Correcting our Rear Suspension.

Which springs to use? This is a question we didn’t dig into until it was too late. Back in 2012, we novices built our first Class 11 car from the ground up. We aren’t able to do anything once. Cage, wiring, transaxle, paint… We learned a lot on the way, some stuff we picked out of books, other things we put our faith into parts suppliers and online forums. A virtual mish-mash of information. We felt that since our car was ‘heavy-duty’ and a full-bodied, caged sedan, that we need the heaviest-dutiest springs for the suspension. We also were not sure on the tire clearance, so a spring that doesn’t protrude outboard was chosen.

We chose Sway-A-Way 1030’s – these are the shortest spring length, with largest diameter available, 30mm.

Immediately we had some issues.

First, no heavy-duty spring plates exist for short bars. Aftermarket plates that are available for this application are dimensionally restricting and have poor material condition.

We experienced very stiff handling. Even with grade 10.9 fine thread bolts, tightened to 87lb/ft, the bolt holes on the spring plates would easily key-hole. This created a scissor-condition in the geometry, where the spring plate would not stay parallel to the trailing arm flange. The scissor condition would change our rear geometry, and the rear end would end up sagging. If the car hit a few harsh bumps, the spring plates were prone to deforming. The bottoms would stretch and the tops would buckle.

During the 2016 Mint 400, the course consisted of a washboard quarry. Very small, harsh bumps were not able to cause our heavy springs to twist – like driving on a solid rear suspension. Even though we didn’t modify the rear travel, we still were not using it to its full advantage. The severe high frequency vibrations didn’t break the spring plates, but rather shook loose the radio, shock reservoirs, and cracked the rear frame horns off! This ultimately took us out of the race.

We repaired the car after the Mint and entered into the CORE Colorado 300 in the summer of 2016. While the car ran great, a driver change and a bad line trashed the spring plates, severely buckling them. This resulted in a dramatic sagging condition, which ruined two tires, cutting their sidewalls on the fenders.

Clearly the rear suspension was over-constrained.

Looking back: we chose those springs based on book information, and a VW off-road part supplier expert recommendation. We should have spent the time to do our homework, gather more information, and make an informed decision. Back to the drawing board.

We began gathering information again. We interviewed some seasoned Class 11 racers, we talked to drag racers, spoke to vehicle dynamics engineers, applied physicists. You want to know what we found out? Nothing! Well not nothing, but we were only able to scrape together a couple anecdotes on what folks are using, but nobody could explain WHY they were using those. I figure that people have been modifying and racing VW’s off-road for 50+ years, so what’s known to work, works.

But I wanted to know WHY. Our car is a little different than everyone else’s, so what will work for us specifically? We already took expert recommendations, which resulted in a car that eats spring plates and shakes itself apart. Engineers didn’t offer too much assistance, probably since this is old technology – if we were working with modern-day McPherson struts and coil springs…that’s another story.

We decided that we had to figure this out on our own.

We began by measuring. Josh was able to borrow some corner scales to get us some ballpark numbers for mass. The car seemed fairly balanced, and with the rear torsion adjuster, we could dial in some balance L/R.

I built a couple CAD models in SolidWorks that could help us understand how much the bar needed to twist, as well as improve the travel from the stock 7in to 9in. The CAD model revealed a couple issues we didn’t consider. The aftermarket spring plate was 100mm wide, meaning that there’s an additional 5mm top and bottom as compared to factory plates. Even though we notched these 100mm plates 10mm to clear the bottom stop, we didn’t really have ANY down-travel (maybe just a 10mm gap between the bottom of the spring plate and the cast iron bottom stop). The CAD also showed us that if factory we would expect 7in wheel travel stop-to-stop, that we’d also expect that would translate to about 25deg of ‘sweep’.

We then turned to the Sway-A-Way catalog information, so that we could start to look at torsion bar options.

Wait a minute. The SAW 1030’s max angle of twist is… 23.8deg. LESS than factory 25deg suspension capability. Great.

I checked some service manuals, and sure enough, I had the upper trailing arm pivot bolt shims in the wrong configuration as well! I remembered that they both belonged on one side, but that’s not the inside. Great.

To recap:

  • Incorrectly installed pivot bolts creating a geometry problem
  • No down travel
  • 10in available shock travel – but only using less than 7in with spring twist limitations
  • Not enough available twist
  • Aftermarket spring plates poor material condition
  • Stiff handling
  • Everything breaks
  • More time fixing/less time racing.

So back to the CAD calculator…

I figured, I would make a stop-to-stop graphic calculator, knowing that we are constrained by the tire OD and shock max travel. I can move the ‘tire’ up and down, which changes the numbers. Travel is in the lower left corner. Max angle the spring needs to twist is mid right, about 32 degrees needed for 9in travel.

Wait a minute. All those anecdotes told me that people were getting 10-10.5in travel, but running bars that can’t twist that much…? For example a 28mm long bar will have a max angle of twist of 32.2deg (according to SAW info and physics). For 10in of travel, you’ll need more like 34deg. This would present the same condition as my 1030’s, trying to twist them past their limit. So either folks are bragging, or they didn’t make a wiz-bang calculator like me.

So the angles were starting to make sense, we have a simple correlation between wheel travel, shock travel and the torsion bar max twist angle. But the question remained: which springs are the right springs for us?

I had to make yet another tool.

A spreadsheet which correlates all the variables, helping us make sense of all the equations and terminology. Fortunately, Sway-A-Way was able to help lend a hand in explaining a lot of these concepts. I figured that since I have to actually choose a real spring that I can buy, I included their catalog data is in the chart.

Wheel rate: This is an interesting concept. The wheel rate actually changes depending on ride height. For example if your ride height is 50% droop/50% bump the wheel rate will be lower than if the ride height is set at 30% droop/70% bump. Defining this wheel rate range for the desired ride heights is key. Wheel rate also relates to suspension frequency. This can be thought of, as the stiffness or harshness of the spring. For a passenger vehicle, you’d expect a softer ride for comfort, and for increased performance, we’d expect the suspension to stiffen. Our 1030 bars have a suspension frequency of 2.4Hz! That’s what we’d expect for a drag racing car, where we don’t want wheel hop or much suspension compression to avoid too much weight shift. We needed to dial it down under 2.0Hz. Factory 22mm long bars are closer to 1.2Hz.

Suspension Frequency

f=(1/2?) [(Kwheel/F)1/2]
<1.0 Hz, for typical passenger cars PLUSH
>1.0 ?1.5 Hz, high performance cars MEDIUM
>1.5 ?2.0 Hz, race cars only STIFF
>2.0 Hz, drag racing SUPER STIFF

According to SAW engineers, for our off-road application a wheel rate between 200 and 300 lb/in is appropriate. That’s more than DOUBLE the wheel rate of factory 22mm long bars, and nearly HALF the wheel rate of the 1030’s.

Download the Suspension Calculator here

To use the spreadsheet calculator:

  1. Enter the unsprung weight on one rear corner.
  2. Enter the distance from the center of the torsion bar to the center or the stub axle. (stock is 16.375-16.5in)
  3. Enter the active bar length – the bars are longer than the active bar length. This is the actual dimension that is twisting (the splines don’t twist).
  4. Don’t change the constants – the degrees and radians values are based on the stock trailing arm length. If you are using longer arms, you may want to recalculate the change in degrees per 1in wheel displacement.
  5. Shear modulus – the alloys that SAW are using have nearly identical shear modulus values of 12,000,000psi. This is not the same as modulus of elasticity, since the bars are in shear, not tension.
  6. Now, put in your wheel travel, remember not to exceed the capability of your car’s geometry, CV max angle, or shock travel capability.
  7. Pi – mmm pi
  8. Bar diameter – This is the trial-and-error part. You can play with different bar diameters found in the chart to see what sort of wheel rates you can kick out. You can also experiment with adding and subtracting weight, which will affect suspension frequency, as well as wheel rate. **be sure to enter a bar diameter that can twist as much as you need!** Since we are now looking for 32 degrees of twist, we need to choose a bar with at least 32+ max angle of twist. Math.

    (this is almost anything in the 300M category)

  9. Wheel rate and ride height – these are some values that help give you an idea of how wheel rate and ride height are related. As ride height increases, so does wheel rate. You can play with the variables (bar diameter, weight, travel, etc.) to plan for best and worst case wheel rate ranges. For us, we probably don’t want to exceed 28% droop if we want to stay within the upper 300lb/in limit of our wheel rate. A 35/65 ratio is common for rough courses.
  10. Preload – If your torsion bar has 30deg max twist available, and you preload it 10 degrees, you only have 20degrees of twist remaining. Choosing the spring based on wheel rate, ride height and frequency will mean we don’t need to worry about preload. We can install the spring plates angled parallel with the bottom stops, and adjust ride height and L/R balance with the center torsion adjuster. Stock 22mm springs were recommended to be preloaded over 20deg for stock ride height!
  11. Center torsion bar adjuster – I made another little calculator that will convert turns of OUR adjuster screw to degrees of twist at the spring. Since no two torsion bar springs are equal, you may find yourself adjusting one side tighter than the other for balance and ride height. If you are really cranking up your adjusters and only getting a very small change in ride height, you may need to step up to a different spring. If you can’t seem to get any down travel, your bars are probably too stiff.

Each bar is a plateau – you find the bar that fits your desired geometry and wheel rate range, and you adjust ride height and damping accordingly.

One thing to remember is that we are not able to add any sort of up-travel. The 235/75-15 tires tend to polish the fender wells on occasion. Instead, we need to add travel in the down direction. Formerly we were slightly notching the bottoms of the spring plates, allowing the spring plate to descend further without modifying the cast iron bottom stops. Now, instead of grinding the bottoms of the spring plates, we decided it would make sense to make some slight modifications to the cast iron. These modifications seem fairly common throughout Class 11.

We began with reinforcing around the lower rear bolt holes. The goal being that we could make a wide bottom-stop that would offer broad surface area. We are planning on still using limiting straps, but in the event a strap breaks, we have a bit of redundancy. The spring plate shouldn’t be hammering against the bottom-stop anyway assuming the correct torsion bars are chosen, ride height is set up properly and shocks are damping properly.

 

I was having trouble seeing through the spring plate, so I made another tool. I had some plexiglass lying around, so I cut it 90mm wide, and made a 1-7/8in hole that would slip over the spring plate collar. Using my protractor, I drew my desired max sweep angle of 32deg onto the plexiglass. All I had to do now was put the spring plate at the top-stop and pivot the plexiglass, aligning the 32deg line with the spring plate, and draw a line onto the cast iron. We only needed to remove a small amount of material to allow the spring plate to descend another 5mm, resulting in 2in travel at the tire.

I also decided that the best spring plate caps were the stock German design. The aftermarket caps seem cheap and poor quality. I reinforced the 2-ply design by TIG welding all the seams. We also installed new urethane bushings with lithium grease.

Comparison of 100mm aftermarket spring plates for 21.75in bars, stock spring plates and Sway-A-Way.

To re-recap:

  • Incorrectly installed pivot bolts creating a geometry problem – fixed – both shims are outboard now.
  • No down travel – fixed – more down travel so that we keep tires in contact with the track
  • 10in available shock travel – but only using less than 7in with spring twist limitations – fixed – by modifying bottom stops we are now using 90% of available shock travel with a little bit of buffer top and bottom.
  • Not enough available twist – fixed – using the calculator, we ordered 1227’s, which have more twist than we need. Ordered the bars pre-stressed, which increased available twist angle by about 10deg! We also don’t need to spend time setting new torsion bar springs.
  • Aftermarket spring plates poor material condition – fixed – new high quality tempered steel plates. No notching required.
  • Stiff handling – TBD!
  • Everything breaks – TBD! (but it IS off-road racing)
  • More time fixing/less time racing – this is the goal! Next will be re-tuning the rear shocks to work with the changes we made

 

A HUGE thanks to Sway-A-Way’s Eric Tice for taking the extra time to explain these concepts. It enabled us to make the various calculation tools as well as make an informed decision on choosing the right torsion bars for my application.

4 Comments

  1. Neil Snyder says:

    Wouldn’t it be better to have equal travel up and down? It seems you are saying that there is very little up travel in your setup.

    • Matt Wilson says:

      Equal travel up/down is all dependent on ride height. We may end up lifting the car a little more for a rough course (like the sharp bowling balls at the Mint 400!) 65/35 ratio, and we may reduce ride height a little bit for Baja 55/45 ratio, since the course is a little smoother. The springs we chose are suited for these ranges for suspension frequency.

      With our special adjuster, we could do the extreme and even dump the car in the rear, we’d then need to increase spring rate (stiffer springs) to avoid clunking the top stops all the time. We’d essentially be using less total twist angle, but need more lbs/deg of twist.

  2. Brando Farmer says:

    Great write-up Matt! Very interesting stuff going on here. Good luck this year.

  3. George K. says:

    Excellent article Matt. I’m glad our team was able to help you and we all really look forward to seeing this thing back in action!

    Best,
    George K.
    Sway-A-Way : Racing Technology

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