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Old 04-06-2004, 08:41 AM   #1
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Arrow double header: short arms vs. long arms, and fact vs. hype

Who will win?

Well, itís no secret that short arms are the rage right now for carpet tracks and tighter outdoor tracks. Alex did it, then Xray, and Corally. I would like to know what the handling qualities brought about by these shorter arms are. More importantly, I would like to know the theory behind the idea of why shorter arms are accountable for these handling qualities.

Typically, and as Carol Smith would point out, shorter arms yield more vertical, and less horizontal, movement of the roll center during chassis roll. Therefore, given identical initial wheel rates and static roll centers, the shorter arms should cause the car to roll deeper into a corner than itís long arm counterpart. But this is not consistent with how users are describing the attributes of the short arms. What then is responsible for this?

Given all the possible alignments that touring cars have today, is it not possible to modify the roll center and camber curves for a long arm setup to produce similar on track qualities to that of the short arms? Is all this short arm stuff just garbage that people blindly follow until the fast guys find a reason to switch back?

Please tell me that the best justification for short arms on a short track isnít simply because they share the same word. Because I saw that crap in the recommendations for using SMALL layshaft pullies on SMALL tracks. What a joke. And people really thought that their cars accelerated faster as a result. What is that? I guess we see what we want to see.

So tell me chassis tuning gurus, is this just another example?
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Old 04-06-2004, 08:46 AM   #2
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Good post

I haven't got any scientific input, neither experience about this.

But I'm also curious about the answers......
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Old 04-06-2004, 09:05 AM   #3
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The only things I can add to this right now is that the arm length of the Xray and other european and maybe Japanese cars was quite a bit longer than the Associated Tc3. Now the XRay arms from the T1M are about the same length as a Tc3.

Another thing I have noticed about the articulation of the suspension of the Xray with stock long arms is very different from how you can move the Tc3 and Losi suspension. By-that and in no scientific terminlogy-the long arm cars can move in many different directions-almost in a full circle of motion without the tires losing contact with the ground-while a Tc3 basically can move the chassis up and down. A video would help here tremendousely!!!
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Old 04-06-2004, 09:08 AM   #4
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I switched to short arms on my Xray and the difference was night and day. The car is very reactive and transitions much faster. It feels like a 1/12 scale car in comparison to the long arms. I race on a large, open track and really didnt notice faster laptimes but on a small tight track it is definately an advantage.
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Old 04-06-2004, 09:35 AM   #5
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I do agree with barnacle and the short arm also make the car stay flatter to the ground in the turn.
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Old 04-06-2004, 09:59 AM   #6
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We have an X-Ray sponsored driver at our track and he has the new car. If I am correct that the new car comes with the shorter arms, you sure could see a difference as how the car carved the turns. Very nice. I wonder what Losi and Associated are thinking about the shorter arms?
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Old 04-06-2004, 10:15 AM   #7
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When you mount the shock on the lower arm, the further in it is, the more leverage the arm will have to compress the shock. By moving the pivot point closer to the shock mounting point, you gain more leverage. If you use the same shock spring as with the long arms, the car will roll more, this is where people are getting the idea of rolling more.
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Old 04-06-2004, 11:27 AM   #8
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yeah randman,

i would have assumed that the short arms would necessitate stiffer springs. if folks aren't changing springs, then of course the car would roll more. the wheel rate would be much softer. but why couldn't a soft long arm setup work similarly?

i am curious as to what pertinent parameter is changing with the short arms that can't be replicated with the lengthy ones. i would think it to be roll center related. it would almost have to be. can someone mention a parameter that i am, thus far, ignorant to?
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Old 04-06-2004, 12:38 PM   #9
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I don't have an Xray, but when switching to the shorter arms does it also shorten the upper camber link by the same amount?
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Old 04-06-2004, 12:40 PM   #10
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Quote:
Originally posted by Ozwald Bates
I don't have an Xray, but when switching to the shorter arms does it also shorten the upper camber link by the same amount?
The upper arm stays the same length as before-though it can be adjusted. Maybe thats a big part of the improvment on the XRay at least.
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Old 04-06-2004, 12:57 PM   #11
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Quote:
Originally posted by Cain
We have an X-Ray sponsored driver at our track and he has the new car. If I am correct that the new car comes with the shorter arms, you sure could see a difference as how the car carved the turns. Very nice. I wonder what Losi and Associated are thinking about the shorter arms?
Long arms are car dependant. It tells me that the other arms were too long for that particular car. The Losi and AE cars seem to handle just fine with their arms
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Old 04-06-2004, 03:41 PM   #12
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I agree with what you are saying Seaball- that this is more an issue of roll center and that you can some what tune the roll changes to react similarly between short or long arms. Obviously though the short arms generate a much more dramatic effect so maybe you need the more dramatic geometry of the short arms when you are running those really stiff shocks that people tend to run on carpet. When you are running on smooth carpet with stiff shocks you might get very little suspension movement compared to bumpier asphalt while the more subtle long arms can get enough roll action from running asphalt.
Another consideration is maybe the short arms are just STIFFER than long arms. Period. Stiff is the way to go on high grip applications right? Whether it be chassis, shocks or suspension....
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Old 04-06-2004, 04:07 PM   #13
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the losi arms are already short. tc3 arms are barely average.

the cars that now have "short arm" conversions are the ones that originally had the longest.
- barracuda
- xray/corally
- atlas, but the new car is still a prototype

if you'll notice, the new arms for those cars are really just a normal length. the short connotation is only relative to the original pieces.

i guess you could look at it with the viewpoint that these cars are now gravitating to what may be a sweet spot for arm length. regardless, i am curious as to where lies the inferiority in the long species.

why?

because with long arms come increased piston displacement for a given wheel travel. this equals better resolution, and the effects of any slop in the system or anomolies associated with the damping are minimized at the wheel. a simple principle of levers.

long arms are also less sensitive to vertical roll center changes. whether the change is dynamic (on the track) or caused by changing tire diameter (common to foam tire racing) long arms minimize the vertical translation of the roll center. that's probably a much overlooked reason why competitive indoor foam users demand a certain diameter tire for their ride. if the roll center has a sweet spot corresponding to how you like your car to feel, that spot can perhaps be expanded by the use of longer arms.

i am not sure where my intrigue for this subject comes from, but it is certainly present.
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Old 04-06-2004, 04:21 PM   #14
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mike - great post, very insightful. perhaps it is not a better design kinematically, but just mechanically. ei. less flex.

and your thought behind the amount of permissible chassis roll for each application is one of valor as well. maybe there is a desired amount of roll center change during cornering, that is INDEPENDENT of chassis roll. for those applications that require little chassis roll, the primary way get a big roll center change durring bump/droop is to shorten the arms.

huh.
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Old 04-06-2004, 04:28 PM   #15
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Default Re: double header: short arms vs. long arms, and fact vs. hype

Quote:
Originally posted by seaball
Who will win?

Well, itís no secret that short arms are the rage right now for carpet tracks and tighter outdoor tracks. Alex did it, then Xray, and Corally. I would like to know what the handling qualities brought about by these shorter arms are. More importantly, I would like to know the theory behind the idea of why shorter arms are accountable for these handling qualities.

Typically, and as Carol Smith would point out, shorter arms yield more vertical, and less horizontal, movement of the roll center during chassis roll. Therefore, given identical initial wheel rates and static roll centers, the shorter arms should cause the car to roll deeper into a corner than itís long arm counterpart. But this is not consistent with how users are describing the attributes of the short arms. What then is responsible for this?

Given all the possible alignments that touring cars have today, is it not possible to modify the roll center and camber curves for a long arm setup to produce similar on track qualities to that of the short arms? Is all this short arm stuff just garbage that people blindly follow until the fast guys find a reason to switch back?

Please tell me that the best justification for short arms on a short track isnít simply because they share the same word. Because I saw that crap in the recommendations for using SMALL layshaft pullies on SMALL tracks. What a joke. And people really thought that their cars accelerated faster as a result. What is that? I guess we see what we want to see.

So tell me chassis tuning gurus, is this just another example?
hey I just gotta ask, didnt Tamiya first use short arms on the TAO4-SS? Or was that a whole different thing entirely?
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