Posted by: drracing | November 13, 2011

Camber and rolling resistance

Here we are…back again! I am always in lack of time to write here and i always have a lot of thing in my head i would like to talk about.

This time, i am writing about a topic which is really not a major one, i have to admit. But some time ago, reading some comments on a forum about this topic i decided i wanted to finally understand about the real answer for a real tire to this question: adding negative camber produces more or less rolling resistance than a more vertical wheel setting? So i decided to look on some real data to find a “reliable” answer (if one really exists).

It is clear when we add camber on our wheels we are normally looking for other major results. The most important target we often try to achieve is an increase of cornering force through the contribution of camber trust. But what is happening with rolling resistance?

To answer this question, as i said, i studied some raw data i had from a radial slick tire. It is actually a small section tire, tested with low vertical loads (it was intended to be used on low weight cars) acting on it at different camber angles. Also tire pressure was quite low, so it was not exactly the case i had encountered on the cars i normally work on but it was the only data like that i had from a radial tire and i though it was worth to analyze its behaviour to understand at least the trend with which rolling resitance changes when changing camber angle.

Of course, because of the test conditions described above, i guess the results are probably not true in an absolute way. Each tire could behave in a different way beacuse of its construction, its inflation pressure and the vertical loads acting on it, just to list only some the parameters that could have an influence on the results. But, at the same time, i think some of the things i saw could be considered true for probably the most of the tires, at least radial ones.

The first thing you could see looking to this test results was that the thing influencing more rolling resistance is (obviously) vertical load. Defining a coefficient like Fx/Fz (in certain way something similar to friction coefficient) to understand how Fz makes rolling resistance to change at different loads, you could see that this coefficient has lower values when load is increased.  In this sense, this tendency could be easily predicted because is the same trend that friction coefficient normally have with all the tires in the world and it is also one of the most important things that make a car tunable with spring and anti roll bars.

The other interested trend, anyway, is connected to how rolling resistance changes with camber for a given vertical load. In this sense, you could see a very slight increasing in Fx as camber became “more negative” (in other terms, Fx/Fz is increasing as you add negative camber).

So yes! Adding camber seems to produce a bigger rolling resistance. At least with THIS tire i was studying. This tendency is clear at all the vertical loads at which THIS tire was tested. Anyway, i must say the difference between the rolling resistance measured at 1° and at 4° is really small. Of course the absolute value of this difference is higher at higher vertical loads, but we are anyway speaking of really minor effects. From an engineering point of view i would say that for THIS tire, rolling resistance absolute value doesn’t change in a very sensitive way with camber. If you anyway analyse the relative variation of rolling resistance with load at a given vertical load you could see differences in the region of 15%. What i mean here is that 15% of a very small number is a very small number as well and i think you would not make a big mistake if, for example in a lap simulation, you would approximate rolling resistance as a variable which is not changing with camber (Of course, this is true for THIS tire!).

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