Definitely DO NOT do this with your hand! Grab a piece of wood or something if you mustOriginally Posted by Z
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Definitely DO NOT do this with your hand! Grab a piece of wood or something if you must
Daniel Muusers
Formula Student Team Delft
2010-2015
I think this is worth looking at too. Also I'm interested in this..
.. is this in aid of unloading your inside rear to help your spool?60-70% Rear LLTD
Mitchell,
All looking very good!
Just fitting the largest legal vertical "end-plates" at the rear (ie. "rudders") will help with the tail-happiness. Then add the wings ... wow!
Z
A few comments/questions,
1) You say the tires are over heating. I would tend to agree as those temperatures are higher than I've seen before (but not by much). In the end, it doesn't really matter what the temperatures are as long as the car does not slow down over the course of an endurance. Maybe 100 C is where they are performing the best. I would suggest looking at your practice endurance times to see if there was a drop off. If no... Then maybe you don't have a problem, other than wearing your tires out quickly, $$$. If yes... There are a few things to work on. See below.
2) Expanding on the point above, I'd watch and ask the driver if the handling changed over the course of an endurance.
How does the car behave at the beginning when the tires are cold? Understeer/oversteer/4 wheel slide? What part of the corner? Braking, turn in, first third of corner, apex, second third of corner, final third of corner (exit)?
How does the car behave in the middle when the tires are warm? Understeer/oversteer/4 wheel slide? What part of the corner? Braking, turn in, first third of corner, apex, second third of corner, final third of corner (exit)?
How does the car behave in the end when the tires are warm/too warm? Understeer/oversteer/4 wheel slide? What part of the corner? Braking, turn in, first third of corner, apex, second third of corner, final third of corner (exit)?
Additionally, asking them how the car behaved in different types of corners may be useful- slow, medium, high speed.
3) From the data presented I suspect some of your issues may be from how the car is handling with cold tires. Cold tires + spool + 60%R + narrower front tires = understeer at turn in and first third. Big engine + cold tires + turn in/first third understeer = mid corner to exit oversteer (as seen in your videos). This will cause the rear tires to heat up quickly, quicker than the fronts and create even worse understeer because the rear tires are hot and sticky. Eventually this may cause your rear tires to overheat.
IMO- this is pretty classic push-kick behavior. Ask the drivers the questions from 2. Most drivers, especially amateur, will only remember the car is oversteering on exit. Have them break down the corner and maybe you will find other areas where the handling can be improved. By fixing these areas the exit oversteer may solve itself.
IF you have turn in / first third understeer add front grip.
4) Driving FSAE cars. I've driven a number of cars, 1-4 cylinders, aero/non aero. They drive like no other vehicle. The best comparison is probably a fast shifter kart but with way more power. I've found that the handling can change dramatically depending on how you drive it. In general applying throttle before apex leads to mid corner/exit oversteer in slow corners or mid corner to exit understeer in high speed corners. Look at your data (steering angle, throttle position, and wheel slip) to help find the problem.
Have them drive deeper and brake harder into slow corners (maximize V in), get the car rotated in the first third of the corner while slightly on the brakes=more front grip, second third come off the brakes while unwinding steering, at apex the steering wheel should be straight and you should apply throttle (a little) to transfer weight to the rear, final third keep rolling on the throttle and track out using all available road.
Cheers
Last edited by dr. ill; 10-12-2015 at 07:57 PM.
Mitchell,
I wouldn't be worried at all. The car setup looks pretty good for something that will have aero added. I agree with Z, next time you go out add a big rear fin (as per your eventual endplates), it should almost certainly get rid of most if not all your instability. Frankly the non-wing teams should be doing this as it allows you to setup a car that accelerates in yaw quite quickly at low speed, but doesn't have any nasty high speed problems as a result.
Once your slip comes into control wear and temperature will be much more manageable. You should have absolutely no problems getting the R25B's to last, and Z is right when he says Aero will make it better not worse.
I also wouldn't be trying to "work around" the spool as a lot of people seem to think. If a car is setup reasonably well (i.e. enough lat G and braking G) then the natural behaviour of the car around the track will do enough unloading of the inside at the right places without having to resort to extreme rear LLT or massive caster angles.
If you did want to have a go at fixing it in the current state (which I would resist - your car is ready for aero). I would think that some camber at the rear and a narrower wheel (as suggested by others) might be a good idea. Also when you tested your beam for toes deflection (and FEA) did you do it with the "as-installed" situation with the links. Incorrectly loading the beam will produce some funny results. I think this is an area where FEA coupled with accurate calculations of link loads will help a lot, given that your toe deflection can change magnitude and direction as you go around the traction circle.
Also: Dampers.
...
All up my take is that a good fsae car will be very responsive at low speed (i.e. mechanical grip only to get around those hairpins) with decent exit traction to avoid spins due to the right pedal, and as aero builds the response reduces to ensure fine control for high speed chicanes. You don't need wings to achieve this behaviour, but you do need a rear fin (i.e. endplates). The non-aero cars tend to have either low speed push or high speed instability depending on the bravery of the driver, and too many aero teams build cars that are good without aero and then add aero and don't revisit their base mechanical setup. The car will be faster, but far from ideal. Aero shouldn't be treated as an add-on.
Kev
Dr. Hayward,
ECU having a surprisingy similar concept to UQR, how did the even softer LC0s fare against the 600's power and the spool's slipping ?
Or was the team prepared for a tyre hungry beast ? Did the LC0s' slip characteristics help ?
P^Squared,
The ECU students are obviously a fan of where UQ have gone, as well as a little worried. UQ have been one of the most innovative and talented group of people in FSAE-A of late. Having them working on the same basic concept puts a lot more pressure on our team.
The ECU car had no problems with tyre wear at all on the LC0's. I should mention that as big winged 4 cylinder cars go the ECU vehicle is quite a light one (184kg). As I mentioned above I don't think that spools are a real issue on these cars, although they can be made to be an issue if you over-compensate. The beam rear end tends to help quite well with wear in the rear, and careful design of the steering geometry can reduce front end wear quite a lot. Note that in some cases KPI can be very useful (even though it seems that most students want to avoid it at all costs).
We have had ludicrous amount of running from our LC0 sets. The aero definitely helps a lot for tyre wear. I have had a few people tell me they were worried about tyre wear due to increased load and grip, but almost always ignoring slip angle. Slip angle is just a measure of sliding velocity compared to vehicle velocity and power is equal to Fv where v in this case is the sliding velocity. The power going into the tyre is what causes heat and wear. Basically for decent wear we want to minimise this power. Given that we want to increase F we can only do this by reducing v. The aewro does a good job of this in two ways. First that it keeps the car very stable due to the moment resisting yaw of the endplates. A non-aero car tends to spend much more time at higher slips. We cannot assume that the driver will always operate at ideal slip angles, and aero/non-aero cars are often driven very differently. Secondly the grip level for an aero car in some locations can be significantly higher than what the driver can reach given previous corners and physical ability (i.e. ability to turn the steering wheel fast enough). This means that for some of the lap the car will be operating below peak slip angles (i.e. reducing sliding velocity).
The second approach to reducing tyre wear is to feed this power into as much rubber as possible. Rubber is a very poor conductor of heat, where you put the power in will be where the rubber heats up. In this respect for wear there is little point in having a 6" wide tyre if you are only using 3" significantly. That rubber will tear away quite easily. This is all kinematics and making sure that the tread is being used, noting that the power during braking can be very high, as well as wheels spinning up. For the latter the power can be incredibly high as the sliding velocity of the rubber can be huge. Designing the camber etc just for peak cornering grip can have a huge detrimental effect on wear. Everything in these cars is transient, and as such the car is only at pure lateral g for the briefest moment, there is almost no point in designing/tuning a car for this point, and that includes wear/temp.
Thirdly vertical transient response matters. The unsprung vibrations can have a noticeable affect on tyre temperature and wear. Every time the vertical load is changed on the tyre while it is generating grip will have an effect on which rubber in the contact patch is slipping and by how much. There are some very well known empirical techniques involving springs and dampers that are used to alter tyre temperature. These are sometimes not considered while making "balance" changes. i.e. increasing the rear spring to increase rear LLT will also change the vibration characteristics. This can cause a doubling down on temperature increases and resulting wear.
...
The R25B's are a different beast as they are noticeably stiffer in the sidewall and more sensitive to wear due to incorrect camber, also much more sensitive to general steering geometry. They require a different compromise between power, contact patch, and vibration characteristics than the LC0's.
In general the LC0's are a great forgiving tyre, but lack the response of the R25B's. Both can be made to work, and wear shouldn't be an issue with any of the tyres.
Beyond my rough observations you would have to ask the students on the team, they are the ones designing and developing the thing.
Kev
p.s. Sorry if some of this post is a little jumbled, in the middle of prepping a dynamics lecture, and I'm not focusing 100%. Although it is amazing that so much of this "race car vehicle dynamics" is just "dynamics"Add in some basic understanding of the friction properties of a chunk of sliding rubber and you are most of the way there.
Last edited by Kevin Hayward; 10-12-2015 at 11:51 PM.
I saw this in the local radio station news blurb:
http://whmi.com/news/article/23808
It reminded me of your video. Ya know, your track and my driveway seem to have something in common: They both look similar. My driveway is crushed recycled asphalt that's been rolled into a surface. Friction and mu stability is horrible. Just what kind of max G can you hold (yeah, I know your car is "Tighty-Loose".
Besides, A car with that much oversteer really will never get you a high g level without shredding the tires from excessive axle sideslip. You need bigger rear tires to reduce the rear cornering compliance until the car's oversteer is less than it's Ackerman Gradient. It will still be "loose" but more managable and have much higher side-bite.
I wish I had a driveway big enough to test friction and mu stability! In a steady state skidpan radius corner we oscillate between about 1.25-1.35G on the black stuff and between 1.3 and 1.35 on concrete. Through actual driving on track you can see peaks out to 1.4-1.45G through the slaloms and bus stops. When I first starting doing this competition I saw people on these forums claiming 1.7-1.8G non aero with hoosiers and thought that we must just be terrible at building cars or driving them. These days I feel like most people overstate their numbers quite a bit and measure acceleration away from the CG (in a dash) without correcting for yaw acceleration.
Considering we've built a car for 10s and already have our tyres for comp there doesn't seem to be much we can do in terms of adding more tyre this year. The next significant jump in size for 10" is either an Avon 16x8.5-10 which comes in harder compounds and is not cheap in Australia or the Hoosier 18x10-10 LC5 which is also apparently a reasonably hard compound. Kev and Z both seem convinced that the car is well setup to receive wings. We love how it feels through the hairpin and the tight stuff, it's just the long radius corners where the car is drift spec and destroying tyres. On that basis I feel like maybe we are on the right path and this is an issue that in our case is best solved with wings. What's your take on this? Don't stress too much about the oversteer until we have wings on the car?
Well what does your tire test data tell you? Conventional wisdom would advise that adding rear downforce will increase the oversteer because a tire is a 'softening spring'. (That's a bad analogy though, its really like a damper). And, in fact, your tires may respond with MORE stiffness from the aero. I didn't mean a "bigger" rear tire, I really meant "more" tire (higher cornering stiffness. Tires which increase stiffness with load (there actually is a Load Sensitivity metric are often used on production high level sporty cars (Corvette for instance). This makes roll bar effects all backwards. If this was the case in your car, MORE rear roll stiffness would help. Again, a simulation with the tire data would tell you. If not, go play car-car and decide: better or worse.
To help resolve some of this, screwing another tire and wheel alongside the existing rears with a hub adapter would be very informative and a lot of fun. Double your Pleasure, Double your fun as they say.
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