TRACTION by Sid Hirst & Trials stories 2001 by Keith Mitchell







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FROM THE MUD by Keith Mitchell

Whilst the Muddies slumber on through Summer and their favourite damp places are devoid of moisture I thought I'd catch up with Sidney Hirst over in New Zealand (Via phone and Email) and see if he had any words of wisdom for us Aussies that were printable.

Sid, as always rose to the occasion and his article is very interesting reading whether you are a past or present OST protagonist or if your vehicle of choice is an Austin 7, modern or special built trials car.

With a contingency of up to 12 New Zealanders and 4 cars crossing the Tasman in August to compete in the ANZ Cup and John Pryce Trial we also see some pre season activity amongst the ranks to improve the competetiveness of some cars.

Dave Milkovic has paid a visit to Fundy Autos for a complete makeover of his rear end. Not only should tis make Dave more desirable amongst the womenfolk, but also his new diff assembly should have him climbing out of never before dreamed of slippy situations. Another to receive Freds expertise is Robert McBain who has also had the wand waved over the rear of the car plus a new powerplant installed up the front.

TRACTION by Sidney Hirst


Traction is the Attraction in Sporting Trials to me. The sport should be about gaining as much traction as possible from your vehicle within the allowed rules. There is also another part of the equation that I term "Getability", this is the ability to get from point A to point B using minimum traction. If all this sounds confusing read on. All this concerns 2WD vehicles and may not necessarily be true with 4WDs.

Things affecting Traction and Getability.

  1. Tires
  2. Tire pressures
  3. Ground conditions
  4. Gearing
  5. Weight on driving wheels in this case rear.
  6. Weight bias
  7. Total car weight
  8. Angle of hill 45 ° = half as much psi of tire forcing into the ground.
  9. Suspension eg, shocks, springs
  10. Suspension locating setup, also independent suspension. Including front wheel isolation.
  11. Engine torque
  12. Driver control ,
  13. Bouncer control


The reason I will start off with tires is that I think this is the most important item. Forget about trying modified suspensions, modified engines etc. until you have a good set of tires. At the moment the Hankook 884 seem as good as any and are reasonably cheap. However the sharp edges do wear off and this can make quite a difference. A new tire doesn't perform to it's full potential until it has done a couple of trials to help increase the flexibility.


As you all know the lower the tire pressure the better the traction, or is it? Sometimes I feel if you go below 3psi you can lose a bit of traction. Two reason, one being that the tire tends to take a concave shape on the ground when the tire pressures are low the centre of the tire is giving very little bite. The second is if you reduce the tire pressure it requires more horse power to spin the wheel fast to dig down to the hard stuff. If you are lacking in the HP department it may be an advantage to increase the pressure to get more wheel speed to dig down to the hard. Also by reducing the tire pressure you are reducing your ground clearance under the diff.


Ground conditions:

You may think this is the same for every one and it is if every one drove in the same wheel tracks and conditions didn't change from one vehicle to another. Some times it is an advantage to take the steeper line in a section if the ground conditions are more tractable. I can remember watching an English video and every one was struggling on this particular section, Julian Fack came through on a different line (although steeper) he made it look like a Sunday drive.

Driving in mud requires a different approach, you have to decide whether the ground is had enough to drive over the top in which case you would probably choose a different line if it looked better. Normally in mud it is better to follow the same line as the person in front if they get through. If you do follow the same line you must not fight the steering wheel, let it follow the wheel tracks the easiest way and in mud if you decide to go for big revs do it as quick as possible and make it rev as high as possible. These two things are most important, if you accelerate quick it tends to throw weight onto the rear wheels and also they will dig straight down to the hard stuff to get traction. The only exception to this rule is if there is no hard bottom it helps to pump the accelerator to get the car to stay on the top of the surface especially if it is swamp. Murray Bridger proved this point at the challenge against Australia at the swamp at Robert George's place.


Probably 65% of the sections I do would be done in 2nd gear which is about 32:1. The reason for such a low gear is not because it hasn't got the power in a higher gear but I can go slow enough to prevent the wheels hopping off the ground. Also because you are going slower it requires less horse power thus requiring less traction to keep forward momentum. So with this you may think why not use 1st gear ? First gear is usually too slow to get the car over even the smallest little rut and you generally get more traction is 2nd. Many years ago I read a test of tractors in a farming magazine, they hitched a rope on and tested how much they could pull, the interesting thing was they would all pull the most in the highest gear possible without stalling. 1st gear can be used at times where there are tight turns, by building up a few revs then reducing the revs suddenly at the same time the side brake is applied you can generally turn sharper by dragging the front around.

Personally I would like to be able use 3rd gear more 22:1 but I would require an engine that would pull down at very low revs. In 3rd you can generally get enough wheel speed to get going once it starts spinning in mud but 2nd is too low for this.

Generally in mud you use the highest gear possible, but keep in mind you are better using a lower gear at full revs with more wheel speed than a high gear that is too high for the power of the motor.


The more weight on the rear wheels the more traction you have. But just because you have more weight on the rear wheels doesn't necessarily mean you will go further. I often see a car stuck in 100 mm or deeper mud and some one will stand on the back to give it more traction it never makes any difference. The reason for this is that the car is trying to lift the rear wheels out of the hole it has dug and by adding more weight on the rear only makes it worse. I will delve into this a bit more latter on.


This is one of the most important things other than tires that will affect the Getability (the ability to get from point A to Point B) of a trials car. But this is one thing that is very difficult to change within the allowed rules. By fitting a rear transaxle does improve the weight bias because you are in effect moving the gearbox from in front of the occupants to behind them. In my opinion we should have about 75% of the weight on the rear wheels with driver and bouncer but my car as well as most of the others only have about 71% on the rear wheels. If the weight bias is increased in the rear it is very important that the centre of gravity is kept as low as possible to prevent the car from rearing over backwards. Rather than add weight to the rear it is far better to reduce it on the front if possible. In my opinion any added weight behind the rear axle is a disadvantage as it makes it more difficult for the rear wheels to climb up banks etc. This is because the inertia required to lift the weight behind the rear wheels when climbing a bank will tend to lift the front at the same time, so it gets to a point where the complete weight of the vehicle is trying to climb the bank all at once.