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Old 04-08-2004, 04:58 PM   #61
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Default lateral roll center

If we continue your left turn example, if the RC was located exactly the same spot as the lower pivot point of the steering knuckle then no, there would be no suspension movement on the left side of the chassis. The suspension would be trying to pivot around that point not more toward the center of the chassis as normally would be the case.

I wonder if we have forgotten that the RC is the imaginary point which the suspension pivots around? If this point moves laterally it influences where the suspension acts on the chassis as well as how much weight it has to over come in order to move.

If we know how changing the relationship between the CG and the RC verticallly affects handling, then shouldn't we be able to apply that knowledge to a lateral shifting of the RC? It would stand to reason the affect would be similar. Although, since an angular relationship would exsist the CG would not have as much leverage on the RC.

I think we could fill numerous pages talking about this subject. Sound like fun to me!!!
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Old 04-08-2004, 05:09 PM   #62
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Guys, I gotta be honest. I'm not quite catching 100% of what's being discussed here. BUT, I'm am catching enough that I'm learning quite a bit and it's giving me ideas. Keep it coming. (Just don't expect me to add anything to it. )
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Old 04-08-2004, 05:16 PM   #63
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Default Vehicle dynamics

Hey seaball check out the SAE book store, I know they have a few books on vehicle dynamics and at least one book on race car dynamics.
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Old 04-08-2004, 07:02 PM   #64
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thanks dude. i have read, actually, a few of them. not cover to cover necessarily, but mostly the suspension sections. despite some of the material discussed, the suspension sections frequently are limited to what i would think are the basics.

i did get an opportunity to go over milliken's book for a few weeks, which is very technical and math oriented. quite frankly, it wore me out.

something interesting that rayhuang and i talked about earlier today was the gap between 1:1 and 1:10 tuning strategies. like josh, and many others, have pointed out, you must be selective with what 1:1 tuning methods you assertain in our hobby.

with that being said, many people take this statement to mean that the effect of a given change, or alignment, produced is not consistent with full scale. while in the end handling may be different, all of the laws of physics still apply. roll centers still drop, slide, etc.

what i'm really getting at is the fundamental idea that there may be large discrepancies, between the two scales, for what we would consider desirable characteristics. much of the full scale literature emphasizes predictability, stability, for a fast car. while those are great characteristics in full scale racing, perhaps our cars benefit from a lack thereof in either or both departments. how many of us have had that car that could be described as both, but was incapable of turning fast laps? predictability is not really a wise thing to compromise on many levels, but stability just may be.

i am not intending for this discussion to digress. but merely to offer insight to determing the best full scale advice to keep in the brain. it's not always cut and dry, but having a focused objective when doing research can help determine what to consider valuable theories.

keep it coming boys and girls.
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Old 04-08-2004, 07:30 PM   #65
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Default Car Set-up

"Loose is fast and on the edge of out of control Cole." Days of thunder. While that stament is overly simplistic it is pretty much a true statement.

Anyway, I think the main diference between the two is TRACTION! I can tell you from experience real cars have nowhere close to the same amount of traction we have. Well on carpet at least.

Also, as Josh pointed out, sidewall flex is considerable in full scale racing. If a 3000 pound stock car had the same amount of traction as we do it would most likely rip the tire right off the rim. The only full size situation that would offer a seemingly like amount of traction would be the surface of a dragstrip. If you walk across the starting line your shoes will stick to the surface. I have actually lost a shoe on one!

Good stuff, keep it up!
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Old 04-08-2004, 07:50 PM   #66
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Old 04-08-2004, 07:51 PM   #67
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Old 04-08-2004, 08:29 PM   #68
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Finally we have diverged into a territory I can put some input in. As Seaball and I discussed today-where we RC racers diverge from full size cars or karts is that we expect very different handling traits out of our RC cars. In fact-what we tune for in a car/kart may in fact have absolutely no corellation to a RC car. Yes-we want good turn in, mid corner speed and on-power steering, but at a much more aggressive level. or at least for everythng to happen much faster than a real car.

So we need to be careful when thinking about rc and cg and all the other things if we are trying to apply them to a car/kart.

We need only be concerned with what is "actually happening" to the rc and cg in an RC car and correlate that to real world measureable things like laptimes, corner speed, g-forces. Things that can be recorded by stop watch or data acqusition.

In simpler terms-we shouldn't put the cart before the horse. Maybe we need to take the data as it is and not try to formulate the ideal. Take the data for a long arm car (all that is measureable at this time), then take the same car with short arms and collect all the data. Then see which configuration is faster. Then make some conclusions.

As ccm39 or seaball said-if we had to actually drive our RC cars sitting in the cockpit, they very well might be completely undriveable to us. Or so aggressive and scary that we'd need a diaper on.
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Old 04-08-2004, 08:36 PM   #69
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Hmmm-and now I am thinking that this is exactly what guys like Hodge, Cyrul, Gil Losi, Teemu, and especially Kinwald comes to mind spend all their time testing. Some with computer and CNC machines and some with a dremel and a pencil!!!

They are going on hunches , ntuition, engineering and then testig it in the real world-then apllying it to future race cars.

Makes you wonder if the XRay Short arm car came about as a product of engineering, or did someone think-hmmmm-do the short arms from the T1M fit on the Evo2?? UMm-YUp-lets try it!

Seems a strange coincidence that they meet IFMAR/ROAR spec when mounted on the outside of the bulkheads.
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Old 04-08-2004, 08:37 PM   #70
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How many diapers would then depend on whether it was stock or mod

I haven't added anything to this discussion so far, but that's mostly because I do not completely understand the physics behind vehicle dynamics yet (junior in HS taking honors physics and AP next year).

At some point however, I think you just have to go with what you know works best from previous testing and experience, rather than trying to think up why it does what it does. IMO, at this level, it'd be better to spend more time getting practice and feeling the changes you make on the track, rather than trying to analyze them conceptually.
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Old 04-08-2004, 08:55 PM   #71
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Brian,

Your comments would be appropriate if not that ccm and seaball practice every week and race every Sunday!! And both have literally improved from 2 or 4 laps behind the TQ to 5 or 6 seconds in the last 6-months. These guys are smart and fast!!!

Also-they have both designed unique cars of there own-so they have a different perspective than say you or me.

Ray
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Old 04-08-2004, 09:01 PM   #72
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You're absolutely right, we want r/c cars to handle very very very differently from full size cars, but the same physics still apply. Weight transfer still exists, roll centers still exist, center of gravity still exists, etc, etc, etc... they all function the same way in an r/c car as they do a full size, we just want them to do different things (if that makes any sense ). But if shorter lower arms on a full size car increase lateral roll center travel, why shouldn't shorter arms have the same effect, but to a smaller magnitude, on an r/c car??? And why wouldn't moving the roll center laterally on a full size car, be the same as moving it laterally on an r/c car??? We may not WANT it to move the same way in both cars, but shouldn't it have the same effect for the most part?

BTW, I found a really informative site on the physics of racing. I'm not sure that it all can be directly applied to r/c cars, but it'll make for some interesting reading for a few of you. http://www.tckc.net/tips_faqs/drivin..._of_racing.htm
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Old 04-08-2004, 09:05 PM   #73
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JeffC-my point exactly-even if it didnt read like that. The principles are the same and may have similar effects on both vehicles, but dont "THINK" about it in terms of what desirable effect it might have on a real car and try and apply it to an RC car. Go the reverse and make the change to the RC car-then measure the rc, weight transfer, cg, etc.

And from those measurements-now make conclusions For example-a change in track width was done to theoretically increase scrub. After testing-you are certain that it did that-Now determine in a scientific way WHY!!!

Now you can apply those findings into a database or into the design of future suspension pieces.

Maybe the real issue is engineers vs. drivers!!! Driver comes into pits and yells at the engineer-car sucks!!! Engineer curls his lip and downloads the data out of the DA!!!





Quote:
Originally posted by JeffC
You're absolutely right, we want r/c cars to handle very very very differently from full size cars, but the same physics still apply. Weight transfer still exists, roll centers still exist, center of gravity still exists, etc, etc, etc... they all function the same way in an r/c car as they do a full size, we just want them to do different things (if that makes any sense ). But if shorter lower arms on a full size car increase lateral roll center travel, why shouldn't shorter arms have the same effect, but to a smaller magnitude, on an r/c car??? And why wouldn't moving the roll center laterally on a full size car, be the same as moving it laterally on an r/c car??? We may not WANT it to move the same way in both cars, but shouldn't it have the same effect for the most part?

BTW, I found a really informative site on the physics of racing. I'm not sure that it all can be directly applied to r/c cars, but it'll make for some interesting reading for a few of you. http://www.tckc.net/tips_faqs/drivin..._of_racing.htm

Last edited by rayhuang; 04-08-2004 at 09:17 PM.
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Old 04-08-2004, 09:08 PM   #74
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Not saying it doesn't help, Ray, I'm sure it does! But not all of us are able to get to the track that often, so our once a week track day would probably be better spent getting track time and getting all the other variables figured out. Believe me if I had the time and the education to test that stuff in relation to vehicle dynamics then i would!
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Old 04-08-2004, 11:21 PM   #75
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Brian McGreevy - I think most of these guys have their stuff sorted out.
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