Chassis Building 101. Ride Height

[RobNoLimit]

More Napkin scratch.

When planning an IFS for a certain chassis, again, wheel size and ride height are right up there. I like to keep the bottom of the front crossmember close to being even with the bottom of the main rail (general rule) If the crossmember is much lower, it will be the first thing to hit the ground (5" dropped C10) If the crossmember is much higher, it will cause drastic eng/trans driveline angles, more than 5 degrees. So, there has to be a balance with the Tire/spindle/rack/crossmember to keep things working good. This is mostly food for thought. You also have to use whats available, as most of us don't have the budget to build 100% spec custom everything (Indy car)

[rchaskin]

Awesome thread.....

[Tincup]

While this is all good stuff, I feel we have gone off on a tangent from the original question.

[Tincup]

While researching Bumpsteer, I did come across this picture which is exactly what Rob was explaining.

[brawls43]

Back to your original question, about setting up the frame and chassis. I do like Rob said, and set the chassis level, its easier working off a level chassis to me. Then set the front spindle height and rear axle based on ride height. So I mock up some welded rods to replace the shocks, to sit at the calculated ride height. This avoids the difference in weight of a bare chassis vs a fully loaded chassis and body. Sure you could calculate the difference in height based on the weight differences, but if you're putting the engine/trans in and doing some work, then taking it out, and your weight is changing, then things seem to shift around. Just my preference.

[RobNoLimit]

I agree, setting building at RH, on a level plain is the easiest. OK, I'm not trying to lead away from the original question, just wanted to go a little farther.

The reason I started out on the IC explanation was to get to the next step (and to help you sort out some steering issues). The location of the IC on each side is what sets the Roll Center (RC). The roll center is the physical roll axis of the suspension/chassis/truck. ** see the drawing

As you RAISE the RC, the truck will lean LESS in a corner, BUT, the trade off is, it will also load the outside tire LESS. This will lead to a 'Push', or under-steer. - less sway bar is needed.
As you LOWER the RC, the truck will lean MORE, BUT it will also load the outside tire MORE, and have less tendency to 'push'. - More sway-bar is needed here.

On our normal performance minded build, a RC of 2" to 2 1/2" is a good starting point. You can manipulate the RC a bit, by raising and lowering the crossmember in the chassis. If the LCA is level, the RC will be appx 1/2 of the distance from the ball joint center or LCA pivot (as these two should be the same if the arm is level) to the ground. If you are building the rails, you have a chance to play with this a bit.

[Rod]

The reason I started out on the IC explanation was to get to the next step (and to help you sort out some steering issues). The location of the IC on each side is what sets the Roll Center (RC). The roll center is the physical roll axis of the suspension/chassis/truck. ** see the drawing

As you RAISE the RC, the truck will lean LESS in a corner, BUT, the trade off is, it will also load the outside tire LESS. This will lead to a 'Push', or under-steer. - less sway bar is needed.
As you LOWER the RC, the truck will lean MORE, BUT it will also load the outside tire MORE, and have less tendency to 'push'. - More sway-bar is needed here.

On our normal performance minded build, a RC of 2" to 2 1/2" is a good starting point. You can manipulate the RC a bit, by raising and lowering the crossmember in the chassis. If the LCA is level, the RC will be appx 1/2 of the distance from the ball joint center or LCA pivot (as these two should be the same if the arm is level) to the ground. If you are building the rails, you have a chance to play with this a bit.

now where cooking!!!

[Rod]

ummm Rob....I'm stealing your art work

[Josh@Ridetech]

Very cool. Good info, Rob!

[Red67Mustang]

Awesome thread! Very informative and useful for tuning the suspension. Thanks!

[RobNoLimit]

For most builds, the front suspension is purchased or re-used, so the shock length is somewhat set. The shock mounting for the rear suspension usually has a bit more freedom. I have two general shock rules that I try to follow.

1) Try to use the longest shock possible (within reason) - but never let the shock be the limiting factor of suspension travel (this is more important in compression, we sometimes allow the shock to be the limiter in 'droop')

2) Try to keep the shock at appx 50% travel when the suspension is at RH. (+/- 5% is well within reason)

I have seen many front suspensions that use short - 3 1/2" stroke - coil-overs, and the coil-over gets the blame for the harsh ride. Here's the quick math. At RH, a 3 1/2" stroke shock has 1.75" of compression travel. It also uses a short spring, usually 7" or so. If there is 850 lbs of the left front corner, and the LCA has a .7 to 1 motion ratio, then there is appx 1200 lbs of force compressing the coil-over at RH. If the spring was pre-loaded onto the shock 1/4", or 150 lbs., then you need a 600 lb spring to hold the car up. And, it rides pretty stiff. If you had a 5" stroke shock with a 12" spring, with 150 lbs pre-load, you would use a 450 spring. These concepts are for comfort and ride quality, But, as we lean more towards a 'soft spring - big bar' set up for performance, this is a winner in for both comfort and performance.

Stroke Compressed Extended Suggested Spring Length Price
2.9" 8.725" 11.625" 8" $250 [Purchase]
3.6 9.425 13.025 8 $250 [Purchase]
4.1 10.125 14.225 10 $250 [Purchase]
5.2 11.225 16.425 12 $250 [Purchase]
6.3 12.325 18.625 14 $250 [Purchase]
6.9 13.125 20.025 14 $250 [Purchase]

I copied this from ridetech's web site. Most shock companies will list the open and closed lengths of their shocks. I like to use their 5.2" stroke coil-over on our WideRide IFS fronts, and the 5.2" or 6.3" in the rear on most trucks. Looking at the 5.2" shock, it has a closed length of 11.225", and an open length of 16.425". The suggested RH for this shock would be 13.75 to 14" (13.825" is 1/2 of the stroke.)

Up front, if you were really going for it, starting with tire height and spindle CL, you could set the RC and crossmember height, then build the rail shape to match the crossmember up front while keeping the body at it's desired RH. Then, choose the desired shock, and fab the upper coil-over mount to set the shock in it's spec RH range.

?? does that make sense?

[Dr G]

Rob - I have read a few times now where you state not to use the shock to limit suspension travel, especially in bump/compression. Yet that is exactly how most road race cars are set up. With bump stops to tune the end of travel. I cannot speak for dirt ovals, Nascar or off road, they use different methods. Can you explain your thoughts on travel limitation? Gustave

[RobNoLimit]

"Race" is the key word here. The street can be a lot more destructive on parts. That being said, you are correct about this, with one exception. They use thick bump rubbers on the shock shaft. On the average street/performance coil-over, when the shock is bottomed out, it's a metal-to-metal hit. - and no, I don't consider the .095" O-ring to be a damper. This metal-to-metal contact can do lots of damage to the shock, shock mounts, control arms,....... The bump rubbers you mentioned are available in different durometer ratings (hardness) and are used to tune in the suspension at the end of the travel. These can be used on the street, but they may have other effects. Most of these are 1" thick or more, and fit around the shock shaft. So, you loose some of the shocks compression travel. - A 5.2" travel shock @ 50% has 2.6" of bump travel, install a medium 1" bump rubber and now you have 1.6" of free travel and maybe another 1/4" of compressing the rubber. in other words, you lost 39% of the available compression travel. Now, if you planned to do this from the very start, you may run a 6.2" stroke shock with different mounts and set it up to get the 2.6" of compression travel, and then hit the bump rubber. ** I am working with pure shock travel numbers. If the LCA has a 70% motion ratio, 1" of shock travel = 1.42" of wheel travel.**

Most race tracks are fairly smooth, all things considered. They don't have speed bumps, curbs, off camber driveways, pot holes, sink holes...... I caution against using the coil-over as a pure travel stop to help shed off impending doom.

[brawls43]

So do you use rubber bump stops on the frame to protect axle on frame hits? This happening before the shock fully compresses? What do you do in front?

[Dr G]

"Race" is the key word here. The street can be a lot more destructive on parts. That being said, you are correct about this, with one exception. They use thick bump rubbers on the shock shaft. On the average street/performance coil-over, when the shock is bottomed out, it's a metal-to-metal hit. - and no, I don't consider the .095" O-ring to be a damper. This metal-to-metal contact can do lots of damage to the shock, shock mounts, control arms,....... The bump rubbers you mentioned are available in different durometer ratings (hardness) and are used to tune in the suspension at the end of the travel. These can be used on the street, but they may have other effects. Most of these are 1" thick or more, and fit around the shock shaft. So, you loose some of the shocks compression travel. - A 5.2" travel shock @ 50% has 2.6" of bump travel, install a medium 1" bump rubber and now you have 1.6" of free travel and maybe another 1/4" of compressing the rubber. in other words, you lost 39% of the available compression travel. Now, if you planned to do this from the very start, you may run a 6.2" stroke shock with different mounts and set it up to get the 2.6" of compression travel, and then hit the bump rubber. ** I am working with pure shock travel numbers. If the LCA has a 70% motion ratio, 1" of shock travel = 1.42" of wheel travel.**

Most race tracks are fairly smooth, all things considered. They don't have speed bumps, curbs, off camber driveways, pot holes, sink holes...... I caution against using the coil-over as a pure travel stop to help shed off impending doom.

Well Rob, I figure you're gonna have to have a bump stop somewhere in the system, and that will always consume part of the "free travel", be it on the damper shaft or mounted to the frame. Assuming it is a progressive stop, not instant. Agreed, the O-rings would make a poor bump stop!

RE Suspension has a good selection of shaft mounted bump stops:

http://store.resuspension.com/home.php?cat=328

You know, now that I think about it a little more, every German car I've owned (BMW, Porsche, Audi) also has the bump stops mounted on the damper shaft. I believe putting bump stops on the frame might be unique to American cars?

Gustave

[Tincup]

Here's another basic question, boxing the chassis. Should it be completely boxed and cross bracing added, or do you want to leave "some" flex in it.

[Dr G]

No, you never "want" flex. The less flex the better. The only thing that should stop you from reducing flex is that it is adding too much weight as part of the process. The bad thing about making trucks handle is that they have a high center of gravity. But the good thing about that is that there is a ton of room between the bottom of the cab/bed and the ground to build a very stiff frame, short of adding a cage. I vote yes, box everything. Look at No Limit frames for examples of how to build an intelligent frame, stiff but not just by throwing weight at it.

[RobNoLimit]

Time teaches you lots of things. Back in the '80's we were boxing chassis end to end. (mostly 53-56 F100 and 55-59 chev) These were almost all 'ladder' style frames. This lesson took 6 or 7 years to learn. My own 56 chev p/u (camaro clip and ladder bars, BBC, Th400) started to crack at the joint areas for the crossmembers. - Now it may be true that I put my truck through harder paces than most, but there was still a problem looming. By this time I was in college, learning about harmonics, stress loads and vector forces..... It all made perfect sense. The Ladder style frames, especially the older ones that came with leafs and beam axles, were DESIGNED to flex. Bad roads, heavy loads and cost dynamics forced this. The ladder frame design is basically two dimensional - length - width, not much height. So, you can make it stiffer and stronger, but if you take out the rail flex, it will crack. Now a days, we box the front, from the firewall/front cab mount area, forward to the front core support, and the rear section, about three ft. from the rear shock crossmember forward over the frame kick, and up to the suspension link mounting area (4-link or 3-link). Then we work on creating an 'X' member in the center of the chassis.

[bergers59]

Why is it that longer shocks are better? I've seen it in a few builds where the shocks are at least 10" and always wondered why.
Also, it's threads like these that keep me coming to this site for the past few years, so thank you.

[Dr G]

Time teaches you lots of things. My own 56 chev p/u (camaro clip and ladder bars, BBC, Th400) started to crack at the joint areas for the crossmembers. - Now it may be true that I put my truck through harder paces than most, but there was still a problem looming. By this time I was in college, learning about harmonics, stress loads and vector forces..... It all made perfect sense. The Ladder style frames, especially the older ones that came with leafs and beam axles, were DESIGNED to flex.

Well I cannot argue with your experience Rob. I guess the boxing creates stress risers at the cross members.

I do know that my current frame is as you suggest, stock with boxing up front and out back. And I hate it. It is so flexible. I guess that's why you sell custom frames right! Gustave