Wheel and Tyre Fitment: Track Width
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Track Width
04.55
| 00:00 | Track width is the distance between the centrelines of the wheels on the same axle. |
| 00:05 | And we have individual front and rear track widths. |
| 00:08 | And these don't need to be the same, in fact they're usually not. |
| 00:12 | In the wheel offset module we briefly mentioned the effect this has on track width, as changes to the offset are directly translated to the movement of the centreline of the wheel. |
| 00:23 | On that note, changes in the width of the wheel won't change the track width if the offset remains the same. |
| 00:30 | As again, this is what determines the location of the centreline of the wheel. |
| 00:35 | Our discussion in this module will be around changing the track width through the offset of the wheels, or the use of spacers, rather than changing the suspension itself to move the hub. |
| 00:47 | So, how does track width factor into vehicles performance? It's all about lateral load transfer. |
| 00:54 | While we'll look into the basics of load transfer here, for a much deeper understanding of what we're about to discuss, I recommend checking out the HPA Suspension Tuning and Optimisation course, which we've linked below this module. |
| 01:09 | So, in basic terms, lateral load transfer consists of two types, unsprung and suspended. |
| 01:16 | Track width factors directly into the unsprung mass calculation, where increasing it will lower the load transfer. |
| 01:24 | Widening the track width also raises the roll centre slightly, all other things unchanged. |
| 01:29 | Since the centre of gravity height will be above this in almost every case where the suspension has been designed properly, this will reduce the roll moment. |
| 01:39 | The result of this is a reduction in the suspended mass lateral load transfer as well, and all of this means more grip due to a phenomenon called tyre load sensitivity. |
| 01:50 | So, if there are changes to the track width on one end of the car relative to the other, this will change the lateral load transfer distribution, or in simple terms, the balance of the vehicle. |
| 02:02 | Widening the front track width relative to the rear will move towards oversteer, and widening the rear relative to the front will tend towards understeer. |
| 02:12 | If we're making changes based on aesthetics to fill out our guards, we need to be aware that we could potentially worsen the handling of the car, making it push or even be unstable. |
| 02:23 | But if we do understand this, then it offers a valuable tuning tool, where we can make small changes to help with performance that might be more practical than changing the suspension setup. |
| 02:35 | So, if a wider track increases grip, why wouldn't we just want to make the track width extremely wide? Assuming of course there's room to do this with clearance to the guards. |
| 02:45 | There are a few different reasons why increasing track width with wheel offset is a bad thing. |
| 02:51 | First, it should be mentioned that increasing the track width can decrease the motion ratio of the suspension to wheel movement. |
| 02:59 | In other words, for that same wheel travel, the suspension spring will move less. |
| 03:04 | This reduces the stiffness and control of our suspension, but in reality this is more academic than practical, as the change will probably be negligible. |
| 03:13 | It will however put more leverage on components like wheel bearings and bushings, shortening their life and leading to more expensive maintenance. |
| 03:22 | If we need to make our bodywork wider to accommodate an increase in track width, which is often the case for large changes, this will increase the frontal area, leading to more drag, all other things being equal. |
| 03:35 | The wheelbase to track width ratio would also be changed, where a wider track width with the same wheelbase will mean a lower ratio, which would make the vehicle more unstable. |
| 03:47 | Typically around 1.6 to 1 works well for most sports cars, but this really depends on a lot of other factors like speed and aero, where we just need to understand that widening the track width will tend towards a more unstable vehicle. |
| 04:03 | If we change the track width significantly on the front axle, this will also completely change the steering feel and handling of the vehicle. |
| 04:11 | This leads into scrub radius, which warrants its own module coming up next, so we'll leave it at that for now and recap the main points. |
| 04:19 | Changes to the offset of wheels will move the distance between their centrelines, which is referred to as track width. |
| 04:26 | A wider track width will decrease lateral load transfer, all other things being equal and since this increases grip, it can also change the load transfer distribution between the front and rear axle and therefore the balance of the car. |
| 04:41 | For the same reason though, track width changes should be made with caution and excessive track width can cause a range of issues around handling and accelerated wear on components. |
