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Old 08-14-2007, 10:47 AM   #76
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Marcos-The reason is that my battery is 7 ounces lighter than a NiMH. I moved the electronics to the battery strap to balance the weight left and right. I am running the car light. It would be better lower but there is no space. I did the same thing on the LosiJRXS by mounting the speed control on the top deck looking for cool air; it was higher than my battery strap. There was no difference noticed on the track. I run my pan car the same way.

Take a look at page 2 for more detail on this.
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Old 08-14-2007, 10:53 AM   #77
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Originally Posted by John Stranahan View Post
Marcos-The reason is that my battery is 7 ounces lighter than a NiMH. I moved the electronics to the battery strap to balance the weight left and right. I am running the car light. It would be better lower but there is no space. I did the same thing on the LosiJRXS by mounting the speed control on the top deck looking for cool air; it was higher than my battery strap. There was no difference noticed on the track. I run my pan car the same way.
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i see!! i had the same problem with the lipo pack on my MI3, so now i have the type R !! Let me know how it worsk for you, also let me know how the A123 cells work for you
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Old 08-14-2007, 12:42 PM   #78
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Originally Posted by adamge View Post
yyh, that won't work because cornering force is generating a torque in the hub. You have a lateral force at the road surface / tire intersection, and it's being solidly resisted by the lower hub pin. That's a torque, and it has to be resisted by the upper turnbuckle. Putting a shock there ensures it won't be solidly resisted, ensuring the hub will flop around.
....

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Old 08-14-2007, 05:37 PM   #79
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Cool thread. John, I also ask why you are running that ESC so high. We all know that keeping the CG low is like the holy grail of racing. Have you looked into the possibility of adjusting your cross weight to compensate for the side-to-side imbalance?

BTW, what racing guides do you own? I am lucky, I have the M&M book
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Old 08-14-2007, 05:42 PM   #80
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Mattnin- see posts 32 and 33 on page 2, like John suggested.
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Old 08-14-2007, 06:26 PM   #81
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Matnin-The electronics are high because they balance the side to side weight better there. Read page 2 of the thread. When you build a race car there are always compromises. If you race F1's you don't put any more weight high than you have to. You even put the brake calipers on the bottom of the wheel. But then you need that big airscoop up high for the engine because thats where the air is. If you run Nascar there is plenty of weight up high. TC's are not really the ultimate electric race machine, they are more like Nascar than F1. Even 1/10 pan cars could be improved, as the suspension is quite primitive. They actually drive better with some weight up high. If I were going to improve TC's I would use smaller shocks and place them lower in the chassis and lighten the beast up to about 43 ounces. If you have to buy a new chassis and top plate to perform well on asphalt, then why do we even need a top plate and all its tweak problems. Maybe its more for appearance than function. I see they are getting smaller with each generation. A little graphite rod to take up the belt tension force would probably be sufficient.

I use this racing guide, which is backed up by 3 full size car books, but it is all in my head and it will leak out periodically during this thread. It is no longer for sale. Many sad marketing tales to be told. A brand new edition was prepared for a publisher which backed out of the deal when they changed editors, and then they broke a promise for a montly column. Needless to say I am not fond of that mag anymore.

"Have you looked into the possibility of adjusting your cross weight to compensate for the side-to-side imbalance?" Well yes. Propose to me your plan to solve a 7 ounce deficit on the right side.

I assume you are referring to Miliken & Miliken. I got some good tire load vs friction data from that book. I think Boomer owns a copy.

John

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Old 08-15-2007, 12:54 AM   #82
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Now I really want to read your book!!!
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Old 08-15-2007, 11:48 AM   #83
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YYhayim-Good luck with your shock experiment. I think you are going the wrong way with the wheel though.

Rezencloud-Thanks.

My next step at the track will be to adjust the roll stiffness of the front and back of the car. To understand why this works you need to understand a little bit about weight transfer and a little bit about rubber tires and when they develop the best friction. I'll start with Weight transfer.

Weight Transfer
RC literature seems to have this stuff backwards so I thought I would post it.
Lets say we have at our disposal a Fullsize Nastruck race car and a medium 4 leg table. First some static experiments. I put the table on a rubber pad. I push on the table top to the side. Some weight will transfer from the near legs and go the far legs. It has nothing to do with roll or shocks or springs. The table has none of these. It has to do with the force I am applying up high. If I apply enough force and have enough traction I can get the table to tip. All the weight will transfer to the farside legs at the moment of lift on the near side legs. Now lets place the table in the bed of the truck and go around a high speed corner. The weight on the table is going to transfer to the outside legs by the action of centrifigal force on the center of mass of the table. If the truck goes fast enough and the bed has enough traction on the legs, the table can transfer 100% weight to the outside legs. The table will traction roll (and fly out of the truck).
The weight transfer in our cars is similar to the weight transfer on the table. It is caused by the centrifugal force on the center of gravity of the car. What affects the amount of lateral weight transfer is how high the center of gravity is and how many g's we pull in the corner (how fast we corner). See the left photo below.

Lateral weight transfer on an axle decreases traction. The reason is that when the weight goes out, the outside tire gains traction at a lesser efficiency than the inside tires loose traction. There is a net loss of traction on the axle. More on this later.

So will more roll increase lateral weight transfer. Mostly no. (There may be a small change in ceter of gravity with roll.)

Increasing roll by using shocks and springs will delay weight transfer, though, so some roll is good. Too much roll makes the car sloppy. Photo 2 shows the forces that cause chassis roll in a car that has springs and shocks.

Very similar to lateral weight transfer, longitudinal weight transfer will kill some forward traction in a fourwheel drive car. This is distinctly different than in a two wheel drive car (my pan car) where we want as much weight to go to the back wheels as possible. In a four wheel drive, If you increase the stiffness of the rear of the car you will speed up longitudinal weight tranfer and kill some forward traction for the same reason I stated above. The rear will gain some traction at a lower efficiency than the front looses traction. There will be a net loss. I see this backwards a lot.

Practical applications next track session.
Attached Thumbnails
Associated Factory Team TC5, Brushless, LiPo, Li-ion Nanophosphate, Tips and Tricks-schematic-lateral-weight-transfer-resized.jpg   Associated Factory Team TC5, Brushless, LiPo, Li-ion Nanophosphate, Tips and Tricks-schematic-chassis-roll-004-resized.jpg  

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Old 08-15-2007, 12:52 PM   #84
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Quote:
Originally Posted by John Stranahan View Post
Practical applications next track session.
John...could you please share your thoughts on the difference in set up needed as far as weight transfer for those running Lipo packs, vs. those running 5 or 6 cell packs. Obviusly those running 5-6 cell packs have about 100-150g of extra weight and will most like need less droop, etc...but those of us running Lipo packs have much less weight and thus require different car set up to ge tthe weight trasfer or traction for a good working set up...

Schumacher Mi3 team driver Paul Wynn has discussed this w/ me over the phone and he does believe that if one wants to run "X" team driver set-up which has 5-6 nimh, but you are running Lipo, its possible running the lighter Lipo pack set up will require set up changes to compensate for the extra weight and transfer...what do you think about this and what advice would you offer in this reguard for those running Lipo
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Old 08-15-2007, 01:08 PM   #85
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Roll Stiffness
7 ounces under is what I am running. This is 13 % under the weight of a Roar Legal TC. I would expect to need about this much less spring rate. Instead of the stock copper and red springs, I plan to start with lighter blue springs (I would have anyway though for asphalt). Instead of 60 and 40 weight oil, I plan to start with 35 weight with beveled pistons. Instead of a high roll center my roll center is nearer the ground. lighter springs and lower roll centers gives you less roll stiffness, more roll. Small droop adjustments for fine tuning after all these are tested and adjusted. This kind of stuff is hard to predict ahead of time. You really need to test it on the track using some signs that the car gives you. If you take a high speed corner and the car sticks then slips then sticks or is imprecise in clearing the marker by just a few inches, the chaces are good the cars roll stiffness is too light. You need to go heavier on the springs front and back or move them out at the top. A little heavier on the shock oil. A little less droop.
If you take the same high speed corner and the car responds very quickly, but then it just slides too much sideways, then you need to go a little softer on these adjustments.
Racers often confuse quick response with traction. A car can respond very quickly to your inputs, but be low in traction at the same time and start sliding just after your input. This means you are too stiff.
A car can be just a little sluggish to respond to your input, but then develop great traction in the corner. This is probably the faster way around the track, a little bit of roll, but not too much.
Our rubber tires seem to develop the best traction when the roll stiffness is just right. Not too high on asphalt, and not too low. The correct roll stiffness allows the tire to follow the bumps better or to have better tire compliance with the road. I will give you a better answer after I run the car.

Splitting the unavoidable lateral weight transfer between the front and the back in the proper proportion will be what allows us to tune the oversteer understeer properties of the car. I will report on this after a track test. A basic understanding of weight transfer helps understanding of why this works.

And then we can discuss the very special case of the locked front diff.
john

Last edited by John Stranahan; 08-16-2007 at 11:23 AM.
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Old 08-15-2007, 01:29 PM   #86
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So here is a weight transfer question. If I put some antisquat, say 2 degrees, on my two wheel drive buggy, the rear suspension gets stiffer on power. Weight transfers to the rear quickly. The car becomes a rocket on the ramp of a jump. I clear the tripple with ease. Should I then put 2 degrees antisquat on the back of my fourwheel drive Independent rear suspension, so that it has more forward traction.


Rear Antisquat
Well the answer is no. On the fourwheel drive the stiffer back end will rob the front pulling wheels of needed weight early on power delivery. You will loose some traction efficiency transferring weight to the back sooner. The car will accelerate more poorly although not very much so as the car is long and low. Coincidently, but for the same reason, you will also loose some rear end cornering traction. So as a tuning aid on a fourwheel drive you only add antisquat to the back to kill some rear cornering traction on Power. If you do go easy on it. Maybe .5 degrees. I don't like the way the car goes through the bumps on power with rear antisquat (its stiffer on power); it bounces around, so I don't use any. Well that's one adjustment out of the way for me. I was able to view a Champ Car (a powerful open wheel fullsize race car) up close. It had no rear antisquat either.
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Old 08-15-2007, 09:03 PM   #87
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I added the second roll schematic photo just above. The roll schematic file was to big originally and it failed to load. The track is open now. I may go out tomorrow if it does not rain.

Roll Center Continued
Here is a newer roll center schematic that you can better visualize the roll center. At the right is a table that you can use to adjust roll center. The roll center is at the point of the big arrow. The instant center is at the large white dot on the far right of the schematic.

Line 1 follows the upper arm
Line 2 follows the lower arm to the instant center where it crosses line 1
Line 3 goes from the bottom of the tire to the instant center. Where line 3 crosses the cars centerline is the roll center about which the car rolls when turning.

Making the roll center higher is a little like using stiffer springs on that end of the car. It reduces cornering traction on that end of the car when using a front ball diff or front one-way.
Attached Thumbnails
Associated Factory Team TC5, Brushless, LiPo, Li-ion Nanophosphate, Tips and Tricks-roll-center-schematic-jrxs001.jpg   Associated Factory Team TC5, Brushless, LiPo, Li-ion Nanophosphate, Tips and Tricks-roll-center-table-jrxs002.jpg  

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Old 08-16-2007, 11:30 AM   #88
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Well it has not rained in two weeks while work on the track was completed. Today we got a tropical storm from the Gulf, it rained 6 inches in the last hour. My street has 18 inches of water in it. What a change. Testing will have to wait again.. I did post new roll center schematics and an adjustment chart with some new text in my previous post.
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Old 08-16-2007, 12:27 PM   #89
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TEAM Xray info on roll-center and its effects (page 24-25, Xray T2 set-up guide)

************************************************** ***
FRONT ROLL CENTER
************************************************** ***
Lower front Roll center:

* Increases on-throttle steering
*Decreases car's responsiveness
*Decreases weight trasfer at front of car, but increases grip
* increases chassis roll

Higher front Roll center:

*opposite of all of the above, but better set-up for smaller, tighter tracks
*good on high traction tracks to avoid traction roll



************************************************** ******
REAR ROLL CENTER
************************************************** ******

Lower:

*Increases on-throttle grip
*decreases weight transfer at rear of the car, but increases rear grip
*increases grip, decreases tire grip
*increases chassis roll
*better on low-traction tracks and used to decrease roll at corner entry

Higher:

*Decreases on-throttle steering
*Increases weight trasfer at front of the car, but decreases grip
*Decreases car's responsiveness
*Lessens chassis roll
*better on high traction conditions to avoid traction rolling on mid-corner and corner exit
*Better on tracks with tight corners and direction changes (chicanes)
================================================== =============

Last edited by yyhayyim; 08-16-2007 at 12:42 PM.
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Old 08-16-2007, 12:39 PM   #90
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Default REAR ANTI-SQUAT

More from team Xray on the subject of anti-squat (Xray T2 set-up book, pg 34)


*Rear suspension resists lifting on corner entry. Off-power steering may be reduced when using stiffer front springs and little rear droop.
*the rear suspension will reach its maxinum roll point quicker. Mid corner steering is reduced untill throttle is applied.
*the rear suspension will resist squatting on corner exit. on-power steering is increased immediately when throttle is applied by transferring less weight rearward and reducing rear end grip on corner exit.
*Increases the rear suspension's ability to handle large or successive bumps

Last edited by yyhayyim; 08-16-2007 at 05:36 PM.
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