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JHF Mod Class KOH racer - Prepping for 2013

65K views 128 replies 57 participants last post by  gtxracer 
#1 ·
When I started this buggy last winter I had every intention of starting a build thread, but it didn't happen before I "finished" it. I didn't take anywhere near as many pictures as I wanted, and I was way behind schedule during the build.

After 3 seasons of racing my first Ultra 4 I had learned a lot about what I liked, what I hated, and what I wanted to change about my car. In 2008, no one really knew where the King of the Hammers was going to take rock sports. At that time builders were trying to combine rock crawlers with trophy trucks.

In the spring of 2011 I started collecting parts for a new Ultra 4 buggy. I was planning to build my version of the ideal KOH racer. I ordered quite a few parts then I had a change of heart. I decided that like so many of us, I couldn't justify a single purpose race buggy. The project kinda hit a stand still before I even started.

When the announcement was made that Ultra 4 was adding a modified class I quickly read through the info about the new classes and decided it wasn't for me. I had no interest in building around a stock frame or body. For the next 3 months I didn't pay any attention to the info about the mod/stock race.

One day I had an idea, and decided to take another look at the mod class rule book. After reading the rules I started making some phone calls. After talking to Chris Rea and Dave Cole I knew what I was going to build. The chassis' that I was already producing and selling could be retrofitted with semi-custom aftermarket frame rails to meet the class rules. I felt like I could build a car that could meet those rules while still being competitive in the main race. I started ordering parts and planning out my new build. The build would be very similar to the Willys pro-mod buggy I built in 2010.

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#2 ·
I wanted this buggy to be simple, light, and compact while maintaining the look of my last Willys buggy. I have to say that in some areas I was willing to sacrifice performance to get the look I wanted. Here are the specs I ended up with -

JHF production chassis
108" wheelbase
79.5"/77.5" track width F/R
19" belly
7"/8" up travel F/R
480hp LS3
4L60 built by Valley Trans, Reno
3.0 Atlas w/shorty 4L60 adapter
Currie F9 chromoly housings
Mark Williams aluminum differentials
6.50 Superior gears
Spool front/ARB rear
Trail Gear unit bearings
14" Trail Gear rotors
Branik axle shafts
CTM u-joints
Sway A Way 14" x 2.5" coilovers, 2" x 4" air bumps (F)
Sway A Way 14" x 2" coilovers, 14" x 2.5" triple bypasses (R)
2" 7075 Branik lower links, 7/8" rod ends
1.5" 7075 Branik upper links, 3/4" rod ends
PSC hydro assist steering system, CBR pump
Trail Gear Creeper Lock wheels
37" Maxxis Trepador tires
PRP low back seats
22 gallon RCI fuel cell
'47 CJ2A dash, cowl, and grill

After doing some research I called TDK Frames about getting some frame rails made. The frame rails I started with are based on early CJ dimensions. Since the frame rails only had to meet factory dimensions from the motor mounts to behind the seats, the rest of the frame was built to my specs. TDK can only build the rails in 2" x 4", 3/16" wall tube. It's completely overkill in my opinion since I was running 2" x 3", 1/8" wall frame rails in my production chassis.

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#4 ·
The modified class faces a handful of limitations. Other than the frame and body rules, vehicles are limited to 14" shocks and must be mounted to the axles. The other big difference between this class and the unlimited class is that you have to use some type of mechanical steering linkage. Full hydro is not allowed.

As a builder I love a challenge. I was excited about building a car to the limits of this class. I already had some ideas on how I would build a steering system to work with a 4-link front suspension. I've got a decent amount of experience designing complicated hydro assist steering systems. The steering was taken into consideration from the start of the build. It would require planning many steps in advance. Even motor placement would be effected by the plans for the steering system.

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Everything was mocked up, cycled, and a few changes were made before the final version of the bell cranks were made. The box was mounted at the base of the steering column.
 
#5 ·
After talking to Mike Shaffer about Jeepspeed suspensions I came up with a way to get the most travel out of my 14" front shocks without mounting them at ineffective angles. I was able to achieve 16.5" of front wheel travel while maintaining ideal shock geometry.

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There are a variety of factors and compromises that were made to allow the most suspension travel and the least amount of bump steer. The steering has 1.7 degrees of bump steer through 16.5" of wheel travel. That amount of bump steer is not at all detectable from the driver's perspective.

Some thought also went into the rear shock mounting. Maximizing travel with 2 shocks is not as easy as it is with one. To get the most out of the rear the geometry isn't perfect, but I was still able to achieve 16" of travel without getting into a noticeably digressive rate. There's only so much you can do to get wheel travel in a limited class.

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#67 ·
After talking to Mike Shaffer about Jeepspeed suspensions I came up with a way to get the most travel out of my 14" front shocks without mounting them at ineffective angles. I was able to achieve 16.5" of front wheel travel while maintaining ideal shock geometry.

View attachment 714288

There are a variety of factors and compromises that were made to allow the most suspension travel and the least amount of bump steer. The steering has 1.7 degrees of bump steer through 16.5" of wheel travel. That amount of bump steer is not at all detectable from the driver's perspective.

Some thought also went into the rear shock mounting. Maximizing travel with 2 shocks is not as easy as it is with one. To get the most out of the rear the geometry isn't perfect, but I was still able to achieve 16" of travel without getting into a noticeably digressive rate. There's only so much you can do to get wheel travel in a limited class.

View attachment 714289
What is the most you can rake them forward before you end up with to much of a digressive rate from your experience? Not trying to pry. I know I am walking a fine line with my questions and I understand if you wont give up your trade secrets. Great build by the way!!!
 
#9 ·
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This is what the bell crank at the frame originally looked like. It pivots between my feet, and space is limited. For clearance reasons I decided to build the mount in single shear. A 3/4" bolt goes through a 3" wide Delrin bushing on the frame with a 1" x .120" wall sleeve. Perfecting this pivot took some work and there were a few revisions that I'll talk about later.

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A 1.75" x 8" hydro assist cylinder was used. The above photo is how the axle bell crank remained for the majority of 2012. That part was completely replaced and redesigned in October.
 
#11 ·
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This photo shows the start of the steering box mount. A vertical tube was added between the box mount and the frame rail. An old school style steering box support was built to hold the neck of the box. You'll see that in updated photos. You can also see the frame side bell crank pivot. The upper link mounts you can see in this picture were revised later. The way the steering pushes and pulls on the axle creates more load than I expected, and lead to some relatively minor issues during KOH last year. A few things were changed later to prevent those issues.

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A UHMW slider was added to the frame rail to support the bell crank. The clearance between the upper link bolt and the bolt for the drag link is about 5/16" when they cross paths. The slider was added to prevent the bell crank from bending in if it took a hit in the rocks.

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One of the neat features of this car is that the coolant runs through the box tube frame rails. The frame rails hold a total of 5 gallons of coolant. -16 fittings were welded near the ends of the frame, and drains were added to the lowest part of the rails. The ends of the rails were double capped to prevent any possible failure. A well shrouded 31" x 19" Ron Davis radiator was mounted low, behind the seats. Dual internal heat exchangers were used to cool the transmission. Later a second trans cooler was added because the heat exchangers weren't up to the task of keeping the 4L60 cool in race conditions.
 
#13 · (Edited)
I have to say that this rig is SWEET!!! I wanted to see how I would fit in one of these because my first 4x4 was a 48 2a. I was in Reno and hit Jesse up to see if I could swing by and see how I would fit and also check out the rig.

Now the reason I was a lil nervous was, I'm 6'11 and Flattys are not known for room! lol Jesse was cool and let me swing by. I'm sure I annoyed the hell out of him by crawling all of the jeep and asking questions while there most of the day. He laughed when I got in a roller that was bout to leave when my head was above the cage but Jesse and a great eye for design said it was no problem to change a few bars here and there to keep the look in proportion and safe!

This rig is flat out simple, to the point and very well thought out. Thanks again Jesse and see you in a few weeks I owe you a few cold ones for letting me swing by.

NOW MOOOOORE PICS!!!! lol
 
#16 · (Edited)
Thanks!

Mounting the shocks where they are helped me take advantage of the axle's rotation through it's travel. The pinion tips up under compression and down when it droops. Mounting behind the axle gives you the opposite effect which increases travel. I drew it all out in CAD ahead of time to see how to make the most of it.
 
#17 ·
The car was barely finished in time to drive it 100 yards and load it up to leave for the 2012 King of the Hammers. Right from the start the engine barely ran, wouldn't rev past 3000 rpm, and would go into limp mode if you attempted to go full throttle. We did everything we could think of to fix the issue, but we didn't get anywhere and missed the EMC race because of it.

Randy Slawson helped me out and got me hooked up with CBM Motors in Ontario, CA. We dyno tuned the car on Monday of KOH week, and everything seemed good with the motor. The next day the limp mode issue came. We battled that issue all week, and Kevin from Pacific Fab went out of his way to fix the problem the night before the big race.

We started the race with probably 15 miles on the car, and had a variety of minor issues throughout the day. Pretty early on in the day I realized I was racing to finish so I drove accordingly. We finished the race about 45 minutes before the cut off, but I was happy that we were able to cross the finish line.

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#18 ·
After KOH there were a few problems with the car that needed to be addressed. We noticed early in the race that the front of the car wasn't centered. We were kinda puzzled when we looked at it in the pits, but eventually we decided the driver side upper link had adjusted itself out. We readjusted the link, but by the end of the race it had started to spin out again. Not only that, the driver side upper link mount at the frame was coming loose and tearing apart. After the race I realized that the stress from the drag link pushing and pulling on the axle was the cause of both issues.

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The frame side upper link mount was modified with a stronger double shear mount. The 3/4" x 5/8" rod end was swapped with a 7/8" x 3/4" joint with a sleeve that allowed me to retain the 5/8" mounting bolt. A washer was welded to the frame to give the rod end a hardened surface to clamp to.

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To keep the upper link bolt from rotating and make it easier to tighten a plate was welded to the frame that fit tightly around the hex head.

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I still had some problems with the upper link wanting to rotate and extend itself. To solve that problem new steel links were made with pinch clamps on the bungs.
 
#19 ·
One of the problems with the steering system was the single shear pivot for the bell crank at the frame. There wasn't a strength issue, but no matter what I did I couldn't get the nut to stay tight. Part of the problem was caused by the .120" wall sleeve mushrooming under the torque of the 3/4" bolt. I eventually increased the thickness off the sleeve about .030". I did the same at the axle bell crank.

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The final solution was a narrow double shear plate that houses a 3/4" uni-ball. The plate has to be removable to get the bell crank out. Keeping the bolt tight is no longer a problem. You can also see the clamp on steering box brace in the last picture.

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At the We-Rock finals in Tennessee I somehow bent the bell crank on the axle. The tire took a hard hit and twisted the last couple inches of the arm. After looking at the damage it was easy to see how I could improve the design so a new arm was built.

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#24 ·
At KOH I wasn't too stoked about the performance of the heat exchanger I was using for the trans. I saw temps as high as 250 during the race. Considering the moderate pace I was running and the outside temperature that day I knew I would have to do something different.

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I decided the best thing to do was to add another trans cooler, but I didn't want to lose rearward visibility. I got a small Griffin cooler from Trent Fab and mounted it on top of the radiator shroud. I leaned the cooler forward to keep it low, but it sees plenty of air flow with the scoop I made to force air through it. I also incorporated the fluid containers into the trans cooler mount. The cooler and fluid containers can be removed and bypassed in about 15 minutes if I feel like taking them off for trail riding or crawling.

I'll update this later with more pics of the cooler.
 
#25 ·
Seems like I've found a lot of problems in the past 10 months! Another improvement I made was to the fuel cell. I was out doing some testing one day and made some bad decisions that lead to a 40 mph rollover. There was only minor damage to the buggy, but the fuel cell didn't handle the roll well. The top of the cell's metal can was bulged out pretty bad. I replaced the original .040" steel top with .090" aluminum. Not only does it look nice, it will be way stronger and should distribute the force better if that happens again.

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You can also see the custom bulkhead and filler neck I made to replace the stock bulkhead and cap.
 
#36 ·
Have you felt any noticeable increased slop/play in the steering due to the couple additional heims between steering box and knuckle?
I haven't noticed any slop even though I've let a few of the rod ends get a little loose. It's still a way tighter feel than full hydro.

You visiting michigan for the holidays?
Already came and left. I was there for 3 days. Went to a Lions game, saw my parents and left. I wish I had a little bit of time to see friends, but not this year.

I think this could easily work better than full hydro. Definitely have more feel.

I remember having trouble twisting sector shafts when I used a box combined with big tires. Make sure to use a check valve at the pump to help the hydro assist combat shock loads to the steering box and pump.
Does it have a beefy sector shaft like 300m?
The steering is hands down better than full hydro. I'd say I feel comfortable enough to drive 10-15 mph faster because the control is so much better. The steering is just way more predictable and easier to correct.

I didn't think that was something to worry about with hydro assist. I don't know the details on the sector shaft, but I'm gonna look into it now. I don't want that ending my day.

Is this how you tricked the front shock into more travel also? Great tech would have never thought of that. "CHEATIN SHIT":smokin:
I wasn't gonna go into detail, but I guess I can explain it. A Jeepspeed with a long arm kit is no different than running a radius arm. Because of the way the axle cycles, mounting the shocks farther behind the axle is just like mounting them on a trailing arm. This idea only works when you have a radius arm or a 4-link that creates a radius arm effect. I made some compromises with front suspension geometry to gain wheel travel. I get some wheel hop in loose sand, and the lack of separation is hard on parts. The other benefit to the radius arm geometry was it helped out with a few things for the steering.

And if your shock mounts move up that little bit, then they're not fully extended and you just gained a little bit more travel. Its axle travel not shock travel you need to be thinking of.

Bitchin car! Love the ideas put into this! Weren't you on the Toy4x4 mailing list way back when with your flatfender?

Kevo
I think so! I barely remember that because it was so long ago.

So when you gain that little bit of "shock" travel doesn't that mean your axle can move a farther distance then with out it? In return giving you more "axle" travel?
You guys are starting to confuse me, but I think you understand what's going on. :D
 
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