850hp and 34" suspension: The Recoil videos

[Whitey Shockie Maker of the Month Award Find all posts View Profile Close Whitey]

Hey all,

Some great (slightly peculiar in some aspects) videos of B.J. Baldwin in his incredible Trophy Truck.

The jumps and the wierdness just get bigger and bigger with each video!

Recoil 1;

https://www.youtube.com/watch?v=Mmh-ew1swD4

Recoil 2;

https://www.youtube.com/watch?v=JXMyZ929lpY

Recoil 3;

https://www.youtube.com/watch?v=wM0qmMb3sCg

Enjoy with the sound right up!!

Best

Mark

[Brodad Junior Member Find all posts View Profile Close Brodad]

Video #2 I'm having trouble deciding if I'd rather be in the truck or in the helicopter on the way to the beach! All in all that is freakin awesome!

[r4ndll Avid PP Poster! Find all posts View Profile Close r4ndll]

I often imagine that is how I drive around in my Prado

[Whitey Shockie Maker of the Month Award Find all posts View Profile Close Whitey]

Buckle up for awsum, Recoil 4;

https://www.youtube.com/watch?v=8D94uZPKQcA

[drwormy Junior Member Find all posts View Profile Close drwormy]

Go on Mark... tell us all about the details of the suspension in a trophy truck - why can they take jumps so well?

[rorza Member Find all posts View Profile Close rorza]

In the same vein as the Recoil vids, bit of an in depth look at a prerunner style rig and a few revs and burnouts for extra measure, such a sweet sounding rig.

https://www.youtube.com/watch?v=oCSJNmRSYac

[Whitey Shockie Maker of the Month Award Find all posts View Profile Close Whitey]

Go on Mark... tell us all about the details of the suspension in a trophy truck - why can they take jumps so well?

Hey drwormy,

I’ll give it a crack!

Designing Trophy Truck (TT) suspension follows the same set of rules for designing any suspension type. You need to know what natural frequencies you will run (which will dictate the coil rate you use), and what shaft velocities and Forces you need to dampen. For a Trophy Truck, these are at the limits of vehicular suspension design.

There are also several other important suspension factors to consider when designing Trophy Truck design.

1. For TT shock absorbers, the Force vs displacement curve is just as important as the Force vs velocity curve. This is because of the high g-load impact Forces experienced by Trophy Trucks as they jump and land. For a linear piston head, the F vs x diagram is parabolic, and the damping dies away at either end of the shaft stroke. This is insufficient for TT’s, as the Forces experienced during bumpout conditions are enormous, and there must be extra damping at the bump/rebound travel shaft limit to prevent damage. A square shape can be used on the compression side of the F vs x diagram by adjusting the bypass tube for that part of the stroke, or with the addition of hydraulic bumpstops.

2. As the impact Forces are extreme, there is typically no rubber bushings or bumpstops in a TT. Steel heims are used everywhere to sustain the dynamic loading. Hydraulic bumpstops are also used, which can add even more Force to the compression side of the F vs x diagram.

3. As the wheel travel is so high on a TT, potentially over 36”, this requires that mid-ranged motion ratios must be used to allow the use of shorter shaft lengths which would be prone to buckling otherwise.

4. As the damping Forces required are so high, only linear piston heads can be used. Due to the inherent mid-range motion ratios used, this means that the amount of low velocity critical damping in TT shocks is typically low. You can easily see this by watching a TT through a corner, there is quite a lot of body roll. After landing, you will see the suspension cycle at least twice and a large rebound heave mode can typically be observed. Low speed kinematics are not important for TT’s when you are driving through 3 foot deep whoops at 150km/h.

5. When choosing natural frequencies for a TT, lower frequencies are normally used < 1.0Hz. For a typical road vehicle, coils would be chosen which give the desired ride height. This is also applicable for TT’s, however, a second important factor is the amount of stored energy in the coil. The less stored energy, the more damping is required in dominant heave modes. As such TT coils must consider both ride height and stored energy, and as such they are a typically lower wheel rate. Rally cars also use a similar principle.

6. Multiple shocks are used on every corner of TT’s. This is because one shock is used as a coilover assembly which holds the TT up, while the second shock controls the damping. The second shock is typically a multiple bypass shock, which allows fine tuning of the damping over every part of the shaft stroke. The coilover shock generally does not dampen.

The starting point for designing TT suspension is then the maximum shaft velocity, typically around 300”/sec, or 7.62m/s (velocity quoted from a King Shock engineer). This is the velocity the shaft will reach on impact after jumping a TT. Once you know the maximum shaft velocity, everything else in the suspension can be designed around it.

I’ll run through a basic calculation with some diagrams to show what g-load impact Force a velocity of 7.62m/s corresponds to. The load will be assumed as entirely vertical. I’ll use the Camburg Kinetic TT as an example, as Camburg have some fantastic photographs and technical drawings of their turn-key Trophy Truck, which can be had for a cool 250K USD. The chassis is shown below;



The front suspension is a double wishbone with 3.0” coilovers and 4.5” bypass, stroking 16” for 27” wheel travel, giving an approximate motion ratio of 16/27 = 0.59, not accounting for shock angle (which is just a few percent).



The rear utilises a 4-link trailing arm setup, with 3.0” coilovers and 4.5” bypass, stroking 18” on the bypass for 32” wheel travel. This gives a rear motion ratio of ca. 18/32 = 0.56. Note the hydraulic bumpstop in the right of the image.



Using the front and rear motion ratios of 0.59 and 0.56, you can calculate what the rebound impact Forces after landing are which through action/reaction produce the rebound heave mode on the sprung mass for a 7.62m/s velocity. This needs to take into account the stored energy in the coil and the amount of suspension travel and work expended. Similarly, you can calculate the compression Force required for a hard impulse impact on the unsprung mass which produces a 7.62m/s velocity. These types of impact Forces are extreme and occur very quickly on a ca. 0.1sec timescale, producing strong acceleration up to 300m/s2. To do this calculation requires a couple of assumptions on some missing details, such as the coil rates and coil travel, both of which must be transformed to the wheel position, and the tire rates. I also need to make an educated guess about the unsprung mass. Rear trussed diff/floating axle can be 800lb or around 360kg, 180kg per corner. Front more like 450lb or around 100kg per corner. Kerb weight is 5200lb or around 2360kg. It’s difficult to interpolate the coil dimensions off the photos, but it’s something like 700lb/in and around 8” of coil travel at shock position. These figures can be used as a starting guide to calculate the type of valving Forces required, as shown in the spreadsheet below;



The bump and heave modes both calculate the amount of stored or released energy in the coil equivalent rate at wheel position. The final valving curves are shown below at shock and wheel position;



You can see that at the large shaft velocities of around 7.5m/s, the Forces at the shock position are around 95kN. The equivalent vertical tire load to produce this is around 9g (nine times the corner weight). These Forces are around 5 times the maximum load you’d see on the bottom strut bush on our Prados. The critical damping for this type of valving is around C/Ccr = 0.07 at the wheel position, not a heap of critical damping, less than 10%, but like I said previously you don’t need it at 150km/h through 3 foot deep whoops.

Keep in mind this type of calculation is really only a starting point, but it might be a good enough valving baseline from which to tune in from with the bypass shocks. This calculation at least gives you an idea of the huge Forces that the dampers must soak up during high speed Trophy Truck whoops driving. The frequencies used are also probably too high for the amount of stored energy in the coils, but with lower frequencies and less stored coil energy the valving will necessarily increase a little. I’ve also used the full wheel travel for this calculation, but the coils won’t use that much via the motion ratio. With less coil travel, the valving will again increase.

If you’re reading this and wondering where our Prados fit in this valving landscape, it’s highly unlikely you’ll see shaft velocities over 1.0m/s in a Prado, unless you try to jump it like a Raptor, and then you’ll be parking it at the wreckers with a bent chassis!

Best

Mark

[Joe Lighty Member Find all posts View Profile Close Joe Lighty]

Whitey - insufficient detail...
Jokes - of course - enjoyed the read!

[Whitey Shockie Maker of the Month Award Find all posts View Profile Close Whitey]

Hey all,

Some great videos of the Baja1000 winning Camburg Trophy Truck;

https://www.youtube.com/watch?v=Z8cT7enz_ps

https://www.youtube.com/watch?v=LXRj3qKs9ps

https://www.youtube.com/watch?v=hSWHaL_Hmj8

https://www.youtube.com/watch?v=jpORZdXRGig

Best

Mark