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NashGTI
NashGTI GRM+ Memberand Reader
3/30/24 4:10 p.m.

After an internal debate about it I decided I'll be OK to post about this build. It's not my personal vehicle, but is being done for a friend of the shop I work for.  I present the "Trackhoe" ( his name not mine I prefer Race Brick) a 96 I think Chevrolet Tahoe autocross/track day build.

 

Trying to pick a logical starting picture, and ended up with this instead.  The engine for this is a 6.2 liter LS aluminum truck block and heads sent out and massaged with valve train trickery, forged rods and pistons, and some port work. Breaking it in with some half assed pulls on a Dyno without the supercharger it did 542hp and 497 foot pounds. We are shooting for 8000 RPM and somewhere acround 900 horsepower.

 

That's backed by a 7 speed BMW dual clutch transmission modified to handle the extra power and run on a stand alone computer. The engine and trans have been moved back in the chassis considerably, nearly a foot and everything has been squeezed vertically. The ride height will be 4 inches to the rockers, the custom tube frame bottoms a quarter inch above that, and the bottom of the transmission pan is a quarter inch above that.

 

 

A quick change independent center section is at the back. This one is just for mocking up and engineering there will be a shiny new one from Winters Performance in the final product. Pushrod actuated rear coil overs will be mounted inboard on either side of the quick change gear set and visible through the diffuser. The fuel tanks are being mounted flat in the center of the vehicle on either side of the drive shaft and exhaust tunnel.

 

 

The exhaust itself ended up being a trick, with the engine so far down and back finding room for the 180 degree cross over collectors meant bringing it forward and then packaging the pipes down the tunnel. All stainless as we weren't allowed the money for titanium here. We were allowed it for the intake side plumbing so from the pair of air boxes to the blower, the blower to the intercooler, and intercooler to the throttle body will be titanium.

 

More of the engine bay with exhaust and steering visible, the steering rack body is centered 2 inches in front of the front axle centerline. The leading edge of the balancer pulley is a half inch behind it, and the engine and trans are offset 3/4 inch to the right hand side.

The front suspension will be pull rod actuated with the coil overs mounted inline with the frame tubes pointed toward the front. Everything has been modeled with 5 inches of suspension travel, 37 degrees of steering (average), 2.5 inch scrub radii and camber gains of .8 degrees per inch front and .65 per inch rear.

 

 

Full size racing buckets from Racetech in both front spots, and it's getting vintage racing half buckets for the rear seats. The owner insists on being able to take his family to dinner in this.

 

Another requirement, 335/30-18 rubber at all four corners. Behind the wheels will be a full set of C6 Corvette carbon ceramic brakes, uprights will be built around C6 style Xtracker bearings. Haltech ECU to control the bulk of everything and incorporate traction control and intercooler sprayers and the 4 fuel pumps. The stand alone for the trans is from HTG and should interface directly with the Haltech.

 

 

It's going to be low, and the track widths have been stretched out to 78 inches front and rear. The filter boxes will feed through ducts at the back of the hood on either side and the radiator fans ducted out through the center of the hood. The thing should only be about 59 inches tall in the end despite being a full size SUV body. The hope is to reskin most of it with h carbon fiber to get the weight down and make the air jacks happy, the owner wants it to retain the look of a Tahoe though so other than some basic concessions we aren't allowed to go crazy with aero work.

That's pretty much up to date though.  To answer the obvious though and end this post, "Why a Tahoe?". The answer I can give is that's just what he wanted, and our paty to getting the raciest Tahoe we can think of is to use the smallest amount of Tahoe we can get away with.

NermalSnert (Forum Supporter)
NermalSnert (Forum Supporter) HalfDork
3/30/24 4:20 p.m.

Da yum!

preach
preach GRM+ Memberand UltraDork
3/30/24 5:30 p.m.

Well, dang...

budget_bandit
budget_bandit Reader
3/30/24 8:15 p.m.

Well i wasn't planning on drooling tonight...

amg_rx7 (Forum Supporter)
amg_rx7 (Forum Supporter) UltraDork
3/30/24 9:04 p.m.

How will humans get into the back seats? :)

Pretty cool build 

rdcyclist
rdcyclist GRM+ Memberand HalfDork
3/30/24 11:15 p.m.

Holee Shinola! That's amazing! Looking forward to seeing how this progresses...

NashGTI
NashGTI GRM+ Memberand Reader
3/31/24 9:54 a.m.

In reply to amg_rx7 (Forum Supporter) :

He said it had to have rear seats but failed to stipulate access or room, the devil is in the details right?  More seriously though, the Bummer trans will be paddle shift and push button operated. The cable operated parking pawl mechanism can go anywhere. The plan is to do a drift style hand brake lever too but it will be mounted close to the driver on the forward section of tunnel. All that being there will be room to climb between the front seats, a full 11 inches in fact to get to the back 

We had to compromise the integrity of the cage structure to make that possible and there is no denying it. It's really the only major contention of the build, us the team building it presenting why it's a bad idea but the owner saying having four seats is the reason it's being built. We've tried to work around and get strength where we can but we can't triangulate anything down the middle.

For what it's worth I wanted the front seats further back 2 inches but got vetoed, and I wanted the CO2 tank and fuel surge tanks between the seats and rear axle but that was impossible with the rear seats.  I'm worried about aerodynamic lift and drag in the rear too and been told I can't have stuff sticking out past where the rear bumper would have been.

Piguin
Piguin Reader
3/31/24 5:50 p.m.

Love it - 4 people in, full race gear... 

 

And even with all the compromises on the cage build it is probably going to be an order of magnitude safer than a stock Tahoe...

 

No idea if it would pass tech without any triangulation, but I guess that depends on where he wants to run it

prodarwin
prodarwin MegaDork
3/31/24 6:24 p.m.
NashGTI said:

It's going to be low, and the track widths have been stretched out to 78 inches front and rear.

surprise

Is that correct?  That's a foot more than a C6.  Its going to be a bitch to slalom, right?

Keith Tanner
Keith Tanner GRM+ Memberand MegaDork
3/31/24 6:43 p.m.

I'll bet you can get that rear to work - I don't think you'll get the lift off that rear that you would off a sloping fastback. But you could maybe cheat and have a way for the air to go from the roof surface out the rear - following that rear diagonal of the cage. Leave the side windows in place but direct air down the center. Then you could even use the remaining roof section as a wing :) I'll sketch that out if it could be useful. It would require obvious venting in the rear, though. Like a perforated rear window.

Mndsm
Mndsm MegaDork
3/31/24 6:43 p.m.

That chassis is a thing of beauty 

NashGTI
NashGTI GRM+ Memberand Reader
3/31/24 7:41 p.m.

In reply to prodarwin :

It's actually only 2 inches wider than the Tahoe itself in the back believe it or not so it's mostly free in terms of footprint.  Front is more.  That's taking the outside edge of the wheel to the edge of the body, actual stock track widths of the Tahoe are far more narrow but the wheels are seriously tucked under on those.  The wide stance of the tires is largely necessary because of the width of those front tires and getting them to have any sort of realistic turning radius.

Amusing side note, owner keeps telling us builders that we need to drive it in anger when it's done.  I've tried to politely decline unless it's at a rented road course.  This thing is going to be way more car than I have any justification to drive around an autocross course I'm just not that good.  He keeps saying nonsense but I'm still not convinced he's aware just how crazy this thing is going to be when it's done.

NashGTI
NashGTI GRM+ Memberand Reader
3/31/24 7:44 p.m.

In reply to Keith Tanner :

No chance of him going for that sort of serious aero work to the top.  I'm pretty confident we can get a wing to work in the back  because the rear glass is fitted to a steel frame that flips up with it, so we should be able to mount the wing to that frame and have it lift with the rear glass and keep access to the rear compartment.  That's another thing that he doesn't want to hang excessively off the back because it'll just look "weird". (His word)

I think we've compromised to this point of having multiple options for a front chin spoiler/splitter that is reasonably interchangable and a rear diffuser that is the same way.  That way we can have stuff large enough to be usefull on a track but then able to relatively quickly change back to the modest stuff when it's time to leave.

NashGTI
NashGTI GRM+ Memberand Reader
3/31/24 7:49 p.m.

In reply to Piguin :

It should be light years better than a stock one, and it keeps getting thrown out that a lot of even SCCA stuff has very minimal safety structure added.  There is talk of doing hill climbs too in addition to the track days and auto cross and safety there is a much greater concern with solid things very close to the racing surface able to be hit.  I think we're going to do a removable cross piece to bridge that between the front seats area when it's at an actual road course or hill climb and nobody will be in the rear seats, but I'd be a lot more comfortable with some X'd bracing coming back from the main hoop and a triagulating piece coming forward between the seats to the tunnel.  Really I'd like to see that X coming down supporting the center spine of the chassis around the rear differential mounting and that can't happen either.

Scotty Con Queso
Scotty Con Queso UltraDork
4/1/24 11:52 a.m.

I was expecting a E36 M3 box Tahoe with a sloppy stage 2 cam and a Chinese turbo, which is awesome but done a million times already. 

I was not expecting this work of art. Carry on. 

nocones
nocones GRM+ Memberand PowerDork
4/1/24 12:05 p.m.

I'm assuming by Track Width you mean Overall width to the outside of the tires.  A 78" TW with 335s would be ~93".  It looks like you are setting the tires basically in the Tahoe body with minor flares which would make sense with a 78" OA width.   Which is approximately a 65" TW (depending on actual width of tires) that is similar to a C6/C7/C8 Corvette.  

Please continue this thing looks fantastic.  

Somebeach (Forum Supporter)
Somebeach (Forum Supporter) Dork
4/1/24 1:06 p.m.

This sea of tubing is fascinating. Just for a reference what size of tubing is being used? 
 

I don't know what types of tubing exists other than DOM and ERW, but what kind of tubing is it? 
 

 
 

 

NashGTI
NashGTI GRM+ Memberand Reader
4/1/24 1:10 p.m.

In reply to nocones :

Yeah, I must be using the term incorrectly, outside of tire to outside of tire 78 inches

NashGTI
NashGTI GRM+ Memberand Reader
4/1/24 1:27 p.m.

In reply to Somebeach (Forum Supporter) :

DOM of a couple sizes, main tubing of the roll cage is 1.75 inch .125 wall and all the bridge work is 1 inch .125 wall. Some of the cage reinforcing pieces and the upper layer of the chassis long tubes are 1.5 inch .125 wall.  The bottom section of chassis is 2 inch .125 wall square tubing  with some 1x2 inch rectangle cross pieces.

That's all the main pieces or at least the important stuff structural pieces I can think of.

Recon1342
Recon1342 SuperDork
4/1/24 1:36 p.m.

Wow.

fouckhest
fouckhest Reader
4/1/24 3:36 p.m.

Super cool build, looking forward to following this!

NashGTI
NashGTI GRM+ Memberand Reader
4/4/24 6:35 p.m.

Light weight racing spindle.

 

 

Obviously not a permanent piece, We took today to make sure what we've modeled 3D in Fusion can actually translate out into the physical world without spending too much time and money making a nice upright that may not work.

 

Wheel fitted, I even tacked a couple nuts to a short piece of tubing to make a tie rod adjustment sleeve so we could turn it.

 

 

Those show the camber change while turning, I thought I had a straight on picture but don't see it now (and while the truck is only 70 or so feet away I am currently sitting down).

 

Steering column tacked in place, quick release steering wheel.

 

While Corey and I were messing with the suspension Jeff has been working with the titanium.

 

And then an overhead view on the mocked up suspension. Control arm mounts are made to bolt on to be able to change the roll center or anti dive without cutting the chassis down the road if need be.

Now I need to get the pull rod coil over mechanism ironed out. 

Keith Tanner
Keith Tanner GRM+ Memberand MegaDork
4/4/24 6:57 p.m.

Loving the detailed updates.

What do you use for your suspension geometry design?

NashGTI
NashGTI GRM+ Memberand Reader
4/4/24 7:14 p.m.

In reply to Keith Tanner :

Me slaving over AutoCad nights and weekends is how this got designed.  Get the geometry I wanted in 2D then translate that into Fusion 360 to have a movable 3D model of the suspension linkage. Once all that was proven there Corey went and designed the arm mounting. There are picture and videos of the CAD stuff on the Infinite Fabworks Instagram account somewhere.  I could put up a screen capture of manic looking master suspension CAD drawing.

Keith Tanner
Keith Tanner GRM+ Memberand MegaDork
4/4/24 7:18 p.m.

Please do!

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