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Old 06-12-2008, 07:39 PM   #2176
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Brian,

I really appreciate the time you took to answer my question, thank you. Thats exactly the info I was looking for.

you got pm,

thanks again,

Casey
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Old 06-12-2008, 07:41 PM   #2177
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Last edited by The Bear; 06-12-2008 at 07:41 PM. Reason: double post
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Old 06-12-2008, 08:44 PM   #2178
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Quote:
Originally Posted by Brian McGreevy View Post
The only thing related to the steering system itself that determines a car's behavior is Ackerman. The center point steering, when set as most people do (inside hole on the steering block), delivers a difference of 9 degrees between the inside and outside wheels at full-lock (steering blocks hit stops on caster blocks), with the inside wheel having a higher angle with respect to the centerline than the outside. The RDX dual-bellcrank steering, when set with the shortest toe links, as most people do, delivers a 6 degree difference between inside and outside. The optimum angle is dependent on the tire. Certain tires require higher degree of slip angle to produce maximum lateral force than others. The right Ackerman setting is largely dependent on the type of track you run on. A very tight track with lots of 180's favors a car with more Ackerman because of the high degree of difference in wheel speeds and corner radii. A more flowing course, with larger radius corners where cornering speed is the absolute priority, requires less Ackerman (more equal steer), but only to a point. See below...

The other thing to consider is scrub. Most RC car tires will produce more lateral force when the slip angle is increased. However, it comes at a cost as too much slip angle will also create excess scrub. In a physical sense, this means that there will be enough of a component of the lateral force in the longitudinal direction to slow the car down. Theoretically, if one could turn the wheels 90 degrees, all the lateral force generated (with respect to the tire) would go into scrub, and that would slow the car immensely. The optimum Ackerman setting is a fine balance between creating lateral force and inducing scrub.
So what book did you copy this from? Nice to see some of that schooling of yours paying off.
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Old 06-12-2008, 08:47 PM   #2179
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Originally Posted by Johnny Wishbone View Post
Brian, can you tell me what the difference in degree is when using the outside holes full locked?

Thanks
As per the instruction manual, 8 degrees .

The inside tire will be at 30 degrees, and the outside at 22.

The inside holes on the steering knuckle yields 30 degrees and 21 degrees, respectively.
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Old 06-12-2008, 08:52 PM   #2180
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So what book did you copy this from? Nice to see some of that schooling of yours paying off.
Hahaha, nice Kraig. Yeah, I'm looking into getting into race engineering when I get out of school (next May, yikes!), so then I'll get paid to do this stuff! Right now Formula SAE (and some books) has taught me a lot about vehicle dynamics, both in a theoretical and practical sense. It's still hard to translate some theory to RC, especially on a foam-tire car, because their behavior is so different from any kind of rubber tire. Some things work backwards in foam compared to how they work for rubber tire and in theory.
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Old 06-12-2008, 10:41 PM   #2181
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Sorry; I think you descrbed Ackerman really well; but, I cannot agree with the premise that "the only thing is... Ackerman".

The 6-8 degree differential between the inside and outside wheels is only a measurement taken while the car is resting in a static position. It does not take into account the up/down angle of the steering links (between the attachment points of the steering lever arm and the steering blocks) while at speed and under deceleration which has a major impact on toe-in/out AND the actual Ackerman effect for the racing condition. Depending on your linkage setup, the degrees of Ackerman can change significantly by just raising (as in accelerating) or lowering (as in slowing) your car.

Run your car with rock hard springs and you can well discount most of my premise. With no up/down motion in the front suspension, it truly will not matter AND it will be mostly Ackerman. But, who has such a setup?

The fore/aft positioning of the steering linkages is another factor. If you go "over center" with the linkages going from lock-to-lock, you can end up with a very floppy (and ineffective) inside wheel. Therefore, fore/aft is BIG in certain situations.

Wish I could recommend a good book on the topic; but, I left the bigger kind of car racing behind many years ago.

Dang! I wrote a preface to a book...LOL

Cheers!
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Old 06-12-2008, 11:31 PM   #2182
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Nice Brian! I think you descrbed Ackerman really well! I have some questions on "the only thing is... Ackerman".

Is the 6-8 degree differential between the inside and outside wheel measurement taken only while the car is resting in a static position? It would seem to me that it does not take into account the up/down angle of the steering links (between the attachment points of the steering lever arm and the steering blocks) while at speed and under deceleration. Wouldn't this have a major impact on toe-in/out AND the actual Ackerman effect for the racing condition? I would think that depending on your linkage setup, the degrees of Ackerman can change significantly by just raising (as in accelerating) or lowering (as in slowing) your car.

So, In theory if you run your car with rock hard springs it would be a 'perfect ackerman' if the question of my premis is correct? With no up/down motion in the front suspension, it truly will not matter AND it will be mostly Ackerman. But, who has such a setup?

I've read through the manual like a GOOD driva and understand the fore/aft positioning of the steering linkages is another factor. If you go "over center" with the linkages going from lock-to-lock, you can end up with a very floppy (and ineffective) inside wheel. Therefore, fore/aft is BIG in certain situations, but would you ever use 'for' steering linkage?

So glad we can have such helpful posts on challenging topics; I left the bigger kind of car racing behind many years ago, but can't get enough of these little rockets!

thanks for all the great info!

Cheers!




This is my version of Bills post. I like it better.
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Old 06-13-2008, 12:16 AM   #2183
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The Bear,

Dang! I should have copyrighted mine...LOL.

Ummmmmm...Second paragraph...I wasn't asking...

'Perfect Ackerman' are absolutely your words...Not mine...No such thing for all racing conditions of going from tight to sweeping corners...2WD or 4WD.

I did get enough of these 'little rockets' in 2006 and dropped 12th scale to make race days less of a challenge...Only race one class and try to help newbies that need it.

Can't wait to get back to my rubber-shod Phi being on carpet this winter.

Good topic!

G-Night.
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Old 06-13-2008, 07:17 AM   #2184
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Bill,

What I meant was the steering system, as in what's bolted to the chassis. I know that bump steer greatly effects the car in jounce and rebound, but I was just trying to explain the differences between the center point and dual bellcrank, just purely as mechanical linkages between the steering blocks and the servo.

Trust me, you're not telling me anything I don't know! I was just trying to help explain in the simplest terms to get the point across. If I had gone into that much detail, he might've gotten lost in my first post!

And yes, I agree there's no such thing as perfect ackerman. We just try to get it as close as possible for the conditions we're given.
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Old 06-13-2008, 07:52 AM   #2185
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Bill,

What I meant was the steering system, as in what's bolted to the chassis. I know that bump steer greatly effects the car in jounce and rebound, but I was just trying to explain the differences between the center point and dual bellcrank, just purely as mechanical linkages between the steering blocks and the servo.

Trust me, you're not telling me anything I don't know! I was just trying to help explain in the simplest terms to get the point across. If I had gone into that much detail, he might've gotten lost in my first post!

And yes, I agree there's no such thing as perfect ackerman. We just try to get it as close as possible for the conditions we're given.
Brian,

And I can absolutely agree with that...Keep it up amigo.

Hope to meet you at the IIC.

Bill
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Old 06-13-2008, 10:14 AM   #2186
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thats right, dumb it down for the rook.....

thanks to you both. Do you guys have plans to hit any carpet races pre-vegas? 360 in NY is looking to put carpet back down........
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Old 06-13-2008, 02:54 PM   #2187
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Default Pulleys

What are the benefits of using 36T pulleys in stead og standard 42T?
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Old 06-13-2008, 03:30 PM   #2188
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Brian,Bill thanks for the info.

I guess if you really wanted to define the steering issue, it should be noted that the single bellcrank on the PHI is not a true center point steering as such. For example the setup used on the 1/12th car where your linkages meet at one location on the servo saver, for one pivot point, is a true cp steering system.
No disrespect or digs intended, just trying to keep technical terms in check for myself.

So again, any thought to making the bellcrank adjustable front and back from the stock location? A new top plate or a flat washer instead of a cs screw for the assembly would make it pretty simple to do, I was thinking about the movement of about the width of the screw itself, so not to get into the diff or the batteries. Same idea as what Xray does but not as complex.
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Old 06-13-2008, 03:32 PM   #2189
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Helps with getting a rollout with 17.5 brushless for starters 55-58mm

with standard 42 -21 (2-1)i need a 39 - 40 pinion with 72 spur
with 36 - 21 (1.7-1) 34-36 pinion 72 spur.

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Old 06-13-2008, 09:12 PM   #2190
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Originally Posted by Johnny Wishbone View Post
Brian,Bill thanks for the info.

I guess if you really wanted to define the steering issue, it should be noted that the single bellcrank on the PHI is not a true center point steering as such. For example the setup used on the 1/12th car where your linkages meet at one location on the servo saver, for one pivot point, is a true cp steering system.
No disrespect or digs intended, just trying to keep technical terms in check for myself.

So again, any thought to making the bellcrank adjustable front and back from the stock location? A new top plate or a flat washer instead of a cs screw for the assembly would make it pretty simple to do, I was thinking about the movement of about the width of the screw itself, so not to get into the diff or the batteries. Same idea as what Xray does but not as complex.
Johnny,

I agree with some of your "center steering" description. I was using Corally's term for their offering of the center steering that was an option for the RDX and is standard on the Phi.

I do take issue on that 1/12th scale, your one location is actually two where the ballcups end up being stacked (one over the other) or fore/aft (one behind the other) which means that the steering links will be on different angles.

Have seen both on full-scale race cars...And neither is true center steering...LOL

Josh Cyrul made an extension for the Phi Steering Lever Arm that is being used by a few Corally drivers. He has since gone over to TOP; so, I will likely mimic his extension in the next couple weeks to test for the Speedworld Cup in July. I now use 36T pulleys (from the Arluck setup we used at the Reedy); so, there is plenty of room to extend the ball ends forward of their stock position.

I am a believer in the steering links being connected to ball ends on the steering arm slightly ahead in relation to those on the steering blocks. And with the links going slightly downward from the center arm to the steering blocks (via different length ballends and/or spacers).

Dang! Another chapter written...

Going off to hear some R&R now...later...

Bill
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