4 replies to this topic

[Bluehair]

When cars corner, the inside set of tires follows an arc with a tighter radius than the outside set of tires. Because of this, I was wondering if the front tires are set to point slightly "toed out". This would point the inner tire in the direction of a tighter radius. Aerodynamically, the opposite (toe-in) seems like it would be desired. Of course anything other than parallel (straight ahead) would cause a certain increase in tire wear, which could be ignored in qualifying conditions.

Likewise, what about the rear of the car? Again, the inside tire follows an arc with a tighter radius than the outer tire. If we assume 70% of the traction comes from the outside tire and 30% from the inside (these are purely hypothetical estimates), then wouldn't it be okay to give up a little more traction from the inside in exchange for an equivalent gain on the outside? Let's assume a hypothetical gain/loss of 3% for an optimum toe-out setting. Then the outside rear gains 3% of the 70% total while the inner rear loses 3% of only 30% total for a net gain of 1.2%. When viewed from above, toe-out seems to have aerodynamic benefits at the rear of the car to help the air converge. Straight line handling may be horrible, and tire wear may suffer as well as straight line traction, so again, the trade off may not be worth the trouble. Does anyone know what the typical toe-in/toe-out settings are at the front and rear of the cars?

[tak]

The notion that the inside tire in a corner needs to turn more than the outside tire is absolutely correct. Adjustment of the physical steering geometry can do exactly that--it's called 'Ackerman'. 100% ackerman geometry means that the inside and outside tires are tangent to their respective turning radius. 0% ackerman means that both front tires remain parallel to each other. Anti-ackerman describes a condition where the outside wheel turns MORE than the inside wheel (useful on banked ovals). According to Carrol Smith's "Tune to Win" book, most race cars run 30% to 60% ackerman. Since the outside wheel carries more load, it runs a higher slip angle, so race cars do not need 100% ackerman.

As to your question on toe in, it is an important adjustment. A little toe out really helps a formula car or a rear drive car on initial turn in. Similarly, at the back of a car, most people run a little toe-in to add stability under heavy braking. I am told by racers that front wheel drive cars need toe-in at the front to help them turn. I personally have no experience with it and have not heard a good explanation why though...

[desmo]

For those who wish to learn more about Ackermann, first do a web search on the term (using two ns!) The current issue(#34) of Racetech has an interesting article on Ackermann as do at least issues #18 & #30. In fact Racetech has writen enough on Ackermann to write a short book on the subject. I thought about writing a quick synopsis of the articles but it would inevitably fail to do the subject justice. In fact I get tired just typing out "Langensperger angle", never mind defining it.

[Ray Bell]

My understanding is that the Ackermann angle (relating to the angle of the steering arms in relation to the axis of the car) is determined by plotting a line from the turning centre of the front tyre contact patch* to the centre of the rear axle...

When the joint on the steering arm is on that line, then the relative turning of the inside and outside wheels is correct.

Of course, we have other factors such as length of tie rods, placement of the rack etc... don't really know if this remains correct for the whole turn.

Can you see the effects then of using an upright from a car with a long wheelbase on a short wheelbase race car?

And fitting wider wheels so that the contact patch moves outwards?


* Not strictly so... from the line of the 'king pin' or the line drawn through the upper and lower ball joints, which gives the above centrepoint... and at the height of the steering arm.

[palmas]

Most of these angles depend on the suspension geometry and specially flexibility, since these angles are measured without load but are made to correct some changes in geometry made be load while curving or acel./braking. In fact, some street cars have some flexible elements to help suspention deformate in a helpfull way (and some can't make a suspention that holds for a few miles).
In race cars, things are a litle more complex for the load variations are huge and there are also some additional needs like keep the tyre hot, or keep the tyre cool, or save the tyres, etc...
Some of the tunning they do to the angles are to elliminate some chassis/suspention problems and are quite difficult to analise just by it self.