Aero Effect of Brake Calcs

[dstamp18]

how does aerodynamic down force factor into dynamic brake calculations? This is the first year we will be running aero on our car and I have been struggling to find much literature on the subject. the closest thing i have found simply adds max downforce loads to the dynamics weight distribution on the front and rear axles. I can only imagine the judges asking why we didnt factor in our aero values when designing the brake system.

D Stamper
San Diego State University

[Racer-X]

Simply put aero gives you down force, down force gives you more vertical load on the tire, meaning you get more grip. So about all aero does for your brakes is make you less likely to lockup at speed. Now you have more grip to take advantage of.

I'm not suspension so about all I can tell you is look at your tire data and expected down force and go from there.

[Dash]

Don't wings have drag? F = M x A

How does the coefficient of lift and drag vary with speed? Will this mean you need more or less braking force from the tires at a certain speed?

If you design the car to be able to stop without the wings, I'm sure you would have no trouble stopping with the wings.

[Owen Thomas]

I believe you are over thinking it. You are correct that downforce adds to the "dynamic weight distribution", but you need look no further than F_f = Mu*(F_g + F_aero). Your aero guys probably already have some info on how much downforce is generated at each axle/wheel for given speeds, so get it from them and add it to your normal force. They might even have it correlated to the tire data already (bonus, most of your work is done!).

Aerodynamic forces will change your pitching moments during braking, but will not affect your geometric weight transfer. Again, if you choose, get this data from the aero guys(lift/drag & location of CP), calculate your dynamic pitch moments, and include them in your calculations.

Note: I have not done this, but this is how I would go about it. Anyone who isn't just pulling crap out their ass like me please have at it.

[Paul Achard]

Just be careful that nothing in the system fails when the driver locks the brakes at top speed(i.e. max negative lift).

[dstamp18]

the article i read was on brake design by race car engineering mag from 2005 i believe. Like i said they added rho (max aero downforce) to the front and rear dynamic axle weights. What threw me was visualizing a free body diagram of the car, whether the front wing down force would create a bending moment around the front axle or cg, as would the rear wing about the rear axle or cg? and they would generate different downforce numbers thus different moment values. I need to ask the aero guys about center of pressure cuz thats new to me. Downforce = V^2 but it makes sense just to use max downforce numbers when making sure the brakes are robust enough. I know an F1 car produces enough drag to decelerate by 1 g or something close to that simply by lifting off the accelerator so clearly drag also plays a role stopping distances. I appreciate the input guys!

[Bemo]

I'd say of course aero can change the distribution of normal loads between front and rear. But I'm also quite sure that you should be on the right track if you just add the aero forces to the dynamic loads. It is important though to know where on the car the aero forces are applied (CP).
To make my point clear (always a bit tricky in a foreign language http://fsae.com/groupee_common/emoticons/icon_wink.gif). The downforce generated by the front wing isn't applied directly to the front axle but to some point in front of it. Same for rear wing and rear axle (although the distance usually should be much smaller there I guess).
What I would do is, just draw a free body diagram of your car in side view. Now you have to add the two aero forces, the gravity force of the car and the decelerating force. You will get the two normal forces for front and rear axle by the two equations you get from this (sum Fz=0, sum Ty=0).
It is a bit tricky as the maximum decelerating force depends on the result, so it is an iterative calculation I guess.

As the others before, I haven't done this calculation myself before, so that is only what comes to my mind. Hope I could help.

[Markus]

Something what I've heard from teams running aero for the first times:

Factor in the extra heat you can generate with the extra grip. So you definetly want to be on the safe side with temp calculations. Also with a large front wing you may have some lack of cooling which you have to consider.

I haven't done the calculations either but would could the same route as Bemo and others. I would use max. downforce i.e. downforce at maximum speed.

[Repmud]

Originally posted by Markus:
Factor in the extra heat you can generate with the extra grip.

How do you generate extra 'heat' when your braking entry speed is lower due to drag, and your braking exit speed is higher due to downforce?

[MCoach]

One thing to consider is power and heat flux. Energy is not begin all, end all.

If you just consider a point mass thermal system (ignore the kinematics side of brakes right now) then your long term brake calcs should end up relatively accurate, or at least in my experience they are. If you want to start calculating shorter and shorter time intervals of braking, such as the curve from ambient or otherwise already elevated brake temperature just before the braking event to somewhere during the brake event. Sure, there may be some calculated energy transferred into the rotor, but how is that being distributed and how quickly? This will quickly start to break down unless you know the rate at which temperature is being transferred into the rotors.

I've advocated the simple calculation that's very applicable to the point mass thermal system, E=1/2*m*v^2, for some of the newer members asking here, but for anyone willing to go as far as considering aero download, might as well consider the changing rate over time.


Having areo download increases the rate at which that energy can be transferred into the rotor (increased Fz), meaning that local heating of pad contact will be higher than areas down near the mounting points, or even across to the non-yet-braked side of the rotor that's currently chilling in the air. The rate at which the rotor material can distribute the energy is a fixed value (well, technically it varies based on temperature but let's keep this simple for now) and will have 'surges' if you consider them as such. You will have high local heating, which is then dissipated more efficiently from convection by having a higher gradient between the air temp and the rotor, and then quickly heated again. This may lead to a shorter fatigue life.

Heat flux is the way to go. Using heat flux, one can calculate application site temperatures rather than assuming the system has already transferred all of the energy into the system and is evenly distributed. That way, you know that even if you're calculating an average temperature of 200-500F, you may still be peaking into the 800-900F-beyond range. I've relied on heat flux to help with cut-out geometry and quick heating/cooling of rotors. If you're new to this, you would be surprised how quickly you can get a rotor to glow a little, but then settle back down to a sustainable temperature.

T_rotor = complicated equation that I don't feel like posting right now involving energy input over time of multiple brake applications. Maybe tomorrow.


TL;DR:
A higher heat CAN be generated because the rate at which the rotor is absorbing heat is increased over non-aero cars.

Discuss.