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Old 01-29-2006, 03:49 PM   #121
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Current State of the Wiring in the Losi JRXS

I have made some changes to the wiring to solve a couple of problems.

Wires and weight are moved more right to reduce the lead ballast weight on right rear.

Positive battery pigtail is rerouted to let the battery drop in the car easier (upside down).

Upgrade wires to 12 gauge. May or may not help lap times with mod motor. You need a soldering iron with a 900F tip to do this kind of work. About $60 bucks for the Weller Model pencil iron that I use. http://www.action-electronics.com/unpl.htm WE-7760 Red Handle, 4033S 1000 F tip which is the one I use.

Increase cooling to the speed control heatsink by unshrouding the lead wire end of the heat sink. Wires are routed out what normally is the bottom of the speed control. Mounted speed control on the side to reduce lead length and move weight to the right.

I made a right angle plastic bracket from a Daysy BB pack box. This is servo taped to the chassis and a tiny tie wrap holds the top to the chassis post. The speed control is held quite securly by servo tape on the back and bottom. Here is a pic. Note blue TC3 springs. Also note the LED's are on the bottom now. I just plan to leave it on profile one, but you can take it off to reset it or do it in a dim room. The speed control is less likely to be damaged in a side impact with this arrangement. The fan, well thats another story. I damaged it already with my finger while it was on top. I hate the little fan, but the speed control has a themal shut down without it in my car even in cool weather.

Note that I offered a Roll Center Calculator Spreadsheet and discussion on the previous page if you are new to this thread.
Attached Thumbnails
Losi Constant Velocity Drive (LCD) vs MIP CVD-upsizing-gtb-6.5-12-gauge-finished-005.jpg  

Last edited by John Stranahan; 02-05-2006 at 09:53 AM.
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Old 01-29-2006, 05:10 PM   #122
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John,

YGPM..............
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Old 01-29-2006, 05:37 PM   #123
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Wow.... space is tight on the Losi huh.
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Old 01-29-2006, 08:39 PM   #124
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Angry Asian- Now don't get mad , but space is Normal on the Losi JRXS. The brushless speed control is a bit on the large and heavy side at this point in time, but it fits and works fine. Welcome to the thread.
Tornado Racing-got your message, replied by e-mail

Roll Center DATA
Here is some roll center data on Jessie's TC3 with a BMI graphite chassis conversion with Foam tires, This is his output of my Spreadsheet that is available by e-mail, after he took measurements. The chassis is always adjusted to the same ride height before each run. The car is dialed with 2.27 inch Diameter Tires and the Roll Moment, which is the torque that tries to roll the car, at 3.20 pound-inch (the last set of numbers below) I suspect if we adjust the car with the tall new tires to have the same roll moment, it might handle similarly. Notice the roll moment with the taller tires right now is higher and the car does tend to traction roll with this setting. I'll try and come up with the proper adjustment and report it here on this post.

CALCULATED VALUES
ROLL CENTER 0.19 inch
Roll Moment Arm 0.69 inch
Roll Moment @1.5 g =3.51 pound-inch
TC3 with 2.42 inch Diameter Tire

ROLL CENTER 0. 25 inch
Roll Moment Arm 0.63 inch
Roll Moment @1.5 g = 3.20 pound-inch
TC3 with 2.27 inch Diameter Tire

Note that these Roll moment torques seem small, but I have measured RC cars on high grip ashpalt at 3.5 g's. It's higher still on carpet and then there are the transient spikes that are higher still, that roll the car on carpet.

Last edited by John Stranahan; 01-31-2006 at 07:00 PM.
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Old 01-31-2006, 06:56 PM   #125
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So here is the suggested change. Up about .025 inches on the rear inner A-arm hinge pivots with the 2.42 inch tires.

CALCULATED VALUES
ROLL CENTER 0.25 inch
Roll Moment Arm 0.64 inch
Roll Moment @1.5 g 3.23 pound-inch


This gives you near identical roll moment as with the short tire. I suspect it will then drive in a similar manner, but I have not gotten results yet. It is quite a bit easier to change the camber link inner pivot on this TC3 which has FTTC4 aluminum camber link blocks than to shim the lower pivots. For this reason it might be better to use a thicker shim under the pivots to make sure you have plenty of adjustability at the camber link inner pivot. Then small chages can be made as the tire wears. On this car the outer camber link pivot is already all the way up.
Also it is not neccesary to use the same chassis height with the tall tires. It may work better a little higher of the ground, or maybe not. My XXXS used to work better at 6 mm than 5 mm.

Any carpet racers with foam tires please make a post on what works for you in this situation.
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Old 02-02-2006, 08:47 PM   #126
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New JRXS Front Arms
If you have followed this thread you know I had some trouble with the shock end rubbing the A-arm and creating suspension friction. I finally put a .030 inch Associated narrow 4-40 steel washer between the shock ball and the arm. Fixed the problem right up. I found out about a week or two ago that Losi has a new front arm that may also fix this problem by placing the shock mount hole farther up (for wheel clearance) and farther out on the arm for better performance on asphalt with rubber tires (less supsension friction). Anyway here is a link.

http://teamlosi.com/contents/index.htm

and a part number.

The LOSA9739A and LOSA9861A revised front and rear suspension arms

These are available at Horizon so you local hobby shop should be able to order them.

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Old 02-02-2006, 08:59 PM   #127
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Default Camber Link lengths

Hi John,

Have you tried experimenting with Camber Link Lengths and Positions? If so, what are the general effects when lengthened or shortened relative to whether it is in high or low roll center? I have seen in some PROs setups in the Corally site that they usually shorten their links. I havent tried doing it yet, hopefully this Saturday. Any info on the subject will be greatly appreciated. Thank you very much for your time

Sonny
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Old 02-02-2006, 09:22 PM   #128
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Sonny-I am going to give a response based on a standard diff (opposite for locked diff). I believe that the major effect of moving the camber links on our cars must be changes to the roll center. I say this because I have found the car not very sensitive to camber change which is also affected by the camber link. I have not made many changes to the camber links on the JRXS as I found on my previous touring cars that I prefered the rear link long on rubber tires on asphalt and high in the wheel. I just put it there to start with.

If you raise the outer camber link the roll center goes up; less traction on that end with a standard diff easier rotation of the car in a turn.

Conversely if you raise the inner link pivot you lower the roll center; more traction on that end of the car. On the JRXS you have to make this inner move in a rather large step, so it's better to tinker with the outer pivot and shims under the suspension arm.

On the JRXS if you shorten the rear link; the instant center moves inward, the roll center is slightly raised. Less traction on that end, More rotation of the rear.

I have the rear shock tower on the front so I can shorten the front link as well, but don't plan too.

After I have the right springs in the car I am going to tinker with chassis stiffness which is the subject of another thread. Right now I have TC3 silver Springs which are 14.5 lb springs. They fit well. I was getting too much roll with the 12.5 lb springs even though I got good traction. This left me with a loss of precision in the steering coming off the fast straight onto the infield. (The blues seemed a little stiff on a different track but may be the right ones at Mikes)
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Old 02-02-2006, 09:53 PM   #129
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Hi John, thank you for the info I will try different settings this weekend and will let you know how it turns out.
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Old 02-02-2006, 10:06 PM   #130
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Quote:
Originally Posted by FBJ's RDX
Hi John,

Have you tried experimenting with Camber Link Lengths and Positions? If so, what are the general effects when lengthened or shortened relative to whether it is in high or low roll center? I have seen in some PROs setups in the Corally site that they usually shorten their links. I havent tried doing it yet, hopefully this Saturday. Any info on the subject will be greatly appreciated. Thank you very much for your time

Sonny
In general...........

Long and flat links give you more forward traction and less stability through a corner due to less camber change. Short and angled links will give you less traction but gain stability. Short links will also react faster on transitions.

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Old 02-02-2006, 10:44 PM   #131
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Quote:
Originally Posted by Jeff Brown
In general...........

Long and flat links give you more forward traction and less stability through a corner due to less camber change. Short and angled links will give you less traction but gain stability. Short links will also react faster on transitions.

Jeff
HI Jeff,
Thank for the info.
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Old 02-03-2006, 06:29 PM   #132
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Jeff-Thanks for the post. I sent you an e-mail.


Torque Steer

When both wheels on an axle receive the same torque then you have no torque steer. If you put your RC car on a dusty floor or loose track it will accelerate straight with no hand on the wheel of the transmitter. If your hand is on the wheel you automatically correct for torque steer by adding opposite steering. On a Losi stock truck, that I used to run, we eliminated the hydradrive and lost 1 ounce off the right rear. The truck now had pronounced torque steer to the right. The left side tire was heavy and had more forward traction. I mounted my transponder module on the right side to eliminate this torque steer and I could accelerate better on looser sections of the track. Eventually Losi incorporated the lighter slipper clutch and moved the motor a little to solve the problem.

Torque Steer and the JRXS

I ran my JRXS (with GTB 6.5) on a slippery track (M&M in Houston). The other day and I had quite a bit of torque steer. I had not noticed it with a stock motor previously. There are two possible causes. One the corner weights are off. With the associated narrow washers under the pivot balls on the bottom of the shocks the suspension is quite a bit looser. Less Stiction. I put the car back on the scales and found I needed another ounce on the right rear. I added this to my ballast stack. GTB is a heavy sucker mounted with its center of gravity a little left of a normal motor. If you draw a line through the center of gravity (pretty much the very middle) of the car to your ballast. The other end of the line will point right to your problem. It's the motor.

Loose left front outdrive

Another possible source of torque steer is a problem with the drive axles. I noticed that every time I inspect the car, the left front outdrive (the non pulley side) is loose. I suspect that this is from heavy braking off the long straight at Mikes. The outdrive has a little bit of rotational play and it rocks back and forth in the diff tube in going from brakes to throttle. This may not be a problem others are having if they use little brakes. If you tighten the T-nut too much, it breaks. Locktite is innefective on the greasy plastic T-Nut. Instead, I made a little T-nut from 3/16 stainless steel stock. I just center punched it by eye and drilled a hole through it on the drill press and tapped it 4-40. Once you have a good tapped hole then sand the ends with the Dremmel until they are the same length and make the nut about 1/16 shorter than the diameter of the diff tube. Put a strip of servo tape on an allen driver to hold it in place while you assemble the part. I used the broken shaft of the T-nut to accurately hold the screw on center. Worked great today. Here is a picture of the finished install. The part is the shiny rod inside the diff tube. I gave the part a good workout today. No movement. No torque steer evident. Really hooked up well on the infield. Still need to go a little stiffer than TC3 silver to handle the high speed turn off the straight.

Note that you can see the eliminated uptravel sway bar mount that are replaced by two plastic bushings. This freed up the supension. The inner hinge pins are a little more poorly supported in the crashes, however.
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Losi Constant Velocity Drive (LCD) vs MIP CVD-stainless-t-nut-sized.jpg  

Last edited by John Stranahan; 02-03-2006 at 09:44 PM.
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Old 02-03-2006, 06:36 PM   #133
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Hey John,

Your gears have shipped so let me know as to when you receive them.

Dude, take a compressor or a small paint brush to that thing. lol lol lol
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Old 02-03-2006, 06:47 PM   #134
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New pic and custom part above in my previous post.



Why a locked diff works so different as far as setup is concerned. This is for four wheel drive cars.

Here is my theory. It is helpful to read my previous post above on torque steer in order to understand this locked diff.

A gear diff provides equal torque to both wheels as long as both axles are the same stiffness. If one wheel is slipping and spinning wildly the engine cannot give it much torque and the other wheel on that axle gets the same amount of torque (almost nothing). You get poor acceleration, but little torque steer.

A ball diff has some friction, but, it acts much like the gear diff, not much torque steer. If you are cornering hard the only way for the front wheels to steer is with side bite (cornering traction). A ball diff on the front causes poor acceleration out of a corner because the inside tire is unloaded reducing the torque it can accept (spins wildly some times) and the outside front tire gets this exact same small amount of torque.

The Torque Steer Theory of the Locked Diff
A locked diff changes everything. You get primarily torque steer with this type of diff. If you load the outside front tire, it has more forward traction than the inside front tire, this causes huge torque steer that is actually used to pull the car around the corner. So to get more steering on power from a locked diff load the outside tire more. This means heavier springs or anti roll bar or higher roll center if front. This is exactly opposite of normal wisdom on these matters, but that is what I have found and it has been verified by a very good driver. This explantions is just my theory at the moment, but it makes sense to me. Please feel free to comment.

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Old 02-03-2006, 06:58 PM   #135
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Tornado Racing-Thanks. I broke the servo arm right at dusk. Brought it home to fix and clean it. I'll give the gears a good test.

Last edited by John Stranahan; 02-05-2006 at 09:24 AM.
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