longitudinal tire stiffness

[shanchan]

what are the general values for longitudinal tire stiffness for slicks??

[MCoach]

Have you tried measuring any?

[shanchan]

Originally posted by MCoach:
Have you tried measuring any?

how do i measure it ??

[M. Nader]

Originally posted by shanchan:
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by MCoach:
Have you tried measuring any?

how do i measure it ?? </div></BLOCKQUOTE>

Do you know what it is you are trying to measure?

[shanchan]

Originally posted by M. Nader:
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by shanchan:
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by MCoach:
Have you tried measuring any?

how do i measure it ?? </div></BLOCKQUOTE>

Do you know what it is you are trying to measure? </div></BLOCKQUOTE>

i am trying to calculate the longitudinal force , for that i need the values for longitudinal tire stiffnes

[MCoach]

With respect to Pacejka's Tire and Vehicle Dynamics, what you are looking for is listed here.
F_x = C_Fk*k

F_x: Longitudinal force
C_Fk: Longitudinal slip stiffness
k: Longitudinal slip ratio

If you are capable of measuring the slip stiffness and the slip ratio, then by all means, calculate the force, but some might say that you are taking the hard way out of this. How about this--just measure the longitudinal force.

Most people find the forces and then calculate the stiffness.

Can you think of a way to measure or estimate the force?

[shanchan]

Originally posted by MCoach:
With respect to Pacejka's Tire and Vehicle Dynamics, what you are looking for is listed here.
F_x = C_Fk*k

F_x: Longitudinal force
C_Fk: Longitudinal slip stiffness
k: Longitudinal slip ratio

If you are capable of measuring the slip stiffness and the slip ratio, then by all means, calculate the force, but some might say that you are taking the hard way out of this. How about this--just measure the longitudinal force.

Most people find the forces and then calculate the stiffness.

Can you think of a way to measure or estimate the force?

as i mentioned above, i need the value of this constant to calculate force.... is there any other alternative way to calculate this force??

[M. Nader]

Are you trying to calculate the max longitudinal force that can be used?
do you have access to the TTC?
can you estimate vehicle weight and weight transfer?


You can estimate you force by estimating an actual coefficient of friction (data for that can be found on the forum as TTC data are quite theoretical) and the weight and weight transfer for each case.

if you have access to TTC, you can get most stiffness-es but won't be accurate unless you use Pacejka scaling factors and know what you are scaling to.

[Zac]

You might be able to make an estimate of longitudinal tire stiffness using the series of equations found in Purdy - Mathematics underlying the design of pneumatic tires. I really wouldn't recommend opening that book though. You'll go cross eyed.

Also, longitudinal stiffness isn't the same as braking stiffness. Unless you are trying to tune a tire design for ABS and don't have access to a flat-track or traction trailer I don't know that longitudinal stiffness is worth figuring out.

[Zac]

if you have access to TTC, you can get most stiffness-es but won't be accurate unless you use Pacejka scaling factors and know what you are scaling to.

Braking stiffness and cornering stiffness are largely surface independent. Small slip angles or slip ratios mean limited slip inside the footprint. Limited slip means friction isn't the primary factor driving your reaction forces.

[shanchan]

Originally posted by M. Nader:
Are you trying to calculate the max longitudinal force that can be used?
do you have access to the TTC?
can you estimate vehicle weight and weight transfer?


You can estimate you force by estimating an actual coefficient of friction (data for that can be found on the forum as TTC data are quite theoretical) and the weight and weight transfer for each case.

if you have access to TTC, you can get most stiffness-es but won't be accurate unless you use Pacejka scaling factors and know what you are scaling to.

can you tell me what is TTC?

[MCoach]

http://www.millikenresearch.com/fsaettc.html

The hard work has already been done. You just need to opt in the $500 for the data (which is a deal far beyond comprehending) and go down the rabbit hole trying to process all of if it.

[M. Nader]

Originally posted by Zac:
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">if you have access to TTC, you can get most stiffness-es but won't be accurate unless you use Pacejka scaling factors and know what you are scaling to.

Braking stiffness and cornering stiffness are largely surface independent. Small slip angles or slip ratios mean limited slip inside the footprint. Limited slip means friction isn't the primary factor driving your reaction forces. </div></BLOCKQUOTE>

Yes i know, that is why i mentioned that he has to know more about the scaling factors and what he is scaling to. TTC surfaces are very smooth and thus the data shows tyres that are very theoretical and what we get on the "real" tracks is about 60% of that.

If the team is new to all this, i suggest just using a realistic estimate/guess to the coefficient of friction to determine their max available longitudinal grip and work from there. Specially if they didn't know about the TTC

[M. Nader]

Originally posted by MCoach:
http://www.millikenresearch.com/fsaettc.html

The hard work has already been done. You just need to opt in the $500 for the data (which is a deal far beyond comprehending) and go down the rabbit hole trying to process all of if it.

And make no mistake, it is one DEEP rabbit hole. make sure that you really need it and have the time before jumping in there. Even Mr. Milliken will tell you that.

[Zac]

Originally posted by M. Nader:
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Zac:
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">if you have access to TTC, you can get most stiffness-es but won't be accurate unless you use Pacejka scaling factors and know what you are scaling to.

Braking stiffness and cornering stiffness are largely surface independent. Small slip angles or slip ratios mean limited slip inside the footprint. Limited slip means friction isn't the primary factor driving your reaction forces. </div></BLOCKQUOTE>

Yes i know, that is why i mentioned that he has to know more about the scaling factors and what he is scaling to. TTC surfaces are very smooth and thus the data shows tyres that are very theoretical and what we get on the "real" tracks is about 60% of that.

If the team is new to all this, i suggest just using a realistic estimate/guess to the coefficient of friction to determine their max available longitudinal grip and work from there. Specially if they didn't know about the TTC </div></BLOCKQUOTE>

I was just clarifying that cornering stiffness and braking stiffness are surface independent (but highly dependent on test procedure) since the question was specifically on stiffnesses and that is what you commented on.

Now if the OP didn't ask the correct question, that is a different matter entirely.

[Warpspeed]

http://i76.photobucket.com/albums/j38/royalshifter/ad6.jpg

[MCoach]

shanclan,

The better option for you currently would probably to try and set up the tire you plan on using and do a brake test. Find the stopping distance of the car and from that you can calculate the deceleration and mu of the tire. Also, a run around a skid pad will also reveal estimated numbers for lateral mu. Don't get wrapped up in tire stiffnesses if this is your first time through, which it sounds like. Just get something built, know that it will be capable of over 1G lateral (keep in mind so that you don't roll over, possibly higher than that!) and then test the car and pull as much data as you can.

[exFSAE]

If you want to know your longitudinal forces, all you need is torque and radius...