Max deceleration

[adekunle]

Hi evryone,

My name is adekunle, m making some scratch calculations for a brake design system and I m wondering whether the front or rear axle max deceleratn is twice that per front/rear corner or its the same on the axle and corner.

Sorry if the question sounds lame.

[CWA]

The question doesn't make sense (to me at least), can you re-phrase?

[adekunle]

Ok,I want to have a max -ve acc of 1.5. So dividing my dynamic weight by force generated by front tires give me 3.0G that mean that the G force at each front corner is 1.5G,ryt

But I was told that its not so, its suppose to b 1.5G for the front axle nt 3G and 1.5G per corner. Do u understand me nw.

[MotorCity1]

Ok,I want to have a max -ve acc of 1.5. So dividing my dynamic weight by force generated by front tires give me 3.0G that mean that the G force at each front corner is 1.5G,ryt

But I was told that its not so, its suppose to b 1.5G for the front axle nt 3G and 1.5G per corner. Do u understand me nw.

adekunle,

Draw a free body diagram.

Car needs to decelerate a 3 g (whoa). You have 4 tires. Each tire contributes to the deceleration. You have a cg height, wheelbase, and static F/R weight distribution. Calculate the wheel loads after weight has been transferred due the deceleration. Assuming the car is symmetrical left to right, you have a coefficient of friction (mu) for the front tires, and a mu for each rear tire. You now have each tire's contribution to your deceleration. Now you do some addition..

[adekunle]

@ motorcity1,

Wel I actually did all dat before I got to my max G final result, c.g height, wheel base, weight transfer and al. Already don dat. So u sayn I need is dat 1.5G is the max G dat each corner tire contributes to the axle dat brakes at 3.0G.ryt?

[MotorCity1]

Yes, i believe that's 'ryt'

I'd check your numbers though...3g braking implies something like 60mph to 0mph in under a second. I'd imagine that is quite hard for your average fsae car.

[CWA]

@ motorcity1,

Wel I actually did all dat before I got to my max G final result, c.g height, wheel base, weight transfer and al. Already don dat. So u sayn I need is dat 1.5G is the max G dat each corner tire contributes to the axle dat brakes at 3.0G.ryt?

adekunle,

I'm still not sure I understand your exact issue.

But in reference to the wording of your above comment specifically, I think there are some fundamentals that you do not convey an awareness of.

There is a single mass (your vehicle), so by F=ma, a single vehicle acceleration. There are four sources of force that act to generate the acceleration of your mass - these are the tyres. If you were to call the Brake Force from your Front Left tyre "BFFL", by summing the forces from all four of your tyres, the acceleration of your car would be found in the equation:

(BFFL+BFFR+BFRL+BFRR) = m.a

Note again - four force values, one acceleration value. You do not sum accelerations at each corner.

I hope this clears up any confusion you may have. If it does not, perhaps if you tell us your 'inputs', or the values you do have (tyre/vehicle parameters?), the values you are trying to attain (vehicle acceleration/braking performance?) and tell us the way you are trying to attain these outputs, we can check whether you are right or wrong, and give specific advice where needed.

[JT A.]

Ok,I want to have a max -ve acc of 1.5. So dividing my dynamic weight by force generated by front tires give me 3.0G that mean that the G force at each front corner is 1.5G,ryt

But I was told that its not so, its suppose to b 1.5G for the front axle nt 3G and 1.5G per corner. Do u understand me nw.

Unless your car is made of slinky's, the whole thing should be decelerating at the same rate. The FORCES from the front and rear tires can be different, but the ACCELERATION at the front is the same as the rear, and same as the CG, and same as any other point on the car.

[Jonny Rochester]

For a car with a static weight balance of 50/50, during deceleration of 1.5G, the normal force on the tyres maybe 80/20 depending on your center of gravity height. As it works out with simple calcs and no aero, if you can brake at 1.5G and no quicker, your tyre mu is also 1.5

[tromoly]

I once had a car that would stop at 1G on the left side but only 0.5G on the right side, it would turn left really well but turning right was a different story.... I had another car that one day the car stopped at 0.8G but the left front wheel stopped at 0G, that was the day my car turned into a trike....

Joking aside, adekunle, what does the "G" you're asking about refer to? How is that "G" calculated?

[adekunle]

Thnks for ur replies guys, really needed that
I was misinformed and ended up confused, I realised this as CWA said

Total front axle weight/dynamic weight = 1G
0.5 of front axle weight/0.5 of dynamic weight=1G

There is no adding up of G force between tire to get some required amount of G for the front axle

[adekunle]

@ tromoly

Funny, bt joking aside ur G is ur max deceleration which is also equals to ur mu btw the tires and the road surface

[stever95]

Dude, your language needs to be refined if you ever want to be taken seriously, like, in life.

[adekunle]

@ stever95

Yep sorry abt dati guess I jus couldnt explain it clearly via text.Bt I wud take note of this next tym. Thanks

[tromoly]

Funny, bt joking aside ur G is ur max deceleration which is also equals to ur mu btw the tires and the road surface

I'm pretty sure Jonny was referencing a comment I made in another thread where I made that statement. Keep in mind there is at least one assumption in that statement that isn't necessarily always true. So to flip that comment around, when is the max deceleration NOT equal to Mu?

[MCoach]

I'm pretty sure Jonny was referencing a comment I made in another thread where I made that statement. Keep in mind there is at least one assumption in that statement that isn't necessarily always true. So to flip that comment around, when is the max deceleration NOT equal to Mu?


Hint: When your tire isn't the only thing braking.

[CWA]

Hint: When your tire isn't the only thing braking.

Other hint - non-linearity w.r.t force into / force out of a tyre. When you start to realise that the simple 'mu' analogy of friction/grip has its limitations when applied to a tyre. It's a good thing we have this mechanism though, otherwise you'd not have much luck with your LLTD tuning; might as well get a three-wheeler..

[Tim.Wright]

Equating the max acceleration to the tyre mu assumes that all the tyres are operating at their grip limit. In braking and mid corner you generally dont operate the rear tyres at their limit because its too unstable for the driver.

How accurate do you need these numbers for anyway? What do you need them for?

[adekunle]

@ tim wright,


Am a rookie to fsae and am designing the brake system. For start am only looking at longitudinal braking, i thought it only had to do with straight line braking, pls if there are other scenarios I avnt taken note of pls I would lyk to know. Thanks.

[Jonny Rochester]

adekunle, brakes has been discussed much on this forum if you search back a few years. I did search, then tried to write up the basics again for my own learning in this thread:
http://www.fsae.com/forums/showthread.php?12049-Brake-design-mu-pressure-pedal-force-heat-capacity-and-disapation

I think the other scenarios people are hinting at, is if you have wings providing downforce, then the normal force on the tyres is greater which can allow the car to see greater acceleration/deceleration than just tyre mu = G force would suggest.

[dynatune]

I have developed a tool for brake system layout that is quite straight forward and is considering also aerodynamic loads. You might find some additional
info for your calculations by downloading the pdf of the tool on this page http://www.dynatune-xl.com/dynatune-bdm.html

cheers
dynatune

[adekunle]

@ jonny,

Wud check d thread. Thanks

[adekunle]

@ dynatune,

Cnt find any link to download it m guessing it isnt free.

[Jay Lawrence]

Dude, pls stp riting n txt spk. It mks u snd lyk a moron.

[Claude Rouelle]

Jay,

Can only agree with you. Using text style in a forum is simply a lack of respect, a lack of understanding (and even trying to understand) of how people converse outside their own country, and simply a lack of communication skills.

[JT A.]

In addition to everything Claude said, the ladies also like it when you can come across somewhat intelligently in a text message

[adekunle]

Ok noted, sorry about that really did not know it had so much implication on the thread. I promise it would not happen again.
Another warning for a rookie.

[stever95]

IT WAS AN ACT THE WHOLE TIME! :cool:

I agree with the others though. Especially in professional environments, it's really discouraging to receive messages and e-mails from people with "u" for you and "4" for "for," etc.

Better that you learned it now rather than later!