Yet another GMC build

[dsraven]

Hydroboost is awesome.

[6DoF]

hydroboost feels different for sure ... but it's like putting an 6-71 blower on your brake booster! having the stopping power of a rig built to drive around at 12,000gvw, but only rolling 4000, works out reeaaalll nice

[dsraven]

AND, its the same all the time unless the engine dies or you blow a hydraulic hose, then you have one boosted brake application before the manual brake feel comes back.

[OKGMC4]

I was car 454 at the track today. Wanted to see how it went with a new set of drag radials on the rear. Best time was 11.7 and I am happy about that.

[6DoF]

NICE!! hard to complain about a solid 11sec truck!

having issues anywhere, or anything you'd change to try for faster?

[OKGMC4]

Originally Posted by 6DoF

NICE!! hard to complain about a solid 11sec truck!

having issues anywhere, or anything you'd change to try for faster?

Could go with a stall converter but the radials might not hold anymore. Could also try more timing or boost with E85 but I like running pump gas and also drag radials might not hold. It would either hook or spin a half turn and go. Just about perfect. The Nittos are expensive but work. Plus, because the tech guy passed me without even looking under hood, I was later warned that if I get faster than 11.5 they will require a roll bar installed. Not sure I want that. Yet. Still would like to make it work on a road course. But I need more brakes, lower in the rear which means frame mods, and track tires. How cool would it be to go track with a bunch of Corvettes and BMWs in an ancient pickup truck?

[6DoF]

ooooo, i always forget that 11.5 is the breakover ... 11.7 is fawkin MINT!

the cornering game can be a deep hole too. moving weight, alignment adjustments, getting springs/shocks/bars tuned just right. worth it.

[Kevin_C]

Thats awsome. I live down the road from New England dragway. Hoping to run mine down the track next year.

[OKGMC4]

Originally Posted by Kevin_C

Thats awsome. I live down the road from New England dragway. Hoping to run mine down the track next year.

Can’t wait to see the time slip!

[OKGMC4]

Want to start improving handling. The truck understeers pretty badly on turn in and I think it is partially due to the rear sitting 2.5” higher than the front. I’m going to make some changes to attempt to fix it.

[OKGMC4]

I'm also going to have to raise the gas tank a bit, and cut the pretty bed floor, and probably modify the exhaust some more. The toe rods need to be relocated downward. Definitely going to go more aggressive on wheel alignment and tires. Looking to move from 275/40r18 to 295/35r18 in a stickier compound. Lots to do.

[6DoF]

my wife's base C7 always pushed bad into corners when we'd go play at the low-key autocross, turns out it just needed a rear sway bar. the base car comes with the lightest front bar and no rear bar. at the suggestion of a guy who races them and did a whole comparison study on what worked well together, we got the stiffest rear bar. made a significant change for the better.

[dsraven]

what rake angle is the front suspension set to? how does the wheelbase on the truck compare to the wheelbase of the donor for front end?

[OKGMC4]

Originally Posted by dsraven

what rake angle is the front suspension set to? how does the wheelbase on the truck compare to the wheelbase of the donor for front end?

I installed the front crossmember per Flatout Engineerings instructions but not sure what rake they intended. But…Flatout also designs around having the rear up high. I could have adjusted that during install, little late now. Don’t want to cut the front loose. These trucks are quite a bit longer wheelbase than a C4. What I get going in is massive understeer followed by massive oversteer. I think wheel alignment and tires will help going in, but that the rear being high makes it swing coming out.

[OKGMC4]

Originally Posted by 6DoF

my wife's base C7 always pushed bad into corners when we'd go play at the low-key autocross, turns out it just needed a rear sway bar. the base car comes with the lightest front bar and no rear bar. at the suggestion of a guy who races them and did a whole comparison study on what worked well together, we got the stiffest rear bar. made a significant change for the better.

Interesting you mention rear bar. I was thinking to put one on but my massive oversteer coming out of a turn makes me second guess that. I thought lowering the rear and reducing rear bar helped corner exit oversteer?

[dsraven]

there are a few things that can affect steering when using a donor front end.
1- wheels with the wrong offset combined with tires that have a different outside diameter compared to the donor. this messes with the scrub angles. basically, if a line were drawn between the upper and lower ball joints and extended down to the floor under the tire, you would want the tire contact patch to be centered on that point, more or less
2-using a donor front end from a short vehicle on a longer vehicle, or vice versa, as this messes with the ackerman angle (google it). what is ideal is when the steering arms that come off the steering spindles, if extended to the rear axle, would intersect the rear axle in the middle. so then, it makes sense that if you put a "short vehicle" IFS into a longer vehicle then the ackeman angle will be different and the front tires may not track in the same circle when cornering
3-installing the donor front end into your project with a rake angle different than what the front end was designed for. mosty the lower control arms are designed to be level at the ride height, viewed from the side and the front. changing this affects how the vehicle steers, especially on front ends with different length upper and lower control arms because they are designed to tip the wheel in/out as the suspension goes through a cycle from full jounce to full compression
4-installing a front end in a component style, like a lot of aftermarket MII set ups, and not paying attention to what is going on with front end geometry. eg: the tie rod inner pivot, with wheels straight ahead, is not in line with an imaginary line drawn from the upper to lower control arm pivots. if this is out of whack then bump steer happens because the tie rods don't pivot in the same arc radius as the control arms
5-installing a front end at a certain rake angle and then, as the project goes forward and time goes by, the rake angle gets changed due to wanting a different look or going with staggered tire sizes front to rear
just my 2 cents. I am sure there will be those that disagree but thats fine too. there are a few sites that explain independent front suspension geometry and how one angle affects another. if interested just do a google search. there is also a tire size calculator that has tabs with different topics and one of them shows scrub angle changes with different wheel offsets and tire diamters. this could be helpfull. finding an alignment tech that knows his/her stuff is also beneficial.
if you don't have stabilizer bars, or are using light ones from the donor and your rig is heavier or has a higher center of gravity, then i suggest to install some that will better match your set up.

[dsraven]

another item to consider is the angles on the rear suspension links. they also work in an arc so as the suspension moves up and down the wheel will move fore to aft. if the vehicle leans as it goes around a corner then this changes the angle of the rear axle center line compared to the frame. if you start off with those links level at ride height then there is less angle change as the suspension moves because the axle is at the longest part of that arc, so small suspension changes dont result in as much axle centerline change as when the liks are already at an exagerated angle at ride height. so, imagine you have those links angled down at ride height. you take a hard turn and there is body lean. the wheel on the inside of the corner drops away from the frame further, so that wheel actually moves forward in relation to the axle center line. the wheel on the outside of the corner comes closer to the frame as the suspension moves up on that side. this moves the tire further rearward in realation to the axle center line at ride height. this amounts to changing the axle center line to be at an angle to the frame, when the vehicle leans, compared to when the vehicle is sitting level. this theory may explain why a heavier stabilizer bar would help a vehicle that suffered poor steering due to excessive body lean on corners. trying different tire pressures and compounds may also help. I understand that corvette was engineered with long and short trailing arms to achieve better antisquat characteristics and the lower arm was engineered to angle up at ride height. copying that angle at ride height would be optimal I suppose.
keep us posted on what you do and how it works, there are quite a few vette suspended 55-59 trucks on this forum.

[OKGMC4]

What you are saying about the rear control arms tracing an arc makes sense. I noticed that when trying to raise the rear knuckles to the desired height. The rear wheels moved rearward 3/4”. I compensated by moving the control arm brackets forward to re-center the wheels. That may explain the oversteer late in a turn. Once the rear starts leaning one wheel moves backward while the other comes forward, inducing that oversteer. I may go ahead and fabricate mounts and try a rear bar, I have a stock one laying around. My front is a 1.25” Ridetech bar, and the front feels very flat. I see what you say about wheels. I’d have to compare to C4 factory wheel offsets, but these are very close to FWD wheels, almost positive offset. The c4 suspension is wide and old school deep dish wheels don’t fit well. It’s harder to find 18” tires now but these wheels fit, are paid for, and I’ll run them until bigger issues are corrected. Most of the c4 trucks on the forum have the engine way forward. At least I’ve got mine shoved about 5 or 6” backwards. The engine is behind the rack where GM designed it, and helps my weight bias. Still too high center of gravity and heavier than a c4, but it has to help. I really need to find someone with scales and weigh each corner. I guessed and put 500# front and 400# springs rear. More than Flatout recommends, but may need stiffer before it’s over.

[OKGMC4]

Just laying rear bar in place on the frame, it will be very easy to build links and mount it.

[dsraven]

will you build your own links with spherical rod ends and adjusters or simply purchase something?

[dsraven]

will you have clearance on the shocks?

[OKGMC4]

Originally Posted by dsraven

will you have clearance on the shocks?

It might be close to the coil overs, but the bar basically sits above the control arms. I can do some frame spacers and or longer links to get clearance if needed. I have some Heims and threaded aluminum tubes somewhere from another project for links, but may have to change length.

[dsraven]

I've seen a few with the car installed behind the axle, maybe that's why.

[OKGMC4]

Bar would fit great on the rear if no rear fuel tank. Easiest way to move tank for me is to unbolt from the bottom and bolt in from the top. Cutting out the bed floor anyhow. Thinking of how I relocated the rear fender lips up 1.5” and how I’m dropping the rear 2.5” …the tires will be tucked in pretty low. Wonder how hard it’s going to be to get a jack under the rear when I’m done?

[OKGMC4]

About 90 percent done with this part. Mostly finish welding. However, still need a brace by the tank to replace the one I cut out. The sway bar will work, just need to order the links. And to remake brake lines. Then on to the bed cutting.