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Old 04-06-2004, 04:41 PM   #16
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Oddly enough, if you do the math, given the same spring and shock location (distance from shock lower mount to wheel), the wheel rate is slightly Higher for the long arms. On the Cuda R2, this makes approx 1 lb/in difference.
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Old 04-06-2004, 04:42 PM   #17
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Default Re: Re: double header: short arms vs. long arms, and fact vs. hype

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Originally posted by trackdesigner71
hey I just gotta ask, didnt Tamiya first use short arms on the TAO4-SS? Or was that a whole different thing entirely?
No, that's something entirely different. The Tamiya SS is short wheel base, not short arms.
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Old 04-06-2004, 05:20 PM   #18
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good info here... keep it coming.
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Old 04-06-2004, 06:37 PM   #19
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Default meat for thought

some years ago (not that many) hpi came out with their progressive rate springs. although they weren't progressive, they were actually dual rate, the intent was to acheive non-linear compliance at the wheel. meaning that a relatively soft suspension at first would get exponentially stiffer during the bump stage, to minimize chassis roll. similar to the effect brought about by declining the shock angle on the tower (laying them down). only the latter is truly progressive. the point here is the desire to get the chassis to increase roll resistance deeper into the roll...

now overall compliance, or resistance, to roll is not just defined by springs or their angle of inclination. part of the equation, of which i do not know all parts, is the distance between the cg (center of gravity) and the rc (roll center). for this discussion being limited to onroad cars, the cg is located above the rc for all stages of cornering. it is the distance that separates them that changes. in most cases the rc drops durring chassis roll more than the cg does. much more. so as the springs are loading, the distance between the two points is increasing. they are working against each other in terms of stopping the chassis roll. in the end the spring wins as it's rate of change outpaces the rate that the roll center is dropping at. but... the point is that the dynamic dropping roll center takes away some of the late gain of stiffening. now this happens regardless of any of the spring/shock/angles. but... as previously mentioned, shorter arms do this much more aggressively than long ones.

as you can see the two scenarios are opposites. the stiffness curves for each (and yes, these are imaginary) are going in different directions as the chassis rolls. insignificant? i think not.

infact, if in both cases the vehicle were tuned to exhibit the same amount of peak chassis roll, that vehicle which is described by the curve with decreasing slope (aka - short arms) would be stiffer for the early section, until near peak. huh. that's a plausible reason for why short arms cause the car to feel more reactive. initial stiffness!

but wait, isn't that also a characteristic of high roll centers? regardless of arm length?

just when i thought we might be getting somewhere...

help

whoops, apparently its "food for thought". my bad.
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Old 04-06-2004, 07:55 PM   #20
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Whoa, it took me a while to read all this but, good posts.
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Old 04-06-2004, 08:24 PM   #21
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What about the suspension weight advantage. Suspension weight directly effects responcivness. On full size performance cars in australia Holden/HSV, steel is used in the suspension parts due to shitty roads. In the US the pontiac GTO(holden Monaro) handles like a dog because they couldn't be bothered making the suspension lighter for the smoother roads in the US.

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Old 04-06-2004, 09:10 PM   #22
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Quote:
Originally posted by fatdoggy
What about the suspension weight advantage. Suspension weight directly effects responcivness. On full size performance cars in australia Holden/HSV, steel is used in the suspension parts due to shitty roads. In the US the pontiac GTO(holden Monaro) handles like a dog because they couldn't be bothered making the suspension lighter for the smoother roads in the US.
Hmm-good point- have seen the Tamiya guys cutting the center ribbing out of the arms on the EvoIII. I thought that was mainly to lighten the suspension.
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Old 04-06-2004, 09:14 PM   #23
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yeah, fatdoggy. i agree. i guess i bypassed that thought since we could use some light materials in the long arms and i wouldn't think it to be an issue. but yes, you bring to light a good point.

i have often wondered if that attribute in some designs has been downplayed. one of the most responsive cars (and we all know what it is) has an ultra low unspring mass. that's why it breaks if you look at it wrong.

really though. that certainly could be a contributing factor, but...i would think the heavy spring steel axles, oversize bearings, and bulky arm construction of the xray to indicate otherwise.

is there enough vibration or bumps in indoor racing to take advantage of a light suspension? what about the fact that we use such high degrees of dampening on carpet? would that not affect/reduce response time? i'll have to think about that.
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Old 04-06-2004, 09:21 PM   #24
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ray,

i hadn't heard of that with the tamiya guys. well, there is certainly enough material to start with.

we'll have to talk more on thursday. i am curious about the findings you present your first post.
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Old 04-06-2004, 09:41 PM   #25
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I belive seaball metioned rollcenter change eailer. Usually when a car rolls, the movement in the suspension arms resaults in the rollcenter dropping. As we all know, low rollcenters yield more grip and allow the car to corner faster. If you consider how the rollcenter changes, the shorter arms resault in more rollcenter change which should yield more grip mid turn than a car with the same setup but longer arms.

As for being faster around the track???? Well its up to the driver to exract the performance for that.. I have been trying a new car recently (atlas ym34si with "long arms") and have found that the biggest advantge is the speed it changes direction. I can make moves on the inside of ppl during s-turns that give me a huge advantage.

So whats better??? Alot of crap is said about this car has this and that car has that. A car is fast because of the driver and that fact will never change. As long as the car is adjustable (modern) you will always have the potential to be competative.
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Old 04-06-2004, 09:48 PM   #26
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I've gotten out of r/c and haven't been around these boards in awhile but I'm sure glad I checked back and found this tonight! I love this stuff!

I played around with some numbers using the Performance Trends Circle Track Analyzer v2.0 software (roll center/chassis setup program for my racecar) trying to come up with some actual numbers and graphs for what changes between a "long arm" and a "short arm" car.

I set everything up as a fab stub late model front end, and made all parameters equal left and right. (closest thing I can get to a full size r/c touring car) I left all parameters equal, changing ONLY the lateral location of the inner pivot for the lower control arm. I then checked roll center location horizontaly and vertically with the car "static", 1 inch dive, 1 inch rebound, 5* roll, and combinations of 5* roll and 1" dive/rebound. This is what I came up with to compare roll center movement in a long arm VS. short arm car.


17.5" lower arm
static rc = 2.8"
rc @ 1" dive, 0* roll = 1.6" above ground
rc @ 1" rebound, 0* roll = 4.1" above ground
rc @ 0" dive, 5* roll = 2.8" above ground, 2.9" INSIDE
rc @ 1" dive, 5* roll = 1.6" above ground, 2.7" INSIDE
rc @ 1" rebound, 5* roll = 4.1" above ground, 2.9" INSIDE

15.5" lower arm
static rc = 2.8"
rc @ 1" dive, 0* roll = 1.3 above ground
rc @ 1" rebound, 0* roll = 4.3" above ground
rc @ 0" dive, 5* roll = 2.6" above ground, 9.4" INSIDE
rc @ 1" dive, 5* roll = 1" above ground, 16.1" INSIDE
rc @ 1" rebound, 5* roll = 4.2" above ground, 7" INSIDE


By going to a 2" shorter lower arm, the static roll center stayed exactly the same at 2.8" above ground, 0" left/right. During dive and rebound, the vertical roll center CHANGE was minimally increased with the shorter arm. During roll (especially dive/roll) the roll center moved to the inside of the corner drastically more with the shorter arm.

Just as a quick test, I did both arm lengths at 1" dive, and only 1* roll... the shorter arm still moved the roll center much further towards the inside of the corner (I wanted to see if this was true during all roll, or if it was an exponential thing)

The roll center moving towards the inside of the corner like that, should make the car roll onto the outside front tire more, especially during mid corner, resulting in more grip from that tire.

In awhile, I'll check camber curve differences, wheel rate changes, etc... with the different arm lengths. I'd like to see visually how all those things change as well.
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Old 04-06-2004, 10:07 PM   #27
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I just realized they've got a downloadable demo of the newer version of the program I used for this. So seaball or whoever else, if you want to play with it a little more and see exactly what's happening you can. http://www.performancetrends.com/Cir...20Analyzer.htm is the page
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Old 04-07-2004, 01:29 AM   #28
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Very interesting post

As far as I can see, 3 points have been made:

Shorter arms are stiffer.
Shorter arms are ligther.
Shorter arms gives more dramatic rollcenter changes midturns, thus giving more grip.

Some considerations:

Stiffness could be achieved in other ways, but that will either mean heavier arms (more material) or more excotic materials. But these solutions havent made a big impact. At least they are not very common, even though there seems to be many people with an anodized fetish.

Ligther arms. This could also be acieved by excotic materials, but it doesnt seem to be worth it. Again, it's not very common.... And compared to the wheel, the arm is already very light, which might indicate that arm weight aint the issue.

So the roll center issue seems to be the single-most important. A video with examples could be interesting.... And Dragonfire's point about making moves on the INSIDE of people during S-turns is definately interesting !

Of all the changes I've made to my car, I find that fast responses combined with predictive reactions is the most important. That incluse making counter actions through an S-turn !

Thanks for reminding me of the HPI progressive springs, they might be usefull for me.
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Old 04-07-2004, 02:12 AM   #29
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Default Re: meat for thought

Quote:
Originally posted by seaball

now overall compliance, or resistance, to roll is not just defined by springs or their angle of inclination. part of the equation, of which i do not know all parts, is the distance between the cg (center of gravity) and the rc (roll center). for this discussion being limited to onroad cars, the cg is located above the rc for all stages of cornering. it is the distance that separates them that changes. in most cases the rc drops durring chassis roll more than the cg does. much more. so as the springs are loading, the distance between the two points is increasing. they are working against each other in terms of stopping the chassis roll. in the end the spring wins as it's rate of change outpaces the rate that the roll center is dropping at. but... the point is that the dynamic dropping roll center takes away some of the late gain of stiffening. now this happens regardless of any of the spring/shock/angles. but... as previously mentioned, shorter arms do this much more aggressively than long ones.

You're absolutely right, Cball, but you're forgetting something. You have to superimpose pitch!
Pitch is important on tight tracks, where the car decellerates quiclky in every corner, even more so on foam tires. On long open sweepers, it's less of an influence.
I also like JefC's idea of lateral movement of the Roll Center. Indeed, with longer arms, the chassis will role more onto the outside (front) tire, which is a) a load on the suspension and 'work' for the damper, and b) possibly more deformation for the outside tire.
Also, the transition back to the 'no roll' state will be slower.

I'm an XRay factory driver, and I've played around with roll centers. IMHO, very, very similar results can be obtained withlong arms and a slightly more angled link. Maybe not exactly the same as short amrs, but very, very similar.
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Old 04-07-2004, 02:20 AM   #30
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Default Re: Re: meat for thought

Quote:
Originally posted by elvo
I'm an XRay factory driver, and I've played around with roll centers. IMHO, very, very similar results can be obtained withlong arms and a slightly more angled link. Maybe not exactly the same as short amrs, but very, very similar.
That's very interesting. I dont think I have the option of "sligthly" angled link (HPI Pro2), but I have the option of changing the angels. I'll try that, since I'm on a tight technical track.

Thanks.

PS: I would like to ensure: When you say "pitch" it translate into the speed of which the roll center changes?
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