Chassis Building 101. Ride Height

[Tincup]

Basic question of the week, shock placement. I know on your Fatbar the shocks are placed outboard of the frame. Most other builders place them inboard, even the OEM places them inboard. On the chassis I have the shocks were mounted inboard, with one shock on either side of the rear, one angled forward, one angled back. What's the reasoning behind the different choices...?

[72BBSwinger]

The forward and rearward placement is almost always on a truck and IMO is for better wrap control under towing conditions.

[Bob in St. Louis]

This is a fantastic thread. So much great information.
Thanks to those of you who are sharing your knowledge.

[RobNoLimit]

Basic question of the week, shock placement. I know on your Fatbar the shocks are placed outboard of the frame. Most other builders place them inboard, even the OEM places them inboard. On the chassis I have the shocks were mounted inboard, with one shock on either side of the rear, one angled forward, one angled back. What's the reasoning behind the different choices...?

Yes, the for/aft shock placement is for axle wrap and harmonics control. We place the shocks outboard for better ride control. Ford and GM are now doing this on their trucks. The closer the springs/shocks are to the center of the car/truck, the more it will tend to act like a 'teeter-totter' in the corners and over bumps. The farther outboard they are mare the shock more reactive and the vehicle more stable.

[Tincup]

So Ideally, you want them mounted outboard of the frame, staggered front & rear....Like a Ford...

[RobNoLimit]

This is a leaf spring set up, so staggered shocks (without the coil spring) will help the natural spring wrap and oscillation. - But, these shocks are not carrying the weight (spring load) of the suspension. If you were to stagger the shocks on a four bar/coil over set up, the two sides would react differently. The coil-over in front would see a higher load, and slightly less travel, the rear coil-over would see a lower load, and slightly more travel. (realistically less than 3% difference) So to get the two sides even, you would need custom springs and valving for each side.
- No, Ford doesn't have a 'left' or 'right' shock. It's the big compromise. this gives them the best ride for the least $$$. Honestly, I would rather put the coil-overs in front of the axle, but that brings in a packaging problem, so, to get everything to fit, we put them on the back side. - Except for our Big-10 chassis, where they are chassis mounted and drivin with a push rod and rocker arm, on the front side.

[Tincup]

So, that being said, if you were to stagger your shocks on a four bar setup, I would imaging the smaller the angle between the two shocks, the smaller the difference ( 1%-2%). The reason I'm asking is my rear ( Dana 60 ) already has the shock mounting brackets for a staggered setup welded on. I was thinking of using them, if they work out...

[RobNoLimit]

This is from a current project we are building for an 89 C1500. With any air suspension the air bag (or air spring) will have a givin Ride Height range, a max open height, and a closed height. Some have internal bump stops, these do not. These bags are 4 1/4" closed, 8" to 9 1/2" RH, and 13" open. In full droop (lifted) the shock will be the limiter. At full compression (dropped) there will be a bump stop on the chassis that contacts the axle. At this point the bag will be at 4 5/8", and the shock will have another 1/2" of travel avail.
When we laid out this chassis, we started with the tire ht @ 28", so 14" to the axle center. The bottom of the rail under the cab is to be appx 5 1/2" @ RH. From the axle center, we star adding. 1 1/2" for the housing, 4 1/2" up travel of the rear axle, 1/2" bump stop. In total we need 14 + 1 1/2 + 4 1/2 + 1/2 = 20 1/2" from the ground to the bottom of the rail (over the top of the axle housing) at RH. So if the center of the frame under the cab is at 5 1/2" at RH, the rails need to rise 15" (bottom of the rails) from the cab area to the top of the kick over the axle.
The shock does loose some of it's dampening laid back like this, and that's part of the plan. In the 'Ride' zone, the shock has lost 18% of it's dampening force compared to being 1 to 1 with the axle travel. For trucks, that have a relatively high ratio of unsprung weight, this helps with the ride quality. We can tune in the rebound with the HQ series shocks. Have you ever thought "Hey. I need more compression dampening on the rear shock?" - No. Now, down at full drop, the shock looses even more (somewhat regressive). And since this is just a 'park', or 'cruise' stance, it's OK, and it will air up quicker without the heavy resistance.
_ Fire away.

[Tincup]

Let's keep this thread going. The topic of the week, Sway bars, should I use splined or standard. What is the theory on splined sway bars, are they better? just for race cars? I only plan on hitting the Autocross once or twice a year, so primarily a street truck. So would standard sway bars be OK?, front & rear?, larger front than rear?, same size?

[David Sloan]

I was really hoping this would keep going as I am at the point of picking bars for my car!
I want to be able to change the bar for tuning / an not sure how to pick the right size to start with.

[RobNoLimit]

I'll get right to it. The main reason to go with a splined bar is tuning. The secondary reason is fitment. That one cuts both ways. A splined bar set up gives the builder the freedom to shape the arm to fit the chassis/tire/suspension. The price tag here is that the builder has to make it work. Splined arms come flat. - you get to shape them. When it comes to the size and shape of the bar/arms, you need to know the range of roll resistance that your after. For that you need to know the range of TRS (Total Roll Stiffness), and for that you need a whole list of chassis and suspension info. (not here, or just yet. Be prepared to buy some books and software for that stuff). So, for now, take a SWAG at it. (Silly Wild Ass Guess). When it comes to effective rates, the bar diameter, wall thickness (if not solid), bar length and arm length can be plotted with relative ease. 'Drift out' of the arms is a lot harder to calculate, so we don't use it much. Once the arms are fitted, you don't change them, so for comparative tuning, this becomes a constant. Just know that the more 'drift out' you build into the bar, the less effective the bar really is.
For most builds, a formed bar is fine. But here is the catch. Can you get one for your build? A Fox body Mustang (example only) uses a formed bar, and there is plenty of TESTED aftermarket support for them. So you can buy different formed bars for more performance tuning. - Now, take a minute to list all of the companies that have done actual track testing on sway bars for your style of truck build, and, using the suspension design your choosing? OK, short list, see where I'm going? What size bar will work for you? Sad as it is, many chassis and suspension builders are churning out product that they have NEVER driven, or worse, NO ONE has ever driven. Just a SWAG from the drawing board straight to your project. Kinda sad and scary. If your using 63-72 or later C10 stuff, there are a few. No Limit, ridetech and Hotchkis. You can guess at sizes based on these examples, but consider arm length as well. Longer arms = less bar.

Here are some picts. Both fronts are No Limit Wide Ride IFS units. One with a splined bar and one formed. The formed bar is 1 1/8" solid. The splined bar is 1 1/4" x ,188" wall. These net close to the same. They have the same tire clearance, and mount in the same locations. One can be tuned later, and one can't. - unless you have another formed bar to change to (like a Fox body would). So, are you the kind of guy that would change bars at a Good-Guys event or SCCA track day? Or will you tune your street ride to get the best ride for everyday use? If the answer is an honest yes, put in a splined bar. If not, go with a formed bar.

[Tincup]

Awesome explanation, thanks....

[David Sloan]

I guess I need to do some reading so that I know were to start as far as size goes. I will want to be able to tune it for different tracks / auto cross , maybe open road racing
Thanks for the info

[RobNoLimit]

The current trend is big tires. But, if the wheel/tire combo interferes with the suspension/chassis, in such a way that it limits the degrees of steering available than the big tire may not be a win. You need to have 28* of steering, or more.

One of my thought trends lately is all about tire size vs. weight. If you have great suspension, and you can keep the tires planted through the corner, your thoughts soon move to the size of the contact patch. The tires contact patch is all you've got to transfer the forces to the ground. Those forces are, Torque (engine HP & gearing) Braking, and Lateral acceleration (cornering forces, in 'G'). Thinking about them separately to make it clearer, you may think like this: Adding Torque helps in acceleration,- and we are accelerating the mass (weight) of the car, but only if the tires can plant it. Big ABS brakes will help braking, again, we are slowing down the total mass of the car, but only as fast as the tires can hold. And Lateral G, which at some point in the formula, when you balance suspension and aero, comes down to contact patch grip vs. mass. The constants in all three concepts are tire contact patch, and mass. And while everyone is trying to stuff bigger tires under the car and build more HP and buy monster brakes, mass is not talked about much. - food for thought.

[Tincup]

Time to revive this thread. Let's discuss engine placement, the further back the better?

[Austin97C1500]

Wow! This is alot of great information. My father and I are doing a 97 C1500 build that will get an engine swap soon. Before we begin we are sorting out suspension. Leaning towards a 3 link out back and between Rob and Ron on the forums and sharing tons of information I feel like I'm getting a better education than the one I paid for!

[RobNoLimit]

Sorry guys, I missed this in all of the Holiday hustle.

As far as engine placement goes, look at it this way. Weight can be positioned over the four tires (the rectangle shaped 'platform') in only three ways. We might call them X, Y, and Z, but I will say Front/Rear, Side/Side, and Up/Down. Lets focus on ride quality and general handling (not drag racing) and say that the goal, with the driver included, is to get close to 50/50 on the Front/rear and Side/Side, and to get the Up/Down as low as reasonable. While your working on that, trying to keep the total weight down. I.E. - don't just add weight to get the Side/Side to 50%, move weight or cut weight. So, the basic answer to the motor question is, down and back.

[RobNoLimit]

OK, you opened the door. This is something I have wanted to discuss for a while. I generally put 'weight' into three categories. 1) Sprung weight. All of the weight that is help 'up' with, or moves to compress, the springs. This is most of it. 2) Un-Sprung Weight. This is all of the weight that is NOT held up by the springs. 3) Rotating Weight. Some of this is "Sprung" and some "Un-Sprung". For now, lets just look at "Sprung Weight" - SW - and "Un-Sprung Weight" - USW. While you think about total weight, and 'lightening' the car/truck for performance, thinking about USW would be smarter.

When the suspension reacts to a change in the road/load/corner... there are two main goals, 1) to keep the contact patch of the tire on the ground as much as possible 2) to get the vehicle/suspension back to it's static RH as quickly - and smoothly - as possible. So, the 'load' compresses the spring with a #force, and the spring wants to go back to it's normal position with an = force. BUT, the ghost in the room is USW. Imagine you are driving down a road and you hit a 2x4 with the left front tire. 1) the RF suspension compresses some, loading the spring more. 2) that additional spring load caused the RF of the truck to 'lift' up some. 3) ** some of the energy from the bump hit the tire, and caused that mass to change direction ** OK, Laws of Physics. .....A force in motion will remain in motion until some other force acts upon it..... So, for a moment, the direction of the wheel/tire (USW) was changed from 'forward', to 'up'. Then the spring/shock have to use some force to control and change that back to 'down' and then 'forward'. The more the wheel/tire (and the rest of the USW) weight, the harder it is for the spring/shock to control. Also, and another negative, is that the higher the percentage of USW, the more the USW can move the rest of the car/truck - SW - around. - a bumpy ride.

This is why there is a term "rides like a Caddy". Cadillacs are really heavy, but, because of all of the SW, the %percentage of USW is relatively low. So, when a Caddy hits a 2x4, the USW doesn't have as much leverage over the SW. But, not too many people ever said "handles like a Caddy" - (current CTS-V owners excepted)

To calculate USW, lets say of the RF of a C10 (OK, I'm going to take LOT of assumptions here) you would need to weight some components separately, and some half supported. I'll go through the list.
Wheel and tire = 70 lbs.
Rotor, bearings, studs, seal = 18 lbs
Caliper, pads, hardware = 12 lbs
Spindle, hardware = 19 lbs
Now, we have to look at what part of the next group is actually moving with the suspension
LCA, - support the pivot end and weigh the ball joint end = 8 lbs
UCA - support the pivot end and weigh the ball joint end = 6 lbs
TRE - support the inner TRE and weigh the tie rod and outer TRE = 4 lbs
Sway bar - support the middle bar and weight the 'arm' and end links - this will weigh both sides, use 1/2 of the weight per side. = 4 lbs
Spring/shock - for now, use 1/2 of the total weight = 5 lbs (coil-over)
So, for the average guess C10 we have 146 lbs of USW. - (this hurts to say) So, if you have the average C10, at 3500 lbs and it has 56% front weight there is 1960 lbs on the front. Or, 980 per side. Now, at first you may think, 980 vs. 146 isn't so bad, but that's not how it is. 980 is the Total weight. To get a % of SW vs. USW you need to take the total weight - 980 and subtract the USW - 146 to get the SW of 834 lbs. now you have a comparison of 146/834, or 17.5%. Now, as a rule of thumb 15% is the good/bad break, and 12% is the good/great line.

So, keep this in mind. Light weight wheels/tires/suspension/brakes are a big help. Also, if you are looking to really compete in some track events, 'truck' based suspension is just too heavy. Now, if you cut some USW, and then you lighten up the front, and move the motor back and change the ft/rr %, then you also reduce the front SW, and the % goes back up. It's a fight. When you get down to 800 total on the RF, you will need to be down to about 85 lbs USW to get a 12% ratio.

This little understood ratio of SW vs. USW is why shock valving is such a big topic in the USCA and Auto-X circles right now. Cars are getting lighter, and the % of USW is going up, making it harder for the shocks to control the forces in motion.

Here is a sketch to help imagine all that is moving around. Have a Happy and great Handling New Years!

[RobNoLimit]

Last night I was thinking about this some more, and I thought of a more relatable example. On this thread we are all at least interested in trucks, so it's safe to say we've all driven at least one. And, we all know the 'bouncy' feeling from the rear. This is mostly due to two things. 1) Too much spring rate. Trucks are for work, and the have to carry a load, so the springs are set to be 'happy' at appx 60% of total load capacity. At full cap. they are max'ed out, and when the truck is empty, they are 'under-loaded'. (too stiff) and 2). a high % of Un-Sprung weight.

Back to our theoretical C10. rear weight appx 1400 lbs (total weight)
Now for the Un-Sprung weight.
Rear axle, gears, brakes = 265 lbs
Wheels and tires , 2, = 140 lbs
Half of the trailing arms = 35 lbs
Half of the coil springs, shocks = 14 lbs
Half of the driveshaft = 16 lbs
Half of the panhard rod, = 6 lbs
Total unsprung weight = 476 lb

So if the total rear weight is 1400 and the Un-Sprung is 476, that leaves only 924 lbs of Sprung weight. Do a little math and you get an Un-Sprung percentage of 51% !!!!! So think about it, when the trucks back tire hits a bump (one side) and 238 of mass is now moving 'up', it's easier to 'lift' the back of the truck, 462 lbs, than it is to compress the spring.

[bovey]

Last night I was thinking about this some more, and I thought of a more relatable example. On this thread we are all at least interested in trucks, so it's safe to say we've all driven at least one. And, we all know the 'bouncy' feeling from the rear. This is mostly due to two things. 1) Too much spring rate. Trucks are for work, and the have to carry a load, so the springs are set to be 'happy' at appx 60% of total load capacity. At full cap. they are max'ed out, and when the truck is empty, they are 'under-loaded'. (too stiff) and 2). a high % of Un-Sprung weight.

Back to our theoretical C10. rear weight appx 1400 lbs (total weight)
Now for the Un-Sprung weight.
Rear axle, gears, brakes = 265 lbs
Wheels and tires , 2, = 140 lbs
Half of the trailing arms = 35 lbs
Half of the coil springs, shocks = 14 lbs
Half of the driveshaft = 16 lbs
Half of the panhard rod, = 6 lbs
Total unsprung weight = 476 lb

So if the total rear weight is 1400 and the Un-Sprung is 476, that leaves only 924 lbs of Sprung weight. Do a little math and you get an Un-Sprung percentage of 51% !!!!! So think about it, when the trucks back tire hits a bump (one side) and 238 of mass is now moving 'up', it's easier to 'lift' the back of the truck, 462 lbs, than it is to compress the spring.

This is great information. The number times I have people ask me about how to deal with the "light" rear of a truck is amazing. People refuse to believe that the lightness is due to spring rate, not physical weight.

I also want to dig up some info I have in a book at home that I believe would be an interesting add to this thread, it talks about sprung vs unsprung weight vs. rotational weight and how they are not created equal when making a vehicle lighter. Rob, your information about USW being 12% is neat, I did not know that. Thanks for the great intel.