Braking calculations

[sahil_tu]

Hi
i am doing calculations related to brake design system, but i am bit confused about maximum deceleration value. Most of the books take 1.5g as max. deceleration but as per book 'Brake design and safety' by LIMPERT he suggested that optimum braking takes place at a deceleration value equal to friction coeff. b/w tire and road surface. but there was not much explanation in support of this point. so i am bit confused about max. dec. value. As per our calculations the max. coeff. of friction b/w tire and road comes out to be 1.8g So what gonna be best fit for optimum braking??

[exFSAE]

as per book 'Brake design and safety' by LIMPERT he suggested that optimum braking takes place at a deceleration value equal to friction coeff. b/w tire and road surface. but there was not much explanation in support of this point.

...really?

Let me put it to you this way - what's the most force a tire can produce at any given point?

[counter]

ya i agree that teams assume 1.5g (I don't know the logic of assuming this value)for braking calculation but maximum deceleration achieved by car depends upon the coefficient of friction of tire, from newton's second law,
W.a = mu*front axle weight + mu* rear axle wight
= mu*(total weight)
==> a=mu
where W is weight of car and a is max. deceleration in terms of g.
I think it would be more logical to take this value equal to mu(comments awaited).

[Edward M. Kasprzak]

Originally posted by sahil_tu:
optimum braking takes place at a deceleration value equal to friction coeff. b/w tire and road surface

A tire's friction coefficient is defined as the maximum force divided by the normal load. Thus, by definition, the force at this value is the optimum for the tire. The maximum force assumes the optimal slip ratio is achieved at the current set of operating conditions (load, inclination, pressure, slip angle, velocity, tread temperature, etc.). Every combination of those operating conditions will have a different friction coefficient.

[Simon Dingle]

Hey, if you're after data for FSAE max acceleration under braking, check out the results from FS-Austria's special event in 2009.

I'll post the link in a second but so you don't have to wait for the link to appear, if you search this forum with "Austria Special Event" you'll find a topic with a list of teams and their average braking acceleration at competition.

[Simon Dingle]

http://fsae.com/eve/forums/a/t...=546101683#546101683 (http://fsae.com/eve/forums/a/tpc/f/125607348/m/530104583?r=546101683#546101683)

[Gruntguru]

Originally posted by sahil_tu:
'Brake design and safety' by LIMPERT he suggested that optimum braking takes place at a deceleration value equal to friction coeff. b/w tire and road surface. This also assumes zero aerodynamic lift.

[Tim.Wright]

Some people might start with a braking acceleration because thats what they've read on their accelerometers during testing or the comp. Thats what we did at the very start.

You can also calculate Max Fx theoretically too but one you've done all the required friction scaling on your unvalidated tyre data would you really trust what it tells you over recorded data?

Tim

[vishal-rvce]

if u r not considering downforce than take deceleration equal to max tire coefficient.Bt if aerodynamic downforce is considered than
max deceleration = u +F*u/w

[The_Man]

Originally posted by vishal-rvce:
if u r not considering downforce than take deceleration equal to max tire coefficient.Bt if aerodynamic downforce is considered than
max deceleration = u +F*u/w

Hi,

I did not get you? Are you asking a question?

I'd suggest in your brake calculations not to involve downforce, your speeds in Formula Student are low anyway and the speed (hence downforce) disappear really fast anyway during braking.

[Edward M. Kasprzak]

Originally posted by vishal-rvce:
if u r not considering downforce than take deceleration equal to max tire coefficient.Bt if aerodynamic downforce is considered than
max deceleration = u +F*u/w

Tire friction coefficient varies with load. Aerodynamic downforce causes this change in load, so using the tire friction coefficient is still correct.

Please define your symbols! If I guess F=downforce, w=weight and u=friction coefficient then your formula assumes a constant friction coefficient--ignoring tire load sensitivity. Make sure you understand the implications of this assumption.

[vishal-rvce]

quote:
Please define your symbols! If I guess F=downforce, w=weight and u=friction coefficient then your formula assumes a constant friction coefficient--ignoring tire load sensitivity. Make sure you understand the implications of this assumption.


yes,F=downforce, w=weight and u= max friction coefficient.since maximum deceleration is required, it will occur at max friction coefficient and hence constant friction coefficient.

[vishal-rvce]

Originally posted by The_Man:
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by vishal-rvce:
if u r not considering downforce than take deceleration equal to max tire coefficient.Bt if aerodynamic downforce is considered than
max deceleration = u +F*u/w

Hi,

I did not get you? Are you asking a question?

I'd suggest in your brake calculations not to involve downforce, your speeds in Formula Student are low anyway and the speed (hence downforce) disappear really fast anyway during braking. </div></BLOCKQUOTE>


I agree speeds in formula student are low but if
you use wing than you will have a significant downforce..
What you think of this formula??? Is it fine to use for break calculation??

[MCoach]

I would recommend not trying to include aerodynamic download on the car unless you plan on running wings similar to Monash Univeristy or Maryland University. Otherwise, plan for zero aerodynamic lift and download and then tune your front/rear bias from there during testing. Drivers should always be able to lock the brakes without excessive effort. If not, you are designing in a region that is too difficult to modulate anyways. If you are still concerned about your design, start doing different testing iterations with different pad compounds (I would recommend that either way if time permits) and record data acquisition acceleration for each run along with driver feedback.

TL;DR:

Listen to Edward Kasprzak. He's been in this business a lot longer than you've been in that book.

[vishal-rvce]

Originally posted by MCoach:
I would recommend not trying to include aerodynamic download on the car unless you plan on running wings similar to Monash Univeristy or Maryland University. Otherwise, plan for zero aerodynamic lift and download and then tune your front/rear bias from there during testing. Drivers should always be able to lock the brakes without excessive effort. If not, you are designing in a region that is too difficult to modulate anyways. If you are still concerned about your design, start doing different testing iterations with different pad compounds (I would recommend that either way if time permits) and record data acquisition acceleration for each run along with driver feedback.

TL;DR:

Listen to Edward Kasprzak. He's been in this business a lot longer than you've been in that book.
thanks for advice MCoach..