novak-adapt.com/knowledge/np242.htm

Chassis weight distribution numbers have been impossible to find for the Comanche. I did find several references to the weight distribution of a Cherokee. Here is one such example.

Quote
Actual vehicle total weight (full tank, no passengers): 3677lbs

Weights at individual wheels:
LF: 1037lbs
RF: 981lbs
LR: 859lbs
RR: 796lbs

Total weight at the front of the vehicle: 2016lbs, 54.9%

Total weight at the rear of the vehicle: 1660lbs, 45.2%
At this point, I can only assume what a Comanches weight distribution would be. I assume a SWB truck, like my 91, would be close to a 60/40 weight split.

Gremlin

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Re: Jeep racetruck ideas
« Reply #6 on: March 04, 2014, 06:35:28 PM »

87Warrior

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Re: Jeep racetruck ideas
« Reply #7 on: March 04, 2014, 06:50:15 PM »
I'm not sure I understand, Gremlin. I have a truck to use. Not only that, it is a High Output truck with the Chrysler multipoint fuel injection system, not that convoluted French designed Renix fuel injection. I see they are all 2wd. If I wanted to go 2wd, I would simply source a 2wd transmission and front axle locally.

Gremlin

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Re: Jeep racetruck ideas
« Reply #8 on: March 04, 2014, 07:09:31 PM »
I'm not sure I understand, Gremlin. I have a truck to use. Not only that, it is a High Output truck with the Chrysler multipoint fuel injection system, not that convoluted French designed Renix fuel injection. I see they are all 2wd. If I wanted to go 2wd, I would simply source a 2wd transmission and front axle locally.
Oh, I forgot you more Comanche trucks than I have AMCs. The Pacific Northwest is full of Comanche trucks with little or no rust. Yah, ignore my last post

MiniVanMan

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Re: Jeep racetruck ideas
« Reply #9 on: March 04, 2014, 10:18:57 PM »
Just sayin

<a href="http://www.youtube.com/watch?v=sf_5kfb0PMA" target="_blank">http://www.youtube.com/watch?v=sf_5kfb0PMA</a>

https://www.facebook.com/JeepXJR

They're winning auto cross events and have under $2k into the build. More proof that the XJ/MJ platform is easily one of the most versatile platforms ever.

FlyinRyan

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Re: Jeep racetruck ideas
« Reply #10 on: May 22, 2014, 11:18:14 PM »
Bump from the dead, but I deal ALOT with the 4.0s. I also helped a bit with the XJR Jeep, as far as supplying them data from factory calibrations and general suggestions regarding tuning. You could repin a OBD1 4.0 harness to use an OBD2 TJ PCM and have full tuning capability.