Tech Time – Leverage

By Kev Rooney

Note – This was originally published in the UKK’s Gazette and is reproduced here with their permission.

According to scientists and historians the understanding of the principles of leverage and ratios have been around since the dawn of time. How else can they explain moving vast lumps of stone over long distances to build monuments like Stonehenge or the Pyramids, unless they had help from little green men?

Leverage and ratios are used in many places on cars i.e. steering, foot brakes, clutch, handbrakes etc. and their influence in transferring components from one vehicle to another is often overlooked. To understand the basic principle of leverage look no further than that mainstay of the kiddies playground, the seesaw. It's a long board with a pivot in its centre, also known as a fulcrum point. Put weight on one end with nothing on the other and that end will lower. Put the same weight on both ends and the board will centralise neither up nor down. If you place a normal 5-year-old on one end and an enthusiastic adult on the other, you have the beginnings of your own space exploration programme! 

What's any of this got to do with cars? Well first we need to understand the principle of ratios. Using the seesaw as our example, no matter how long the board is, providing the pivot or fulcrum point remains in the middle, the ratio will always be 1:1. To get this ratio you divide the distance from the fulcrum point to point of load in one direction, by the distance from the fulcrum point to point of load in the other direction. The answer will always be one.

However, by moving the fulcrum point we alter this ratio and also gain a mechanical advantage. Say our seesaw is 12 feet long with the previous example. The fulcrum point is in the middle and therefore 6 ¸ 6 = 1. Moving the fulcrum point say 2 feet in one direction produces a reading of 8 ¸ 4 = 2 (that's a ratio of 2:1) or by moving it 4 feet in one direction, again from our original example, gives 10 ¸ 2 = 5:1.

Whilst to maintain the seesaw in equilibrium (both ends off the ground) at its original length, the same weight at either end is required, once you introduce ratios the weight alters on one end, depending on the ratio. If the weight on both ends was 5 stone in example one, then in example 2 only 2˝ stone is required at the long end to maintain equilibrium, (5 stone at 2:1 ratio). And in example 3 only 1 stone is required to achieve the same result (5 stone at 5:1 ratio).

Now instead of weight we introduce the concept of force being used and multiplied by the ratio effect. This is how we apply this principle to both the foot and handbrake system of the car. As we've covered in past issues many different factors affect brake selection, but if you've used your brains (and those of original equipment suppliers) you'll find that a ratio of between 4:1 and 5:1 works best on foot brakes. Too far either way will result in either a pedal that's far too sensitive or one that requires the use of both feet on the pedal to produce any slowing effort (reminds me of a drum braked 70s Firebird I once drove!).

Generally this ratio won't need altering but if you channel or section your motor bear this in mind before cutting pedals about. If the pedals are too long once sectioned/channelled, it's better to recess the floor to allow for full length pedals to remain (and if we're talking about manuals here the clutch effort required can be enormous, making for a very tiring drive).

Transferring handbrakes can lead to MoT problems with either insufficient or excessive travel. Remember it's 5 clicks maximum travel when your rear brakes are adjusted correctly. As a starting point to get correct leverage, look at the handbrake that came with the rear axle/brake set up and use/duplicate that. Remember that the rivet that the handbrake rotates around is your fulcrum point. If you've installed a brake and no matter how hard you try you can't get that first click, then the hole connecting your cable is too far from the rivet. On the other hand if the handbrake button is pointing at the roof with no braking effort, then the distance is too small.

The same principle of leverage and ratios also explains why when fitting a smaller steering wheel, the effort to move goes up but the reaction time speeds up as well. Ignore the fact you have a round wheel, the point where the nut fits is the fulcrum point and the width of the steering wheel is the length of the seesaw, but in this application all the load is applied on one side of the radius, converted to radial movement by the box/rack and back to leverage by the steering gear at the bottom of the column.

So using the same principles that our forebears used, we too can build large monuments to be wondered about by future generations, i.e. how did they ever get that 2-ton Cadillac to stop, steer and park on a 3:1 hill?

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