+ Reply to Thread
Page 1 of 5 1 2 3 ... LastLast
Results 1 to 10 of 49

Thread: Front and rear tire choice and weight distribution

  1. #1

    Front and rear tire choice and weight distribution

    I realized today that I had dozens of PM in my Inbox. I never looked at this Inbox. Sorry guys I will not answer those. Simply too busy. Just post your question on the forum for the benefit of the whole community.

    One caught my attention tough: it was about the front and rear tire choice and the weight distribution, and a statement I made during recent OptimumG seminars and in informal conversation at the FSAE / FS competitions.

    If I read in a design specification sheet that the students have the same front and rear tire (I am fine with that) but the weight distribution is 42 % front I will probably not read the rest of the spec. sheet with a lot of attention The car is already hill born and the only thing they could do to have a decent handling will be to create a patch on patch.

    There are clear vehicle dynamics principles that show that your front and rear tire cornering stiffness ratio should not be far away, just a few %, from your weight distribution. And that your front and rear roll stiffness distribution ratio should be very close to your suspended weight distribution. If you don't then you must have tons of compliance and you won't get good performance. You could maybe get a balanced car with a patch on patch but you won't get a lot of grip. Or you would get a lot o potential grip but with a car very difficult to drive.

    There is a reason why even a cheap car like the Smart do not have the same front and rear tires.

    Food for thoughts.
    Claude Rouelle
    OptimumG president
    Vehicle Dynamics & Race Car Engineering
    Training / Consulting / Simulation Software
    FS & FSAE design judge USA / Canada / UK / Germany / Spain / Italy / China / Brazil / Australia
    [url]www.optimumg.com[/u

  2. #2
    Senior Member
    Join Date
    Mar 2005
    Location
    Modena, Italy
    Posts
    363
    Quote Originally Posted by Claude Rouelle View Post
    If I read in a design specification sheet that the students have the same front and rear tire (I am fine with that) but the weight distribution is 42 % front I will probably not read the rest of the spec. sheet with a lot of attention The car is already hill born and the only thing they could do to have a decent handling will be to create a patch on patch.

    There are clear vehicle dynamics principles that show that your front and rear tire cornering stiffness ratio should not be far away, just a few %, from your weight distribution...
    I'd counter that and say that putting the same tyre front and rear doesn't mean you have the same cornering stiffness front and rear. Something that practically EVERYONE seem to miss is that cornering stiffness is roughly proportional to the vertical load (e.g. your static weight loads!!!).

    So if you have 42%F mass distribution and the same tyres front and rear, then (leaving out other K&C effects) your cornering stiffness distribution is going to be more or less 42%F too - save for a for percentage points change due to load sensitivity.
    Last edited by Tim.Wright; 11-04-2015 at 11:06 AM.

  3. #3
    Senior Member
    Join Date
    Mar 2005
    Location
    Modena, Italy
    Posts
    363
    Further to this - there is no real reason why roll stiffness or LLTD distributions should be similar to the mass distribution. Its just a rule of thumb and like all rules of thumb that I have ever come across in this field it's wrong.

    There are 3 main (first order) effects in this discussion: tyre behaviour (cornering stiffness and load sensitivity), weight distribution and lateral load transfer distribution. Theoretically you can control all of them (especially in FSAE) but in reality there is a much smaller practically acheivable range of these 3 things. It's important that working together - these 3 effects give you the desired reseponse (objectively measured - e.g. using the static margin or similar) in the complete range of operation of the vehicle (linear, transition and saturation/limit ranges).

    For example - here is a very common setup for front engined road vehicles (e.g. the not so cheap Audi TT):
    1: Equal tyres front and rear for cost reduction
    2: Large front mass distribution (60%F) which give a baseline understeer response on the equally sized tyres (due to tyre load sensitivity)
    3: Roll distribution significantly more rearward than the mass distribution to reduce the level of understeer in the transition and limit ranges.

    The 3 things work together to give a response which has a large static margin in the linear range which then reduces to a lower, but still positive level in the transition and limit range. If you look at the vertical loads on the highly loaded external wheels, you will see that their front:rear bias starts equal to the mass distribution and then moves rearward to be closer to 50% (matching the tyre size distribution) near the limit. If you would have left the LLTD equal to the mass distribution (like the books say) the car would be an understeering pig at the limit.

    By the way, this particular rule of thumb regarding the roll stiffness relation to the suspended mass distribution took me approximately 3 years to unlearn.

  4. #4
    Tim,

    I do agree with you that there are some shortcuts or simplification (especially in the tire linear range – low slip angle) made between on one hand weight distribution and front and rear tire cornering stiffness distribution and on the other hand weight distribution and roll stiffness distribution. The three should be look at together.

    I also agree with you that all other things being equal or ignored (aero, camber, compliance for instance) if you move your CG forward you need either to soften your front suspension or stiffen your rear one to keep the initial balance.

    However, let me know if you have similar perspectives or not…

    1. Do you agree that the ideal car performance occurs when each tire is used at the maximum available grip (if we only think lateral grip that means peak slip angle) AND we have the yaw moment you want when you want.
    The ideal yaw moment you want is zero on the skid pad (or for a fraction of a second at or near the corner apex)
    But in transient (corner entry, exit) the amount of ideal yaw moment depends on the variation of the speed and the variation of radius. In simple words too much yaw moment (or yaw acceleration) = oversteer and too little yaw moment (or yaw acceleration) = understeer.

    2. If we exclude the effect of camber and pressure and we agree the front/rear tire width ratio (same front and rear compound, let’s not complicate things) is very similar to front/rear tire cornering stiffness ratio….
    And if I take your Audi TT example….
    And you have the same front and rear tire…..
    And we look only at the front and rear outside tires…..
    If the car is at the limit (that means each front and rear tire is close to its peak slip angle… that means that the front and rear cornering stiffness is zero…ouch ratio of zero Vs zero… how do we go about that? A bit of academic here…)
    And you manage, despite the front weight bias distribution, and thanks to your TLLTD to get the same vertical load…
    Well ….same front and rear tire, same (peak) slip angle and same vertical load … that means same front and rear lateral force… (let's exclude the cosine of the steering angle)
    But a and b are different so the yaw moment around the CG is not zero. If you are at the limit we should have 1000 Nm of yaw moment one way or the other

    What is right or wrong with my perspectives?

    Claude

  5. #5
    To go in the same direction as Tim first reply... maybe in other words
    When we change our weight distribution we change FOUR parameters; your front and rear tire cornering stiffness - at least in the linear range = small slip angle (and I agree with Tim; it follows pretty much the same ratio change as the weight distribution change) but also a (distance front axle to CG) and b (distance CG to rear axle). a and b will change in a much higher percentage tan the cornering stiffness ratio. Not only the tire front an rear lateral grip Fyf and Fy b change but as a and b are the leverage arms of the front and rear grip... that changes the Yaw moment about the CG

  6. #6
    Claude,

    I don't want to hijack this thread, but coming from "front and rear tire choice":

    A couple of years ago, you made the hypothesis that there is a "tire out there, which noone uses but could be the perfect one for FSAE".
    Zurich is searching since the fall of 2012 for a tire to replace the Hoosier and was not able to find one so far. As a team that normally finds stuff, they are still wondering.

    Care to reveal your secret?
    -------------------------------------------
    Alumnus
    AMZ Racing
    ETH Zürich

    2010-2011: Suspension
    2012: Aerodynamics
    2013: Technical Lead

    2014: FSA Engineering Design Judge

  7. #7
    Senior Member
    Join Date
    Aug 2011
    Location
    Brisbane, Australia
    Posts
    132
    I think the secret was that Claude has no idea if there is or not, but hopes to inspire people to look beyond the solutions everyone else is using.

  8. #8
    Well of course, that's what we are thinking as well.

    But calling his "bluff" two years ago resulted in a "no, no, you just did not look at the right place". So I'll try again
    -------------------------------------------
    Alumnus
    AMZ Racing
    ETH Zürich

    2010-2011: Suspension
    2012: Aerodynamics
    2013: Technical Lead

    2014: FSA Engineering Design Judge

  9. #9
    Senior Member
    Join Date
    Feb 2011
    Location
    North Carolina
    Posts
    114
    Quote Originally Posted by JulianH View Post
    Claude,

    I don't want to hijack this thread, but coming from "front and rear tire choice":

    A couple of years ago, you made the hypothesis that there is a "tire out there, which noone uses but could be the perfect one for FSAE".
    Zurich is searching since the fall of 2012 for a tire to replace the Hoosier and was not able to find one so far. As a team that normally finds stuff, they are still wondering.

    Care to reveal your secret?
    Yup same here. We searched for awhile and I think the only company we came across that had something suitable that no one is running was American Racer. Many hours were spent on Google trying to find the magic tire. haha
    Trent Strunk
    University of Kansas
    Jayhawk Motorsports
    2010-2014

    Now in NASCAR land. Boogity.
    Opinions Are My Own

  10. #10
    Still.... That tire does exist and to my knowledge (and I attend at least 6 or 7 FS competitions each yea) it has never have been used in any FSAE / FS competition. One team was close to the answer and then gave up a few years ago. Just think about the type of car and the type of race duration that are similar to FS in the racing world. The search is not that difficult. Well it could depends on how many languages you know. Think outside the box. It is there.
    Claude Rouelle
    OptimumG president
    Vehicle Dynamics & Race Car Engineering
    Training / Consulting / Simulation Software
    FS & FSAE design judge USA / Canada / UK / Germany / Spain / Italy / China / Brazil / Australia
    [url]www.optimumg.com[/u

+ Reply to Thread
Page 1 of 5 1 2 3 ... LastLast

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts