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Awesome Mack. You gotta love this type of mod!


Truck vs. Tractor braking. Trucks have a SBM (Spring Brake Modulator) Inversion valve. Given most trucks don't pull a trailer, the brake system has to function incase the primary (Drive) brake system fails. Tractors rely on the trailer to help stop the vehicle if this condition occurs, therefore not requiring the SBM.


If the primary fails, this valve relies on the secondary (Steer) for air supply, and uses the spring (park) brakes to slow/stop the vehicle. It keeps the spring brake charged, then depending how much the brake pedal is depressed in the cab, proportionally releases air out of the chamber so the spring actuates the foundations. When the pedal is released, air is applied caging the spring, allowing the truck to roll for a few brake applications. Of course, the intent is to allow enough air to move the chassis to the side of the road.


When shops convert tractors into a straight truck (mostly dumps), it's imperative this brake system be adapted. If not, when the primary fails, the spring brakes will lock up the drive axle/axles. Some OEM's utilize spring brakes on one axle (tandem tractor). Therefore with one axle, it's imperative that a second set of spring brakes be added, which is STD on all trucks.


FMVSS 121 requires a fully truck to park on a 20% grade up/down for five minutes without moving. If lift axles are installed, they must be raised during this event, therefore the generally used 30/30 cans be changed to 30/36.


If purchasing a converted tractor (check the VIN#), you might verify it meets federal requirements. ESP/ESC is required on most all tractors, not trucks. If this system remains on the modified truck, it won't meet the design criteria.

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Gary's Pete will cause a stir at the pump. One awesome piece of equipment.


Fuel tanks are rated at useable, or volume gallons. There is no standard. Regardless, tanks can only be filled to 95% to allow for expansion as hot fuel is returned from the engine. FMCSA 393.67 identifies the requirements.


Couple Examples:



Diesel fuel crossover, return, and withdrawal lines which extend below the bottom of the tank or sump must be protected against damage from impact.



For diesel-fueled vehicles, the fill pipe and vents of a fuel tank having a capacity of more than 94.75 L (25 gallons) of fuel must permit filling the tank with fuel at a rate of at least 75.8 L/m (20 gallons per minute) without fuel spillage.



Use a calculator. Example, a 25" diameter tank would be 56.5" long, for 120 gallon (volume). Useable, would be 10% less or 108 gallons. 5% on top for expansion, and 5% on the bottom for contamination.



If your 25" diameter tank is 62.5" long by volume, that would be 133 gallons (X .90), or 120 gallon useable. Both are identified as a 120 gallon tank.


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These wide tires can be referred to as floaters. In this case, it's all about spreading the load out.


Some drivers are also referred to as floaters. Those with manual transmissions who do not use the clutch, aside from starting/stopped.


What happens to the disc/s torsion springs during launch.... they compress. When do they return to the natural length? When (IF) the clutch is depressed.


Unless you depress the clutch, at least once, the springs in the clutch remain compressed. It doesn't matter when you use the clutch pedal, but the sooner after gear selection the better.


If this is not your practice, eventually the springs become frozen at shorter length set and fall out.

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