John Stranahan

iTrader: (27)

Sonny-You have similar equipment now to what I observed the Trans Am and CART full size race car guys use in the paddock area of the Houston Grand Prix to setup their cars before the race.

You will notice that the numbers move around a little. Theirs do to. This is caused by friction in the shocks and stiction in the suspension joints, but you have the right method. Bounce the car up and down pushing on the center of the shock tower or between the wheels, front and back) before you take a reading. When I do this to my 1/10 scale touring cars I get about a .2 to .3 ounce variance. This is more than good enough to setup the cars. you don't need perfection. If you have more variance than this remove the bottom shock fastener and use your finger to move the suspension. It should drop freely by gravity alone. If not fix the problem. This free movement is essential to good traction on the small cars. Push the shock in by hand and make sure it moves easy and does not have a bent shaft.

First set the ride height on the car. You normally only need to set height on three corners. Put the car on the scales. Adjust the front or rear shock collars (I assume you have the threaded kind) until the front scales read even (plus or minus .2 to .3 ounces). If the weight is right in the car then the rear scales will automatically read even (within the .2 or .3 ounces). If not you can move some weight or just leave the front even. Shock collar down 1/8 turn to increase weight on a corner. Also shock collar down 1/8 turn on the opposite corner (diagonal corner) to increase weight on a scale. A tweak station will do this last step very well. It will leave the front even if the front is on the beam, but it cannot be used to place the weight right in the car. Good luck. Be glad to add more detail. I have some additional detail earlier in the thread. I'll put a link in a bit.

Its a bit easier if you follow the whole procedure, then you find out exactly where your ballast needs to be.

Side to Side weight first. Two balances, Two beams. Left side tires on one beam, Right side on the other beam. Move weight to get it even.

Front to back weight next. Two balances, front tires on beam rear tires on the other beam. Make this even or put a couple extra ounces toward the back if possible.

Finally corner weights with four scales.

Here is the link.

http://www.rctech.net/forum/showthre...es#post1977808

You can also use corner scales to preload the left front a certain amount for the oval. This gets you consistent performance after you have made changes to he car. You can do this with the springs (or tweak screws) and it is then called adding or subtracting wedge (Nascar). On a touring car you call it tweak and go for zero tweak.

I usually use balances at home to set the weight right. It is quick and easy. At the track I use a tweak station, because it is compact and more rugged. Some oval guys have posted previously that they use scales at the track.

John Stranahan

iTrader: (27)

Future Test- Flat wire vs Round wire Cobalt 10 x 1. GTX. Here is a peek.

Fatboy Joe

Thanks John for the information

thedudeaturdoor

the flat will spool up faster
(aka a good drifting motor)

and the round will be faster over all

and the flat will have better acc.

John Stranahan

iTrader: (27)

Sonny-you are welcome. Tell us how it went.

Losi JRXS Rub Points

I have quite a few packs on the car now. Time to look for problem areas.

Front Shock End Rub
I had one lower shock end that was rubbing the A-arm after an A-arm replacement. Pass you X-acto knife on the inside of the shock end and make sure there is clearance between the end and the widened part of the A-arm. The rear belt was rubbing the chassis a little. I was running it low. Take the Dremmel with a 3/16 inch chain saw sharpening stone and bevel the top of the chassis on the inner narrow portion of the elongated hole where the belt can rub. I did this by pushing the belt out of the way with one hand and had the car fully assembled. Not too much trouble. This is very important for stock.

Front Drive Pulley
My front drive pulley was rubbing the front shock tower on the sides of the pulley slot. I have some axial play on the whole diff tube pulley assembly. The problem was caused by a loose left outdrive cup screw. I retightened it and will check it once in a while. I have a feeling this is caused by heavy use of brakes with a locked diff. This outdrive can rock back and forth in the diff tube. I might need to add some purple (low strength) locktite to the inner teeth on the oudrive or blue loktite to the screw head.

Rear Shock Tower on Front
I have completed this mod. It works fine. The outer hole cannot be used but you get two holes further out than with the front tower. The shock is not past perpendicular to the arm on these two holes so movement out still stiffens the roll stiffness. The inner most of these two holes is maybe 1/2 or 1/4 of the normal spacing as the rear is drilled on a different starting point. Thats the hole I am in now. Upper shock mount second hole out on rear shock tower up front. Worked out nice. Track report to come. There is an extra set of camber link holes ouboard. Just disregard these. Roll bar is off for now.

Rear Diff
I removed the rear diff completely. The thrust washers and caged bearing was the problem. The balls were no where to be found!. Thats a new one for me. Pieces of the cage were still there between the two grooved thrust washers. Normally a rear diff last a long time on these cars.

The dude at your door-. We posted together. Did you get a chance to actually run the two motors?

thedudeaturdoor

nope,

but i know some1 who did

he said that the flat is better for small tracks like the manufactors say

so im getting a flat wire when im rich enough

John Stranahan

iTrader: (27)

dudatyourdoor: thanks. I have the two motors in hand. The round wire has about 4 more Watts and 1000 more peak RPM than the flat wire. Brushes are not fully seated so this may mean nothing. Kind of like spinning my GTB up on the Bench.

JRXS Tweak Report
The chassis has stayed flat and tweak free since I put loctite on the last 8 screws that hold the front and rear plastic bulkheads. When I twist the chassis now it does not creak, nice and solid. Just springs back. Always near perfect on the tweak station.

Made a couple changes to the setup. The rear shock tower on the front worked for me. Having the ability to move the shock another half hole out was nice. I was able to remove the front swaybar. Plenty of steering traction now. This reduces my need for brakes. I am in profile 1 on the GTB 6.5 now with 20% drag brake.

Paragon Traction Action
I tried Paragon Traction action and it did not work for me. 27's dry worked very well. My GTB 6.5 motor has a rather powerful first third of trigger travel, Cristians 10 x 1 is more linear, the 10 x 1 motor comes on a little softer. This could explain the difference in the way the tires work. I have some Corally Jack the gripper to try.

Roll Center Adjustment
I had a little oversteer with a locked diff after the shock tower change. I raised the rear roll center by adding one shim under the rear inner hinge pin pivots. This corrected the problem except on very heavy throttle exiting the corner. I may add one more shim. These shims make a tiny change to your oversteer understeer balance.

Rear toe
I added another 1/2 degree of rear toe. This tends to reduce oversteer. Cristian and I both had tuned out a similar problem of on power oversteer at Reflex coming onto the straight from a tight corner in mod. This helped matters at the beginning of my run, but then the effect wore off. I might have been overheating the rear tires. I plan to go back to 2.5 degrees.

Captured Inner hinge pins Fix
One thing I did not like particurlarly well on this car is that the plastic swaybar mounts tend to capture the inner hinge pin and require the hinge pin to rotate with suspension travel. I made some bushings out of 4 spare sway bar mounts (just cut away everything that does not look like a bushing). Loose the little button head screws. This allowed the hinge pin to remain stationary and let the arms pivot on the pin. The car felt really good. This is probably not as strong as the stock setup, but I am hitting things a lot less already. I have more overall traction.
If you want to do this as a test without making a part. There are two spacers in an inexpensive Losi shock kit LOS A5015 that will equal the thickness of the sway bar mount and the thin spacer. You would need four of these part numbers. About $2.00 each. It would help to look at the package in case there have been changes.


Overall impressions JRXS at this point in time

The car has about twice the grip at all four corners than it did only a week ago. Some of this is finding the right roll stiffness and then there are probably the mysterious forces at work. Same Tires and temperatures. The car looked really good, supple yet responsive, tracked well, only a tiny bit loose on heavy power (corner exit oversteer), good trigger control will eliminate this, but I will try a couple more things to tune this out. I am quite happy with the car now that the chassis is tweak free.

Here is a link to the current seup on this thread. I have included the position of my Skoda II body. This body is a bit too thin to hold up well, I may change to a protoform body.
Current JRXS Setup

FBJ's RDX

Hi John, so by raising the roll center of a car whether it may be in the front or rear will generally add traction to the side where it is raised? I am quite confused with the roll center thingy. Some say that when you raise the roll center, it will lessen traction on that end, I am I correct? The reason why I am asking is because I tried raising the rear roll center of my car (RDX) to add more steering because I was informed and read in the manual that i will give the car more steering because it will will lessen traction on that end but will feel stable. Is there a difference between stability and plain traction? I hope you can shed some light on the differences, Thank you very much for your time

John Stranahan

iTrader: (27)

FBJ's RDX

Actually it's true both ways. It depends on what kind of front diff is onboard. I am using a locked diff. Both Cristian and I have done a simple experiment where we raised the rear roll center (with a locked diff). We saw increased cornering traction at the rear. It is exactly the opposite with a standard diff. Look at my post a few posts back on roll stiffness. Whenever I do these test, I start with a little change, then medium, then huge change. This exagerates any effect and I can pin it down pretty well.

If you read the Losi manual and then the associated manual they say exactly opposite things some times. I don't trust the manuals much. I do my own experiment on each tuning option. The main goal of a manual is to simplify. This is at the expense of accuracy.

Here is the link to my post on this thread.
Roll Stiffness

Note: that I am now using a long ball stud on the rear upright of the JRXS. I had to trim the top of the outer camber link ball cup for clearance on some RP 30 wheel and tire combo that I have in hand now. This longer ballstud has the effect of raising the roll center slightly like the shims under the rear hinge pivot blocks.

John Stranahan

iTrader: (27)

Traction Compounds at Mikeshobbyshop.com
I had a couple more flavors of traction compound to try. Corally Jack the Gripper and Paragon Traction Action (the new white label can as opposed to the old black label can. A good tire today (18C) was the CS 27. The Corally Jack the Gripper had no effect. The Paragon Traction action had a positive effect. I could note higher corner speeds and also the air dam started rubbing from more body roll caused by the higher traction. This formula has been changed from the old black can. Smells more like blended gasoline now with just a hint of wintergreen or something similar. My hands don't stink.

I tried a couple of flavors of tires, RP 30's and Pitz 30,s. Neither hooked up. I tried the Paragon on the Pitz 30's and they still would not hook up. Still too cold.

Notes from a Chemist
All of these traction compounds have made an effort to reduce the smell. Unfortunately the smell is not the only thing that gives you headaches, sinus irritation and drainage etc. (Kills you slowly so to speak) The culprit is the solvents. So here are some tips.

Turn the last tire with the spur gear to keep the stuff off your hands.

Wipe the tires off with a paper towel instead of a rag so that you can throw it away outside.

Seal the four tires in a quart Zip Loc Freezer baggy after your practice or race, to keep the solvents out of your pit bag.

I have my traction compounds sealed in a small tupperware container so that they don't stink up the takle box. This keeps the solvents out of your house.

Treat this stuff like you would gasoline.

JRXS setup - I added one more shim under the rear pivots. Profile #1 on the GTB with 20 % drag brake now that the car will steer.

John Stranahan

iTrader: (27)

Front toe
I normally run 0 toe. Today I tinkered with the on power oversteer out of the corners. I was going to try toe out. I put my caliper on the wheels and they were already toed out. Tried 0 toe next. Better. Tried about 1/16 to 1/32 of toe in and the problem was cured.

JRXS Shock Piston
I had a shock piston come completely loose from the shaft the other day. Two others were loose. They need to be tight. Squeeze the shaft with a lead strip and pliers on the shaft and tighten it next time you check your fluid or before the race.

Protoform Dodge Stratus Body and Wing
I tested a Protoform Stratus body. On the first run, I thought I lacked steering off the straight. There is a cutout line on the rear wing to cut about 1/4 by 3 inch piece out of the top center. This cured the problem on the straight, but now I oversteered on some of the other corners. Can't uncut the wing, so I raised it up with two 1/8 spacers. The car was back in balance. Had one wreck dead center on the first bumper off the straight at almost full power. I had misjudged the angle of the car when I nailed the throttle. No damage to the body. The Losi Skoda body would have split all the way to the hood. Picture of the wing in my next post.

FBJ's RDX

Hi John, thank you for the very nice information on the roll centers. It has help me alot in understanding how to fine tune my settings. Thanks

John Stranahan

iTrader: (27)

Saturday Practice- No one showed because of 80% rain forcast. It did not rain, track was dry and had high grip untill at least 1:00. My car is now strapped. Two thumbs up on the JRXS. I added a thin shim under the front pivot blocks to add a tiny bit of steering (Locked Diff). I'll change my post on the JRXS setup.

FBJ's RDX-Thanks for the post.

Roll Center
I thought I would discuss roll center a bit since it can be used to fine tune your roll stiffness.
The rollcenter is the point about which the chassis rotates when it rolls in the corner. There is a front and back roll center. If you connect the two points you have the roll axis.
When the car corners the mass in the car is forced outward acting through the center of mass. The chassis suspension and tires resists this outward force acting through the roll center. Because the roll center is usually below the center of gravity, the chassis rolls (low roll stiffness). If the roll center and center of gravity were the same height, the chassis would roll very little or not at all (very high roll stiffness).
There are some advantages to having some roll in the car. Roll delays weight transfer. When you delay weight transfer you gain some extra cornering grip. You want good roll (low roll stiffness) on poor traction surfaces (use lighter springs) and no roll (high roll stiffness) on very high traction surfaces like carpet (use very stiff springs)

So the question is where is the roll ceter and how can you adjust it. First a few common adjustments. If you have followed this thread you have seen that I have added a few shims under my pivot blocks (JRXS), as has Cristian (FTTC4). Note that my blocks are also flipped so that a low roll center is obtained. Generally on touring cars, (I have used XXXS, TC3, TC4, JRXS)

Raising the lower (A-arm) inner hinge pivots raises the roll center.
(I am using low roll center blocks, but raised up by a couple of shims in back (one thick one thin), and one (thin) shim in front.

Raising the outer camber link pivot raises the roll center
(I have also used a long ball stud on the rear outer camber link pivot, with a ball cup shaved off on top). There are good reasons to have this as high as possible and still fit in the wheel.

Lowering the inner camber link raises the roll center
(I have not made a change here. I am in the center hole.)

Changing the length of the camber link changes the roll center a tiny bit. You would really have to calculate the difference because it is so small. This also changes the way the camber on the tire changes with roll, but camber has a small effect on these cars compared to roll stiffness.

There are good reasons to have the roll center at or near the ground. The carpet racers know this. If you run a high roll center the cornering force not only pushes the car around the corner, but it also jacks the chassis upward. On carpet this can cause a roll over. If the roll center is at the ground almost 100 percent of the cornering force is used to make the car corner and little is wasted in chassis jacking.

So being curious, I wanted to know where my roll center ended up. You don't really need to know this; it's just an academic question. I have previously written a roll center calculator spreadsheet. You can put the cars axles on four blocks and measure the location of the suspension points with a dial caliper (and a compass or divider, if you have one, to make a couple of the measurements easier). My JRXS rear roll center is about 1/16 inch below the ground for the static case. It moves slightly up and out some with roll so this is close enough to the ground for me (notice that I am done tuning before I calculated this). If I make any more changes it will be to add stiffer springs as the traction picks up with warmer weather. Anyway if you want a copy of this Microsof Excel Spreadsheet I have two versions, the TC3/TC4, and the JRXS. It will work for any car or truck, even full size cars with double wishbone suspension. These two version are just filled in with my numbers which can serve as a basis to check for errors. Send me an e-mail to [email protected] You do have to already have Excel.

The pic shows the graphical part of the output: The center line of the car, The suspension points that you measured (easy measurements). The instant center, and the roll center. There are also some numbers output so that you don't really need the graph. The sketch shows you what you measure.
Also in the pic is my cutout raised wing. It works really good like this at Mikes.

Any comments on this subject are welcome.

John Stranahan

iTrader: (27)

Roll Center Simulations

Here are some simulations using the (Microsof Excel Spreadsheet) roll center calculator available by e-mail free in my previous post

What if I use the lower inner camber link pivot hole The roll center rises to .451 inch from -.06 inch. This is a huge change.

What if I use the upper inner camber link pivot hole?. The roll center drops to -.25 inch. This is a huge change.

What if I add another .030 inch shim under my inner hinge pin pivots?. The roll center changes from -.06 to -.05. This is a subtle change and is more in line with what you should use roll center adjustments for.

Swaybars
The purpose of sway bars is to increase the roll stiffness on a car without having to use springs so heavy that the tire does not follow the road irregularities well (to have good tire compliance), or for a small fine tune of roll stiffness. I find even the tiniest pivot friction from a bad part really hurts the cornering traction. Our sway bars add to this pivot friction on 1/10 scale, not that much on 1/8 scale or full size cars. There has to be a compelling reason for me to add roll bars on a 1/10 scale, like I don't have another hole on the shock tower and the next spring is too stiff.

Tuning Order
First get the springs in the right holes, Get the rear toe right, get the front toe right, then tinker with the roll centers a little. And then Argh (Ackerman). Cristian had some good luck here on the FTTC4 by adding two washers under the bell crank cross-arm ball studs. Thankfully my Ackerman is not adjustable. I could tinker with bump steer, but the car is good.

JRXS Idiosynchrases (Chassis short Circuits Battery, Locked Diff Problems)
The chassis is somewhat conductive as are the molded graphite chassis. I caught my XXXS sizzling from battery bar contact. I relieved the area with the Dremmel. I caught the JRXS sizzling on the rear cell as I was about to finally seat the battery. You really need to arrange things so the battery goes in easy. I made a slight change to my positive pigtail angle (more directly up). to fix the problem. I sanded the end of the pigtail at an angle at the battery to slim the battery a little. Now the battery drops in. No sizzle.

The locked diff compresses the plastic pulley and the pulley oozes away from the pressure with time. This slightly alters the location of the pulley as will a loose outdrive on the non pulley side. I have had the pulley ride up on top of the chassis twice now. I beveled the chassis in this area to prevent rubbing and will replace this pulley more often.

Losi JRXS Spool?
I await the nice Aluminum or lightweight steel spool that will take the standard Losi One Way Outdrives and Pulley. Please.

GTB 6.5 in Mod 10 turn class
Two thumbs up, but could use more brakes. My kit must be 10 pounds lighter without all the brushed motors stuff. Consistently good performance.

Spektrum
Two thumbs and two big toes up. Great advance.

Springs TC3 on a JRXS
I just put TC3 blue Springs on the JRXS now for a test. They are a little less stiff (actual wheel rate rather than manufacturers specification) than the Losi 17.5 lb spring, but stiffer than the 15 lb spring. They actually fit with a few minor modifications. I used two washers under the lower shock pivot ball. I used a thicker plastic spacer on the upper shock pivot. I relieved the caster blocks slightly to run the blue springs.

John Stranahan

iTrader: (27)

OK. I have a few of the roll center Spreadsheets e-mailed. See the post above for details. To get the approximate center of gravity, balance the car on the side with a pencil eraser holding the car up (if the body will allow it) Use the other hand to reposition and guide the car. Move the pencil up and down the side of the body until the car does not have a tendency to fall in a particular direction. Use the position of center of the eraser as the center of gravity. Put the car tires down on the table now and measure from the ground to this point. This is accurate enough for our purposes. On the JRXS the drive pulley is centered in the car. I used a point jabbed lightly into the spur and moved it up and down by rotating the spur until the point was at the center of gravity. I then put the car on the bench and measured to where my point was. This is in error slightly due to lack of the body which weighs about 100 grams. I did put a 100 gram weight slightly back of center on the car to take my suspension measurements.

I get good reproducibility on the roll center to about 5% on repeat measurements.


We are working the new foam tire vs worn foam tire TC4 problem at present.