weight transfer during braking

[vandit]

during a high decceleration braking there is quiet a considerable amount of weight transfer front rear to front of a car.....and hence less braking is required at the rear then at the front to actually prevent locking up of rear tyres.........can anyone help me out as to how to calculate this weight transfer and related pressure differnces needed to for appropriate breaking........

[vandit]

during a high decceleration braking there is quiet a considerable amount of weight transfer front rear to front of a car.....and hence less braking is required at the rear then at the front to actually prevent locking up of rear tyres.........can anyone help me out as to how to calculate this weight transfer and related pressure differnces needed to for appropriate breaking........

[B Lewis @ PE Engine Management]

The easiest thing to do is just use an adjustable proportioning valve in the system and dial it in once the car is mobile. The calculations are certainly possible, but I think that you would end up making so many assumptions that your results wouldn't mean much.

[scooter2131]

A quick way to do this would be to assume your front brakes take about 60-70% of the braking force. Given the mass of your car and the traction limits of your tires you can assume a deceleration and consequently a resultant force that acts at the tire contact patches. Slap all that on a FBD and you should get a crude estimate for load tranfer to your front wheels under SS braking. If you want something more accurate or time/velocity dependent you have to learn how to use ADAMS, which I haven't learned myself yet. Hope this helps as a start.

[Homemade WRX]

I saw the equation this week when I was doing some brake calc's but don't recall what book it was, I'll see if I can find it for you and post it up.
Micah

[kwancho]

Dr Bob Woods wrote a book called Modeling and Simulation of Dynamic Systems that outlines the whole brake calculation process very well.

[terra_dactile]

hI
I could be mistaken but in order to get the load tranfer towards the front tire you can do a simple free body diagram of you car which give the eqation
Load transfer front tires =
(unsprung mass*deceleration*height CofG)/
Distance from front axle to C of G
i did an aproximation at 1.6 g<s and it gave me roughly 175 lbs transfered to the front tire combined. If you reuse this equation at each interval of G's you will be able to graph your load distribution front/rear depnding on G load of deceleration.

This is just a though not a claim, i would like it if some one would check my equation, i could be mistaken!

Jude Berthault
ETS Formula SAE
Steering Director

[Storbeck]

It's total mass of the car, and it's the total wheelbase, not just the distance from cg to front.

add the result to the front static weight and subtract it from rear static weight and you have your new weight distribution

[Buckingham]

Unless you are "king of the late-brakers" you won't be attempting to brake at 1.6g's very often. When performing braking calculations you must consider the braking situations when your driver is most concerned about vehicle balance (braking and turning). You must also consider your driver's individual driving styles, as brake balance can be critical to how the driver likes to turn-in.

[Homemade WRX]

to get the true max, wouldn't you need to calculate for suspension compression up front as this would further transfer weight forward....I figure it would be minimal but if you really wanted to

[Agent4573]

I have to agree with b lewis on this one. I would just assume about 60% of braking is done up front, then get a bias controller. If you want to use numbers accurately don't forget the important things: height of CoG, wheel base, shock damper coeffs, spring rates, roll center, engine braking, etc....


We use identical rotors fron and rear on our car, only there are two in front and one in rear, this equates to a 66% front brake bias, with a bias bar on our brake pedal, and a few turns, all 4 wheels lock evenly, with no complicated equations to go through.(At least not for this part of the braking system)

[Cement Legs]

Ditto...

Why lock so many other things into contrainsts set up by your brake systems (such as f:r weight ratio). Design an adjustable systems with either an in line pressure regulator or a balance bar on separate master cylinders. If you run into unforeseen packaging problems down the road and have to move 20lbs of weight forward or backward you can make the brake pressure adjustments and test.

[Matt Gignac]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Agent4573:
I have to agree with b lewis on this one. I would just assume about 60% of braking is done up front, then get a bias controller. If you want to use numbers accurately don't forget the important things: height of CoG, wheel base, shock damper coeffs, spring rates, roll center, engine braking, etc....
</div></BLOCKQUOTE>

Shocks, springs and roll centers will not affect the amount of weight transferred during braking. They will affect the angle your chassis sits at, and this may affect your static weight distribution, but I don't think it will be significant compared to dynamic weight transfer due to braking.

Even if you go adjustable though, I'd calculate how much distribution you need front to back anyways. I assumed some coefficients of friction for tires and pads, the rest obtained accurately from CAD or measurements, used the formula for weight transfer (mass*acceleration*CGheight/wheelbase), figured out the normal and tractive forces at each wheel, translated that to braking torques needed, then sized everything to meet these torques. Did this for a few different decelerations, looked at g-g data to see what kind of braking we like to do, done!

Honestly, it took us an hour to make that excel sheet, so I think it makes sense to try this instead of assuming some proportion.

We still have the bias bar tho, to adjust for driver preferences or track conditions.

Matt Gignac
McGill Racing Team

[kozak]

Agent4573 if you don;t mind me asking do you use the same calipers in the front as in back or do you step the rear caliper up a bit.

[Agent4573]

same calipers front and rear. wildwood dynalite singles

[kozak]

yeah thats what we were thinking of using too.

[Homemade WRX]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Agent4573:
same calipers front and rear. wildwood dynalite singles </div></BLOCKQUOTE>
single floaters I'm assuming...?...

I'm looking at running the dynalite single floaters up front and ps-1's inboards out back...torque differences will be made up with rotor size...

[vandit]

well i figured out the weight transfer on my car.........and is there anybody using HOOSIER TIRES of 20.0*6.0-13 R25A specs.....can anyone tell me what is the coefficient of friction used in calculating braking force......

[Kurt Bilinski]

Look at it this way. What's your time worth? How much time do you have to complete your car? Aren't there more important things to be working on?

Take a guess and move on... it only takes a couple hard braking runs to establish what it's *really* supposed to be, far less time then you calculating a number - that *still* must be tested.

[Storbeck]

A rough calculation of your weight transfer can be done in about ten minues, a more in depth spreadsheet that plots weight distribution verses braking deceleration can be done in an hour easily. I'd say it's worth it, just to get the balance bar near the center to start with.

I did it while I was drinking and procrastinating on homework, so I was multitasking and I figure it was a near zero drain on my time resourses. http://fsae.com/groupee_common/emoticons/icon_smile.gif

[Marc Jaxa-Rozen]

We had to change master cylinders on the first car I worked on because I'd forgotten to look at the fluid displacement in the cylinders...that definitely took more time than a simple calculation http://fsae.com/groupee_common/emoticons/icon_frown.gif

Not only is the brake system vital, but it's also easier to calculate than pretty much anything else on the car, so I figure it's probably worth it at least to make sure you're in the ballpark with braking distribution.

Marc Jaxa-Rozen
École Polytechnique de Montréal

[kozak]

well is their any quick calcs that i can do to make sure that the brakes and master cylinder that i use will get me past the rules. right now i am thinking 10" rotors up front and 1 10" in the rear and dynalite singles all around. as for master cyl i have no idea.

[Homemade WRX]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by kozak:
well is their any quick calcs that i can do to make sure that the brakes and master cylinder that i use will get me past the rules. right now i am thinking 10" rotors up front and 1 10" in the rear and dynalite singles all around. as for master cyl i have no idea. </div></BLOCKQUOTE>
last year we ran 10.3" alum rotors up front and one in the back, dynalite single floaters up front and dynalite single outback on Q pads...5/8ths MC...and we had tons of brake...

[kozak]

were thoes rotors wilwood or custom, is there a huge diff in coef of friction between steel and alum.

[Homemade WRX]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by kozak:
were thoes rotors wilwood or custom, is there a huge diff in coef of friction between steel and alum. </div></BLOCKQUOTE>
wilwoods...and yes there is a huge Cf difference

[kozak]

so what has the better c.f.