View Full Version : Anti-Ackermann? Is anyone using it?
guzzman
08-02-2007, 06:16 AM
Hey everyone, im currently design the steering system for the Curtin Motorsport Team in Australia, and from our tire data (Hoosier 20.5 x 7 - 13") i have determined that the tires prefer anti-Ackerman.
I have completed many hours of reading on the Ackerman argument, however I can't seem to make a decision. I know there are other teams using this tire so i was wondering if anyone out there is running with anti-Ackerman, or if you are running with Ackerman what is your justification if your tire data says otherwise?
My thoughts are that the anti Ackermann is preferred by the tires because a lightly loaded inside tire has maximum grip at a lesser slip angle (or more heavily loaded tire requires a higher slip angle)
From this statement, I was hoping someone could enlighten me as to why teams go with Pro ackermann anyway
Thanks for your help, and yes ive read all the other topics http://fsae.com/groupee_common/emoticons/icon_smile.gif
guzzman
08-02-2007, 06:16 AM
Hey everyone, im currently design the steering system for the Curtin Motorsport Team in Australia, and from our tire data (Hoosier 20.5 x 7 - 13") i have determined that the tires prefer anti-Ackerman.
I have completed many hours of reading on the Ackerman argument, however I can't seem to make a decision. I know there are other teams using this tire so i was wondering if anyone out there is running with anti-Ackerman, or if you are running with Ackerman what is your justification if your tire data says otherwise?
My thoughts are that the anti Ackermann is preferred by the tires because a lightly loaded inside tire has maximum grip at a lesser slip angle (or more heavily loaded tire requires a higher slip angle)
From this statement, I was hoping someone could enlighten me as to why teams go with Pro ackermann anyway
Thanks for your help, and yes ive read all the other topics http://fsae.com/groupee_common/emoticons/icon_smile.gif
JHarshbarger
08-02-2007, 06:31 AM
I think most teams realize the issue of the outside tire having more lateral acceleration at higher slip angles. Few teams go as far as running anti-ackerman geometry, but most will run reduced pro-ackerman or have geometries closer to parallel-steer. If you are designing for a large amount of weight transfer, your car may very well prefer anti-ackerman. I would just make it adjustable or have replaceable parts so you can fine tune it.
flavorPacket
08-02-2007, 07:36 AM
people run pro ackermann because this competition is about yaw. you can take it from there.
js10coastr
08-02-2007, 08:43 AM
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by guzzman:
From this statement, I was hoping someone could enlighten me as to why teams go with Pro ackermann anyway
</div></BLOCKQUOTE>
"I'd rather go with a pro ackerman than with an amateur ackerman."
Thanks folks, I'll be here all night!
Goran Malmberg
08-02-2007, 12:11 PM
I would like to add that it does matters how fast the track is concerning the Ackermann angle. So there is no fixed receipt.
Regards
Goran Malmberg
James Montupet
08-02-2007, 02:22 PM
http://students.washington.edu/dennyt/fsae/zapletal/
The toe article will have what you're looking for. I'd recommend reading the whole thing but if you feel like just skipping down to some arguments against anti-ackermann, find "large slip-angles" in the text file; just a convenient header to look for, I'd say the large slip angles argument doesn't apply to us.
James Montupet
Duke University Motorsports
guzzman
08-02-2007, 08:06 PM
Thanks for the comments.
I have that article already, and reading it only confirms my suspicion that my tires like anti-ackermann, i guess i just need to run tests with pro and anti ackermann and see how the car goes in the FSAE type tracks..
There is the statement that anti-ackermann wont like sharp corners but how sharp is sharp!?
James Montupet
08-02-2007, 09:53 PM
Guzzman-
Clearly, the best approach is the one you proposed: have adjustable ackermann and test it.
But... this could add weight, complexity and increase design time. Also, with certain packaging constraints, you may not have the range of adjustability you'd want.
So, that comes right on back to the question of getting a good baseline. And to do that, you've got to get around the "how sharp is sharp" question.
One approach is to use the kinematic steer angle method highlighted in the paper. This will give you "effective" dynamic toe, rather than just a dynamic toe value you can't really relate to anything. This will allow you to approximate optimal dynamic toe for different corner radii.
Another approach to combine with the one above is to use the tire data to calculate the yaw moment associated with slip angle drag; look carefully at the magnitude and direction of your "lateral" force, effectively. You can then compare it to something like tapping on the brakes mid corner to see if you think its a serious performance gain or loss.
James Montupet
Duke University Motorsports
guzzman
08-03-2007, 02:43 AM
Thanks James, quite helpful stuff.
After running a few simulations, Im going to look at the yaw moment of the car using the tyre data (lateral forces) during steady state corning, and the effects a change in ackermann has on this. The transient state will also have to be considered at the beginning and exit of the corner. Hopefully after a bit more thought i can figure out how to measure the transient yaw moment (since the weight transfer is moving towards the front at some rate)
Just putting out some ideas which i will start to tackle over the weekend.
guzzman
08-03-2007, 02:59 AM
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by James Montupet:
http://students.washington.edu/dennyt/fsae/zapletal/
I'd say the large slip angles argument doesn't apply to us. </div></BLOCKQUOTE>
Can you explain why you think this? The largest slip angle im calculating is around 7.4 deg
murpia
08-03-2007, 04:11 AM
I'm wondering what kind of static toe settings teams are running?
My experience is that static toe out at the front works well for braking stability and predictability of the car. Of course this is heavily influenced by Ackermann setting, as mentioned above.
Anyone care to share their Ackermann and static front toe settings?
Static toe in at the rear is pretty universal, although I hear FWD racers go for rear toe out to get good turn-in...
Regards, Ian
"Our tyres want anti-ackermann"
What does your car want though? If the effect of anti-ackermann is slower yaw response it's not automatically what you want.
RCVD cogently argues the anti-ackermann case, but the specific example is superspeedways. In this environment slip angle drag can cause you to be power limited in a turn. You are never going to be power limited in a turn in FSAE, so minimising slip angle drag might not give you an advantage.
Ben
James Montupet
08-03-2007, 06:40 AM
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by guzzman:
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by James Montupet:
http://students.washington.edu/dennyt/fsae/zapletal/
I'd say the large slip angles argument doesn't apply to us. </div></BLOCKQUOTE>
Can you explain why you think this? The largest slip angle im calculating is around 7.4 deg </div></BLOCKQUOTE>
Couldn't put it better than what Ben said above.
James Montupet
Duke University Motorsports
guzzman
08-03-2007, 06:48 AM
Ah okay, well i went through simulation today and the yaw rate / response is very similar when comparing anti and pro, with the anti setting giving a higher average yaw rate and quicker response time.. i will have to check the numbers and go through the simulation again, but so far anti looks like the go!
James Montupet
08-03-2007, 06:59 AM
Feel free to omit whatever details you want, but just out of curiosity:
What type of corners? Did you include a slalom or a hairpin?
Home-built simulation, commercial, type (doesn't sound like a steady state model), etc?
James Montupet
Duke University Motorsports
guzzman
08-03-2007, 07:44 AM
No problem, must admit ive only done some initial testing. So far i've only simulated a step steer situation, but will be running slalom and skid pad also to see how it fairs through that. We are using the veDYNA model
guzzman
08-14-2007, 10:26 PM
just thought id post my progress on this..
after discovering that the tyre data in the simulation program is not reflective of that from MRA, the simulation isnt any good for transient testing...
Ive hit a wall now, and im back where i started.. Ive tried to compare yaw moment for different geometries however the lateral force values are not known for transient cornering since it includes the elastic stage of weight transfer.. any ideas how to simplify it?
Mike Cook
08-17-2007, 11:49 AM
I'm pretty sure that tire doesn't want anti-ackerman. Toe out is going to have a much larger affect on transients the ackerman since transients start at small steer angles and at small steer angles ackerman has little to no effect. So, it seems to me that you have little to no pratical experience with these cars. If I were you I would start somewhere closer to 100% ackerman. Look at pictures you can basically see what the good teams are running. From there, run a skid pad and adjust your toe rods to get a feel for what the car wants steady state. Then use toe to change your transient behavior. This will produce a decent car, and if you make it this far then you can start to worry about about the transient effects of ackerman steering.
Oh, and toe out is not stable under braking. But with these cars its not usually a huge concern.
flavorPacket
08-17-2007, 04:01 PM
here's a free lesson. design finalists 2007:
http://img518.imageshack.us/img518/5861/dsc04100nt3.jpg (http://imageshack.us)
The one on the left had so much ackermann that it made the other cars feel bad, so it just kept the wheels straight.
duckei
08-26-2007, 01:30 PM
I like it. I like it a lot.
Christopher Catto
09-01-2007, 06:55 AM
It is good that your simulation can give you that kind of accurate information. Interpreting it may be somewhat harder. Although all tyres are different, it is a BIT unlikely to run anti-ackermann on an FSAE car unless the track is very open. The driver should really test this because simple things like how much steering lock he needs to use to conduct a turn or a slalom are quite important. Not just factors like cockpit/elbow space but also how fast he has to turn the wheel to get his desired response.
As for parts manufacturing, it is not too hard to make the steering bracket with 3 holes, or plugs which each have the hole in a different position, or simply a slot (if you do the bolt up tight there is no way the bolt will slip, only in case your tyre hits a high kerb). then you can have some special in-house rod ends with long thread (threaded rod with TIG-welded cup on the end, then press-in a bearing)to allow for the large amount of adjustment you need.
Practical experiments are the best way to validate your simulations. The judges will like it. Just dont spend 6 months just to figure out that your car does not mind -50% to +100% Ackermann.
Drill the holes, make the steering arms, go testing, time some figure of 8s and a complex track. Compare laptimes. Write brief report. Move on. Solve next problem on the list.
Kyle Roggenkamp
09-01-2007, 11:39 PM
Here is our car from 2005. Not the best picture, but you can kind of see our ackerman angle.
http://img403.imageshack.us/img403/9330/fsaepictures403copynq4.th.jpg (http://img403.imageshack.us/my.php?image=fsaepictures403copynq4.jpg)
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