- Tyes are obviously one of the most important things we have when it comes
to racecar performance.
I've got Jared here from University of Canterbuty Motorsport and we're going
to be talking about tyre tuning and tyre selection on one of their Formula Student
cars.
Hey Jared, we've got a couple of your cars here behind us, I can see one's a combustion
and one's an electric and obviously here we've got wet tyres fitted to one car and
slicks to the other.
But can you talk a little bit about how you guys go through a tyre selection
process, what sort of parameters you're looking for, how do you actually choose
what you're going to build the car around?
- Yeah absolutely so a big important thing for us is about the grip levels we're able to
get out of the car given the events we're going to be doing.
So within Formula SAE we have a range of events given from acceleration, very simple
straight line test, straight through to full endurance running.
So we want a tyre that's able to give us the most grip over a series of different events.
- And what sort of tyres have you got available to you, I presume it looks like
these tyres are made specifically for these sorts of cars, so what sort of range of
options have you got, brands, sizes, big, small, wide, narrow?
- There's a real difference between a lot of tyres throughout the FSAE community
should I say.
Everything from an Avon semi slick which is similar to what you'll see on a Formula Ford
straight through to the Formula SAE specific tyres we see that we have from Hoosier here.
So there's a lot of different brands, there's a lot of widths, lot of heights and that's very
dependent on what people are looking for in terms of tyre temperature.
How much RPM you're running, what your wheel speeds are going to be, how much
you want to try and work the tyre, what your car weight is, all that kind of stuff really
builds into how we decide where we're going to get to on our final tyre.
- And where does that tyre selection process come in as far as the whole car design?
Is it a case of designing the whole car around the tyre, is it designing some of
the car and then saying this is the tyre for us, where does that come in in the
design process?
- It's definitely been an iterative process over the years so we've changed our
compounds in the tyre we use based on our performance from previous years.
So you take what we've learned from the previous years, we kind of go OK we're
happy with the performance but our longevity wasn't enough, let's look at a
slightly harder compound that's going to last longer.
As we know our car is going to be similar in weight and similar in performance to the
year before, hopefully a little bit better but it's mainly that we look at what we've
done previously and base it off that.
- So obviously being part of the university, you guys have got enormous access to
resources as far as simulation and computing power, probably much more so than the vast
majority of race teams really.
So how do you guys leverage that, what sort of information do you get about the tyres
and does that come from the tyre manufacturer, do you have to test
yourselves, where does that come from?
- So there's a Formula SAE consortium which gets data from Calspan International out of
Europe, so they do all the test rig for tyres specific to Formula SAE.
We're able to take all that data and make our own MATLAB scripts and that is able
to extract all the data we want out of it and gain predictive models of the tyre
in multiple ways.
- So when you say tyre model, what exactly does that mean, what information
are you getting from those tyre tests and when you say model, what is that
going from and to, what are you giving yourself there?
- So it's all about, for the tyre model, is about having basic outputs and inputs.
So for our inputs we have our normal force, which is the tyre acting down
from the load on the car and you're also looking at the slip angle going through
the tyre.
So from that data, we're able to look at it and go OK what's our lateral force coming
out, what's our longitudinal force at our lining moment and our over turning moment?
So those are based off of testing from a big belt with a test rig where they have all the
data and sensors, they record all of that and all we do is data fitting to that through
a MATLAB script and then that's transferred through.
- So this is essentially taking discreet points of data and fitting an analytical curve through it
so you can, to essentially look up the forces at any combination of vertical load, camber,
slip angle, stuff like that?
- Yeah absolutely so our specific model is known as the Hans Pacejka magic formula
which a lot of people use and a lot of professional teams use to semi imperically
define the performance curve of the tyre.
- Got you and what about validation, so we've talked a bit about the selection process,
how you guys go about that, but when you get onto track, what are you using as far
as sensors or back ending that validation to really understand whether you made
the right decision?
- Yeah so we've got our linear potentiometers acting on the dampers which is looking at
our loads acting through the corner.
We've done a bit of work going on the normal force acting through all of our push rods
so that we can start to interpolate how much normal force we're getting through and then
we're able to look at what we should be expecting in terms of grip levels
through corners and we're able to use that in terms of our GPS and all our other sensors
to bring that together to go OK this is what we should be expecting from a lap time,
how good are we doing comparitive to that?
- And how does that validation process usually work, do you find the tyres are
usually behaving as you expected based on the data you gathered before from the simulation
or is there usually quite a big difference?
- Relatively, we're slowly working through an Adams model of the car to simulate a
lot of quasi static setup for the car and how it's going to do our lap times.
We're getting pretty close between our physical model, our MATLAB model and our Adams model
so we're pretty happy with it at the moment but it's a very long process to get that to a
full track simulation.
- What about tyre pressure, what's a typical hot tyre pressure you guys would be targeting
for a car like this, what sort of range?
I'm sure you don't want to give away all your secrets but give me some idea
of where you are?
- Yeah and this is probably a really big thing comparative to normal racing.
We're running really low, like we're talking 13, 14 psi hot, maybe a little
bit more but yeah because of how light the cars are and how wide the tyre is
relative to that, we're looking at really low pressures to try and really work the
tyre and gain the most grip out of it.
- Got you, and like you said before, there's a range of different styles of events you
guys do, you do an acceleration event which is essentially a drag race and you've got
auto cross and endurance which is much closer to something like circuit racing or
a sprint, are there many differences in the tyre pressures in the way you set up the
tyres for those different types of events?
- Yeah absolutely so it's all that kind of stuff that we're used to like camber and toe
and tyre pressure, we're basing all of that relative to the events we're doing.
The way we set up for acceleration is totally different, we want to have the car to be able
to run well in a straight line, we want to be able to squat nicely, we want to be able to
get the most grip out of the back of the car mainly.
So you know we're lowering the pressures in the back, we're making sure that we can get
optimum traction off the line and then from there, moving all the way say through to endurance
we're looking at having a more steady car throughout the distance of the race.
We want to be able to make sure we're not increasing too much tyre wear, we want to not
be putting too much camber so that we're not wearing the inside wall, all those kind
of things that you would normally see in normal racing.
- So from what I understand, the auto cross event is essentially a single lap
and the endurance is a similar lap but many times over, I don't know how long
you go for but is the way you set the pressures for the auto cross, does that
mean you're starting with a lot higher cold pressures in order to achieve the
right hot pressures within one lap?
- Yes definitely so because we want the car to be on straight away, we're looking
at increasing our pressures straight out the gate so that the car is on straight out and
we're able to have within the first 300 m we've already got hot tyres.
We put that in with a bit more increased toe so that we're able to just generate
a lot more heat so that we can make the most out of a one lap dash.
- Interesting, so how much, what sort of difference in toe gives you a measurable
difference in your initial tyre temperature like that?
- Ooh hard to say, as you said I don't want to say too many of our secrets.
But we're not talking too much, we're talking a couple of degrees is really enough
to work these tyres, they are a super soft compound so really especially with the 4WD
systems, it's working in a lot of different ways and it gets a lot of heat quickly.
So you don't need to do anything majorly drastic to the car.
- So from what I understand, the electric car behind us here uses a torque vectoring
system, you were saying earlier that you were one of the drivers for the competition.
So for that torque vectoring system, how does it feel for you in the car, do you have to drive
it quite differently from say a normal combustion rear wheel drive car, what is that like in the
car for you?
- It's totally different, one, going to the electric is a totally different kind of beast
really, we're talking about something that's got instant torque, wherever you put your
foot, you're going to get the power you want, relative to waiting for a combustion
car with its power curve where you're working through your rev range, this has
the same power throughout that whole range up to maximum speed so first of
all it's about learning that and getting used to that working well and then once we
implement our torque vectoring like we have on the car, we're able to load the car up
and we're able to keep on the gas through the corner even up to the apex a lot more
because we're actually able to generate more grip when we've got a little bit of
gas on.
The only hard thing with that is is that if you start to get a little bit of oversteer
actually going on, as soon as you lift off the gas like most people will, it'll just
really come around on you.
- Is that something you find, is it much more difficult to drive or is it just
something that you've really got to get used to it?
- I think it's just a matter of change.
It's a matter of understanding what's underneath you, taking a little bit of
time to adjust but in terms of the car being different, it's not harder in any
other way, it's just a matter of changing your style.
- And what about a performance increase, have you been in a position where you've
been able to back to back the car with and without torque vectoring, what sort of
percentage increase in performance do you get?
- Oh major, we've probably seen over say a 30 second lap stint that we do at our test
track we're talking about a 1 second difference of drop in time.
- That's pretty massive.
- Oh it's huge.
- From a driver's perspective, with the electric vehicle over the endurance distance, is that
a situation where you have to essentially energy save because of a certain amount
of energy in the battery that you've got available?
- Yep so we also use regenerative braking so first of all we turn our power down
and we use regenerative brakings so we're able to use a multiple system so we're
not using as much power but we're also trying to bring power back into the system
when we're braking.
So one that means we wear our brakes a lot less and we're able to trail brake
the car in with regenerative braking but we're able to power ourselves up which
hopefully in time once we start to understand our electrical systems a bit better, we'll
be able to reduce the size of our electrical package and reduce the weight of our car.
- And as far as the amount of rengenerative braking you can get, are you able to harvest
everything, do you still have, do some of the conventional brakes still have to do
some work, is there a limit to that?
- Usually when we're doing an endurance, we base ourselves to only brake with
regenerative braking so we're talking about 15 kW of braking power which in one of
these is near on lock up.
- OK so when you talked about having to turn the power down for an endurance
event, is that more of an energy limitation of how much energy you've got to complete
an endurance event or is it more about maybe a reliability thing, making sure you
don't cook anything during that run?
- Yeah so it's a little bit of both.
So temperature is a really big thing, the more power we pull through the
electrical system and the batteries, the hotter it gets and we do run into
issues if we have any of that but as well it's about part of the endurance attempt
is to have the best efficiency possible.
So we're trying to get a matter of trying to get the best compromise between getting
a really good endurance time and finishing well in that as well as having our best
efficiency possible.
- Got you.
Thanks very much for the talk Jared that was really interesting, some really
fascinating stuff going on with these cars.
- Cheers mate.
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