Wheel and Tyre Fitment: Balance
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Balance
05.05
| 00:00 | The geometry and structure of a wheel is never going to be perfect, and these imperfections are going to result in imbalances. |
| 00:08 | What this means is that the weight is not perfectly distributed around the wheel. |
| 00:12 | So, when rotating at high speeds, this will result in variations in the force from rotating the wheel. |
| 00:19 | Generally speaking, the worse the imbalance or the higher the speed, the worse this will get. |
| 00:24 | It will also increase significantly at a speed that coincides with the natural frequency, but this will be different for each case. |
| 00:33 | One of the root causes for this is what's referred to as runout. |
| 00:37 | Radial runout is where the wheel is out of round, resulting in variation in forces up and down. |
| 00:44 | Lateral runout is where the wheel isn't straight on the plane it rotates on, and results in variation in the force from side to side. |
| 00:53 | Aside from runout, imbalance is caused by heavy and light spots, simply as a result of the manufacturing process. |
| 01:01 | A light or heavy spot on the circumference of the wheel is referred to as static imbalance, and will cause a similar up and down force to radial runout. |
| 01:11 | We also have dynamic imbalance to deal with, which occurs when the balance on each side of the wheel centreline is different. |
| 01:19 | Like lateral runout, this causes sideways forces. |
| 01:22 | The outcome of these forces is vibrations that the driver will feel through the chassis and the steering wheel. |
| 01:28 | Besides being uncomfortable, these can be transmitted through the tyre and disrupt the contact patch, causing grip and handling issues, and also introducing noise to the steering and braking feel, taking away from the driver's control. |
| 01:44 | Further to this, it can also shorten the life of tyres, shock absorbers, bearings and bushings. |
| 01:50 | This discussion is specifically around wheel imbalance, and what's important to understand regarding the wheel. |
| 01:57 | But let's deviate for a moment, as the tyre will also present imbalance issues, and in a real situation, they're always going to be used together, so need to be considered as an assembly. |
| 02:09 | In saying that, it's always ideal if both the wheel and tyres are well balanced individually as well. |
| 02:16 | How well balanced a wheel is when new comes down to the manufacturing process, where more advanced methods and increased quality control will generally result in a better product. |
| 02:27 | For example, the use of CNC machining with forged wheels produces a much more accurate geometry than a simple casting process. |
| 02:36 | The design of the wheel shouldn't play too much of a role here, like the number of spokes for example, simply because the design follows a consistent pattern and the mass is distributed evenly around the wheel centreline. |
| 02:50 | Obviously, if the wheel is well used and has been damaged, this can cause imbalances, but that should be expected. |
| 02:57 | If we have wheels that are as good as possible from factory and in good condition, then this forms a solid foundation. |
| 03:04 | Think of it like building a house on a flat and level pad, as opposed to a patch of potholes and undulations. |
| 03:11 | One is clearly going to be a better starting point and give a better result. |
| 03:16 | This is important in order to minimise the need for balancing the assembly together when the tyre is fitted. |
| 03:23 | On road cars this is almost always done when fitting the tyre and we use small weights that stick or fix to the barrel. |
| 03:30 | We also use a tyre balancing machine that can measure the static and dynamic imbalance to achieve this. |
| 03:36 | In race applications though, it's not uncommon for the tyre to slip on the bead and rotate relative to the wheel. |
| 03:44 | For example, if we mark the tyre and the wheel with a straight line in the radial direction and then go out for a session, when we come back, the line on the wheel and tyre are often no longer in alignment. |
| 03:57 | So, if we balance the assembly like we would for a road car, then we've probably made the imbalance worse by adding extra weight to parts of the wheel that are no longer in the correct location. |
| 04:09 | Adding to this, rubber pick up from the track in a randomly distributed manner can further imbalance the wheel. |
| 04:16 | This is why it's even more important for the wheel to be as close to perfect balance as possible for a race car, because we can't use balance weights as a bandaid. |
| 04:26 | It's also worth noting that a quality race tyre will usually be balanced better than a road tyre anyway, but again, our focus is wheels. |
| 04:34 | In summary, wheel imbalances are caused by run out and uneven weight distribution, and this can lead to vibrations that affect the handling, control and lifespan of components. |
| 04:46 | It's important that the wheel is as well balanced as possible, as in motorsport applications, we won't always be able to use balancing weights to bandage the issue. |
| 04:55 | Like most things, this comes down to the manufacturing method and the quality of the process, as well as a good design. |
