Make it handle

[robnolimit]

Originally Posted by Hop

Not sure if this has been addressed. looking to add coil overs. Are new upper and lower control arms required or will the current setup met my needs. 64 Step Side 2 1/2 spindle w/3 in drop spring in front, 4 in drop spring in rear. I want to lower the vehicle another inch all around without changing much.

Before I say what I feel I need to, remember that I am really trying to help. OK. So, my first question is, "What are you trying to accomplish?" I.E. - What are your goals for the suspension and/or the truck? This information has a lot to do with the right answer. Here's why.

IMHO (or, my opinion being fairly well educated on this subject) Adapting to a coil-over is seldom the best choice. (this is different that using a coil-over from a fresh design standpoint) What could you gain from a coil-over. 1. Compact design. uses less space and weight than a conventional coil/shock. 2. Easy mounting, - from a fresh design standpoint. 3. Adjust-ability of spring height via the threaded spring collar. 4. Typically built with high quality shocks, and/or with some valving adjustment.
And for the down side, LENGTH. To get 5" of travel from a coil-over, the open and closed measurements would be around 12" closed and 17" open. (eye to eye of mounts) Travel = ride quality. The shorter the travel available, the higher the spring rate needs to be to stop the vehicle from bottoming out on compression (bump/dip). The worst offender in the ride quality game are the 'Hy-brid' or 'direct fit' coil-overs. The ones that bolt to the LCA and the spring tapers out to fit the OE upper spring cup, with the upper shock mounting into the OE hole. These typically have 3" to 3 1/2" total travel, and I will now ask anyone, "If you, or anyone you have ever known, or heard of, has ever swapped in a pair of these, on any vehicle, and been happy with the ride quality, please let us know. As they may be the Unicorn, the only one.
Now, if ride quality isn't an issue, and you just want it lower, and maybe some adjustment to fix other problems, OK, they will work.

On to real Coil-overs shocks. The video shown is the best way to fit in a pair. By raising the upper shock mount they have created room for a longer shock to get back to the OE travel range. (lets say 4" to 4 1/2" of shock travel) that shock would be 11" to 15" long (closed vs open). But, this is a fair amount of work. There are kits avail, like the one from RideTech, and if this is your goal, that may be the best bet.

Now, on to your desire for 1" more of drop. If you do a coil-over swap in 'kit' form, and then back off of the spring collar 1/2" to drop the truck 1", you are loosing potential compression travel. And you didn't have a lot to start with. If the shocks and springs - or - coil-overs are installed correctly, then they are NOT the limiting factor of the suspension, the actual suspension design is, say, bump stops. Again, you need a very stiff spring to stop the vehicle from bottoming out.

You could get the same from a 4" drop spring and installing a high quality adjustable shock, BUT, the OE C10 shock mounts don't allow for a shock when the suspension is that low. Porterbuilt has a raised upper shock mount for that. BUT, the suspension itself is still almost bottomed out. You can trim down the bump stops (please don't just take them out) But by now, I'm sure you have noticed the front crossmember smacking the ground from time to time as it is now, and dropping another 1" will make that even worse.

Sorry for the heart-braking. But I want to be honest, and to give you the knowledge you need to make a good plan. If you want a coil-over because of the cool factor, then great. If you want another 1" drop, just trim the spring that you have, but know you will loose clearance and ride quality. Please stay away from the direct fit type coil-over swap kits. OR, you can go farther. Raise the crossmember up, or consider an aftermarket IFS with more travel and clearance. Sorry again for going on, and being a bit harsh, but I hate seeing people spend money and get a lot less than they were sold. Keep Truckin!

[rapid49]

How low is too low for my panhard bar? What are the advantages / disadvantages of lowering the bar. I am going to install a Fat Bar 4 link on my 56 GMC. I currently have a 25 gallon blazer tank installed with little room between the tank and my rearend cover. I would prefer not to change the tank and can fab new mounts for the panhard bar. I can make more room for the bar by mounting it lower. I need to know what effect this would have on the handling.

[robnolimit]

OK. EVERYONE should read this. (smart question BTW) The location of the PHR will determine the rear Roll Center. This is easily mapped out. The RC will be in the center of the bar (left to right) and at an elevation equal to the average of the two mounts at Ride Height. had to say it that way to include bent bars. If the bar is straight, it's the center of the bar. There are a couple of basic PHR rules we try to follow.

** Try to keep the bar level (mount to mount) at RH.
** Try to keep the center of the bar in the center of the WMS (Wheel Mount Surface) - or, center of the car/truck.

*** Some adjustments to these rules are commonly made for weight balance and track condition, but let's stay with the basics.

So, What about RC height. ** Lowering the roll center increases the Roll Angle of the chassis. So, a lower RC will increase body roll. (that's the bad part) BUT, the increased body roll puts more load on the outside tire (good for light back trucks) and may increase traction. (up to a point of diminishing returns) On the other side, raising the RC will reduce body roll, and keep tire loading more even (good if you have even ft/rr weight balance and sticky tires).

I think you would be OK to drop the PHR, but you will need to brace the dropped chassis mount. Please post up some picts when you are done.

[rapid49]

Thanks Rob for your quick reply . I will definitely post pics I'm looking for all the feedback I can get. I'm getting an education on this thread , thanks to all who share .

[rapid49]

I ended up lowering the panhard bar about 5" After the dust had settled I realized I probably could have done 4" Nothing is welded out at this point . I am also guessing at ride height. It came with 200# springs . How much can I expect the shocks to compress at ride height ? Any opinions on lowering the roll center this much ?

[robnolimit]

Looks Good. One thing to check is the bar at R.H. Those coil-overs should be appx 14 1/4" at R.H. (appx 1/2 of the shock compressed.) Here's the quick way. Right now the shock is full extended, with 5" stroke. Measure the distance from the top of the axle housing tube to the bottom of the frame rail (notch) just above the housing tube. - looks to be around 6 1/2" or so. Subtract 2 1/2" from that measurement (1/2 of the shock travel) Cut two pieces of 2x4 to that length. Place a jack under the axle housing/gear set and remove the coil-overs. Put the 2x4's on top of the housing tube and use the jack to lift the housing until the 2x4's hit the bottom of the frame rail. Now the housing is very close to Ride Height. Make sure that the bar is level in this position.

But the looks of it, rear R.C. height will be around 8". (total guess, but, 27" tire = 13 1/2" to the axle center from the ground. Bar looks to be about 4" below the axle C.L. subtract another 1/2" for tire squat, and your at about 8") Your front R.C. will be around 4" to 5", so this is fine.

[rapid49]

Once again thanks for your quick reply Rob. You mentioned the front roll center. The front is a 81 Camaro clip with Hotchkiss springs and a 1 5/16" sway bar. It has circle track upper control arms that are moved back 1/2" for added caster. I am also running the original 15/16" front sway bar on the rear. Can you explain the relationship with the front and rear roll centers.
I also want to thank you for the quick shipping. I ordered the fat bar kit mid morning Tues. and it was waiting for me when I got home from work Thurs. afternoon .

[clemsonteg]

Originally Posted by robnolimit

OK. EVERYONE should read this. (smart question BTW) The location of the PHR will determine the rear Roll Center. This is easily mapped out. The RC will be in the center of the bar (left to right) and at an elevation equal to the average of the two mounts at Ride Height. had to say it that way to include bent bars. If the bar is straight, it's the center of the bar. There are a couple of basic PHR rules we try to follow.

** Try to keep the bar level (mount to mount) at RH.
** Try to keep the center of the bar in the center of the WMS (Wheel Mount Surface) - or, center of the car/truck.

*** Some adjustments to these rules are commonly made for weight balance and track condition, but let's stay with the basics.

So, What about RC height. ** Lowering the roll center increases the Roll Angle of the chassis. So, a lower RC will increase body roll. (that's the bad part) BUT, the increased body roll puts more load on the outside tire (good for light back trucks) and may increase traction. (up to a point of diminishing returns) On the other side, raising the RC will reduce body roll, and keep tire loading more even (good if you have even ft/rr weight balance and sticky tires).

I think you would be OK to drop the PHR, but you will need to brace the dropped chassis mount. Please post up some picts when you are done.

When you say to keep the center of the bar in the center of the WMS, do you mean left to right? I think going from the frame to the trailing arm on opposite side is the best that can be done for the in a stock type setup, right? That would put the bar off center to the drivers side?

[robnolimit]

Yes, it would be nice if the bar were centered left/right. Usually not 100% realistic with a lot of fab work, or shortening the bar (bad). So, it is acceptable to have the PHB center 1" or 2" from the vehicle CL

[Ddkeeler]

So this is off the current topic a little, but I remember rob saying something about flipping the leaf spring shackle hangers on the 73-87's. What is the reasoning behind this?

[rapid49]

Originally Posted by Ddkeeler

So this is off the current topic a little, but I remember rob saying something about flipping the leaf spring shackle hangers on the 73-87's. What is the reasoning behind this?

With the weight of the truck above the shackle instead of below it ,gravity is working for you instead of against you in regards to the leaf spring being able to flex therefore letting the suspension work more freely.

[SierraMtns]

Hey Rob,

FRAMES

I wanted to ask you about boxing frames and adding X-braces. I understand we want these frames stiff to handle the HP and let the suspension work better.

Is it needed to box in the whole frame or just some key areas? I have 3/16" x6" steel plate to box in the frame and a 20' stick of that stuff weighs 76 lbs. I remember my auto shop teacher saying that it takes 10 HP to move 100 lbs. At what point is boxing the frame and adding X-braces slowing us down?

Thanks

[G8DAN]

Yes. Any more information on the X-Braces?

[robnolimit]

Boxing, X bracing and weight. The best weight-to-performance saying that I know is this: 100 lbs = 1/10th of a second. Now, this not an absolute, but if you have a 10 to 13 sec car, it's really close. On the flip side, Weight down low, below the Center of Gravity, will help the base handling. Also, the stronger chassis will help in traction, braking, handling and a big improvement in ride quality. So, in total gain, I would add 100 lbs of chassis stiffener weight with no thought about a negative to the weight.
So, yes, box the whole thing, but make sure it's square and level when you weld. Standard crossmembers make squares, or ladder frames. - not very strong. Try to make triangles. They are strong. We do sell the X member from our Pro-Tech chassis for $595 that can be fitted into the OE chassis. (not a 100% drop in, but not too bad to fit). I know that PB also has a pre fab X kit.

[G8DAN]

Hey Rob,

Anymore tips with making X Braces? Should they cover a certain length? Type of steel used?

[Ben_Yeaton]

I have a few questions regarding bump steer on a stock crossmember, stock a-arm truck equipped with dropped spindles and coils. This seems to be an extremely popular option and I wonder if flipping the outer tie rod to the bottom of the steering knuckle would alleviate any bump steer issues? Is this the universal fix for the bump steer issue as long as it flattens out tie rod angle? I know using a heim joint has been discussed, but isn't desirable for a street truck. Reeming a new tapered hole from the bottom isn't possible, so I've been on the search for a tapered insert to use that fits a stock outer tie rod. There may be something out there, but I haven't found it. Also, it may be even more useful to do the flip after doing the "caster mod" and seeing the tie rod mount rotate up slightly. Thoughts?

[robnolimit]

Drop spindles (theoretically), have the steering arm positioned to keep the tie rod in the OE geometry. So, they should have appx the same bump curve as a stock truck. I would think that moving the tie rod end to the bottom of the steering arm (using stock steering components for the rest of the system) would produce much worse bump steer, as it would be way too much of a change. keeping the tie rod end 'level' is not really the goal. Having it match to the I.C. (instant center) is, and it may not 'look' right, even when it is.

[Bowtie001]

http://67-72chevytrucks.com/vboard/s...17#post8155517

FYI

[Ben_Yeaton]

Originally Posted by robnolimit

Drop spindles (theoretically), have the steering arm positioned to keep the tie rod in the OE geometry. So, they should have appx the same bump curve as a stock truck. I would think that moving the tie rod end to the bottom of the steering arm (using stock steering components for the rest of the system) would produce much worse bump steer, as it would be way too much of a change. keeping the tie rod end 'level' is not really the goal. Having it match to the I.C. (instant center) is, and it may not 'look' right, even when it is.

That's great information, thanks for answering. I guess it would be best to get under and measure for my I.C. and go from there. Right now the lower arms are close to flat, and the uppers are much higher at the spindle. The tie rod angle does look steep, but I'll do my best to get a rough I.C. value and then see if it would be better above or below the steering knuckle. As you mentioned, just because it looks wrong doesn't mean it's not spot on.



Is there any preference to have the I.C. error on the high or low side if there must be an error? Or is the "closest to perfect" approach applied?

When measuring for I.C. is it best to work with a protractor and get angles of the arms and tie rods, or measure up from level ground for locations to map out and calculate from there?

As I mentioned before, I'll be doing the caster mod soon, so that would only raise the tie rod mounting point slightly from where it is now it seems. This MAY give more reason to mount below if the I.C. shows the values are close.

When I get a chance to go out and get some measurements I'll be sure to post what I find. Thanks again!

[Custom 68]

and a follow up for clarification on measurements. (at least for me)
if we are measuring up from the ground where would we measure on the ball joint and tie rod end locations?
I have added three red lines to the lower ball joint. I am going to assume it is the lower line I added in this case as that is really where the pivot is. I dont think it would be possible to use a protractor so the measure and calculate should be the better option IMO. but I wanted to confirm where they do indeed get measured to.
the connection of the a-arm to the frame is understood but I just want to confirm on the other ends...
Thanks
Dave

[robnolimit]

Yes Dave, thats right. When we measure to ball joints, for geometry, we are measuring to the center of the pivot ball. Mostly this will be your best estimate. At you lowest red line, or a little lower.

[Ben_Yeaton]

Originally Posted by robnolimit

Yes Dave, thats right. When we measure to ball joints, for geometry, we are measuring to the center of the pivot ball. Mostly this will be your best estimate. At you lowest red line, or a little lower.

Thanks for clarifying. I tried to do some measuring and mapping to get my I.C. and I'm thinking I should go back and measure again. The numbers I got show that the ball joints are closer together than the inner arm pivot points, so the intersection point is outboard of the vehicle rather than inward. All of the diagrams I see show it the opposite way with the planes traveling closer together towards the center of the truck, not away from it. I'll go back and try again. Do you have any tips to measure some of the locations that can't be easily accessed? Laser level and a tape measure? From what I did see though, the tie rod is very similarly angled to both arms, but I'll try and map it out to see exactly.

[Custom 68]

Thanks Rob, I think I will do some measuring and draw something up to show where I currently am.
This is some good information and you make it easy to understand.
Dave

[67_ShortFleet]

What are your thoughts on steering gearbox choice? I'm thinking about the CPP 500 series. Or a variable ratio. Thoughts..

[robnolimit]

To be honest, I have no experience with the CPP box, so I can't render any opinion. So, depends on what you are after. If you are looking for a performance series box, I would look into AGR or Lee.