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Old 10-29-2009, 09:07 PM   #31
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Originally Posted by John Stranahan View Post
Thanks for the posts guys.

First there are no real ground rules for my threads. I like them to stay on tech rather than chat. That's about it.

I did notice the plastic in the RTR is a different color and probably cheaper as every single part of the car is stamped RTR so there is no confusion buying packaged Losi parts. My "calibrated" hands, with which I twist every car that I look at, by grasping the car around both shock towers and giving the car a twist tell me the chassis is pretty close to stifzell in torsional stiffness. Maybe not quite so tough when you hit the boards, though. Stiffer than a TC3 less stiff than a Losi JRXS. The setup may not be much different than on a Stiffzell car. We never really setup the stiffzell car any differently than the graphite plus cars. Anyway there are probably lots of these RTR's around.

I never had the type R. I wish the introduction had been delayed and they had not produced the first version. I had trouble getting enough rear weight from the get go. I got the TC5 next.

with a RTR chassis, RTR arms and other components you may get mroe traction but less steering than a kit cars.

any way continue your Journey we all watching closley.
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Old 10-29-2009, 09:22 PM   #32
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I find the X simple to work. I'm not saying it's as easy to access things as some of the newer cars....but it isn't brain surgery. Most screws are 4/40 so no tricks there. But things like removing diffs just require a few more screws to come out. On the upside....remember how nice it is to have a fully sealed drivetrain. The belts are a subject of discussion still. Each color (Blue, Yellow and Black) relates to a level of performance ( basically because each belt was made from different materials and/or construction) Blue was most basic. Good for RTR cars or street bashers with low performance motors. Yellow was an upgrade and meant for higher performing motors. Maybe 27t or 19t in race conditions. The Black Long belt is the top dawg. meant to handle just about any motor for sanctioned racing. It was also longer (777mm) and requires the appropriate belt adjuster and delrin pulley. I believe the pulley was also black vs. the white 1 for the blue and yellow belts.
As you've read in my posts I run the black belt and have never had a problem but.....I may try the yellow belt and see if the drivetrain is any free-er or smoother. The construction of the yellow belt might make it more pliable and therefore create less friction/drag. We'll see

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Old 10-30-2009, 01:19 AM   #33
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Im currently running my XXX-SG+ on asphault and wanted too know your opinion, if a RTR or stifazel chassis would have a bit more flex and possibly help with traction.

track can be found at www.rrcsa.org.au

would i notice any change too the car if i kept it all the same bar the chassis?

cheers
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Old 10-30-2009, 08:59 AM   #34
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Paraletic-I would not trade cars. There are adjustments to be made that can compensate.


Roll Stiffness
I noted in the last post that I made some changes to the car to suit my track better. Practice days are usually low traction. I thought I would decrease roll stiffness while keeping the 14.5 lb springs which seemed about right.

Roll stiffness is how hard it is to roll the car (lean the car) in a turn. The more roll stiffness you have, the quicker a car responds. Because the tires are more stiffly supported in the turns with more roll stiffness, there is less compliance of the rubber to the road over the minute bumps in the road. Thus you also have less traction. Use a stiff car for carpet or high traction asphalt like during a big multi day race.

On a dusty parking lot track a low roll stiffness might be in order to pick up some valuable cornering traction.

We have several handles on roll stiffness.

Stiffer springs give higher roll stiffness
Thicker anti roll bars give higher roll stiffness
High roll centers give higher roll stiffness

If you increase roll stiffness on only one end of the car, there is less tire compliance (tire conforming to the road) there and other weight transfer factors that cause less traction on that end. That is a very valuable tool in itself.

Roll Center
Most of the above may make sense but then there is this roll center business. The roll center is the point in space about which the chassis rolls when it is cornering. When a car corners tire friction is developed which forces the car inward to make the corner. These tire and suspension forces act on the roll center which is usually low in the car. 0 to 1/4 inch high on our touring cars.
When a car corners centrifugal force also acts on the car. It acts on the center of gravity which is usually higher in the car. About 3/4 inch high in our touring cars.
The straight line between the low roll center and the higher center of gravity forms a torque arm. When the cornering forces act on this torque arm the car rolls. If the torque arm or lever is long the forces develop more torque, the car rolls easier. If the torque arm is short the forces develop less torque, the car acts stiffer. See the first diagram. We change the length of this arm by moving the roll center up or down.

The Losi comes with a high roll center. This is good for medium traction asphalt. I decided to lower the roll stiffness by adding low roll center blocks to the car. In combination with the proper pivots on the end of the car, they lower the inner hinge pins. This lowers the roll center. The car will get more supple. It should develop better traction on low grip asphalt. The last picture shows the front of the car. As was mentioned earlier you can use some of the rear blocks on the front to lengthen the car, and to remove drive axle angle for less friction and slightly more front traction out of the corner. I really need this, so I am giving it a try. I also removed considerable kickup or upward tilt to the front of these hinge pins.

Suspension Jacking
To understand more fully what roll center to use you need to understand another property of roll centers, Suspension Jacking. The fourth pic shows the tire frictional force pushing against the roll center as I described above. This large force is broken down into two components as it is not parallel to the ground by the high roll center. The large horizontal component helps the car corner (the good component) the smaller but significant upward component raises the chassis up in the corner. For touring cars on high traction surfaces this jacking force can raise the car up enough that it does a traction roll from being to high.
so for low traction use a low roll center
for medium traction any roll center goes
For high traction carpet back to a low roll center.


pic 1, car is making left turn. (White) Centrifugal force acts on center of gravity. (Black) Tire frictional force acts through the suspension on the roll center and pushes it left. Result cars top rolls to the right.


Pic 2, form line one along upper camber link
form line two along lower A-arm, this locates instant center far right.
form line three from bottom of tire to instant center
BIG ARROW, shows roll center. A roll center at the ground is most effective at cornering. There is no wasted suspension jacking.

Chart showing effects of moving suspension points on roll center.

pic 4. Car is making a right turn. Tire friction acts through the suspension at an upward angle to meet high roll center. a component (short fat arrow) drives the suspension upwards. The horizontal component drives the car around the corner. The horizontal component is maximized with a low roll center.

last pic. Front of the car right now. 4R rear pivot in front, low roll center blocks at back of hinge pins.
Attached Thumbnails
LOSI XXX-S, Tips and Tricks, Open Mod, etc-schematic-roll-007.jpg   LOSI XXX-S, Tips and Tricks, Open Mod, etc-schematic-roll-center002.jpg   LOSI XXX-S, Tips and Tricks, Open Mod, etc-roll-center-adjustment-chart.jpg   LOSI XXX-S, Tips and Tricks, Open Mod, etc-schematic-suspension-jacking003.jpg   LOSI XXX-S, Tips and Tricks, Open Mod, etc-front-car-4r-rear-pivot-block-installed.jpg  

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Old 10-30-2009, 11:08 AM   #35
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Just so I'm clear on the last pic. Front of the car: Losi low roll blocks. Used the 4R pivot block (top holes of diff cover?). I know there's a difference between the the front & rear pivots ( can't remember @ work) so does this result in any angle of your hinge pins?

Also if lowering the front roll center seems to be the right direction in performance maybe try the Matt Francis front blocks (TMF 1107) which are suppose to be 0.150 lower than the high blocks instead of the 0.075 the losi ones are.
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Old 10-30-2009, 11:49 AM   #36
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when i was running the yellow belt i took a toe nail clipper and cut everyother tooh off. i did free up the drive train a little bit. that was with a stock motor though
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Old 10-30-2009, 12:05 PM   #37
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Sorry to be off topic but Ijust found a xxx on craigslist and have some questions. How old is this chassis? Who has parts? I didn't find any on losi's site. Is this a good first car for vintage or stock TC? Thanks
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Old 10-30-2009, 12:52 PM   #38
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Sorry to be off topic but Ijust found a xxx on craigslist and have some questions. How old is this chassis? Who has parts? I didn't find any on losi's site. Is this a good first car for vintage or stock TC? Thanks
Great car for stock AND vintage. I've raced both with it. The design goes back at least 7-8 years but was well designed and way ahead of its time. Parts ARE still available from Losi but not for the Graphite car. The newer xxxs was the Drift R version. Same car just with cutesy drift stuff. Check Ebay, Losi Parts House, Stormer Hobbies, Ultimate Hobbies.....Many, many others. Easy to get and inexpensive.
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Old 10-30-2009, 01:07 PM   #39
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Just so I'm clear on the last pic. Front of the car: Losi low roll blocks. Used the 4R pivot block (top holes of diff cover?). I know there's a difference between the the front & rear pivots ( can't remember @ work) so does this result in any angle of your hinge pins?

Also if lowering the front roll center seems to be the right direction in performance maybe try the Matt Francis front blocks (TMF 1107) which are suppose to be 0.150 lower than the high blocks instead of the 0.075 the losi ones are.
The front inner hinge pins appear to be parralel. Yes, 4R rear block, upper holes of diff cover. This took out the rearward slant of the arms. I did notice these low roll center blocks were not as low as those I used to use. If there is an improvement I'll see If I can find Matt Francis blocks. None popped up on my searches.

I put links to a few current sources in a previous post.
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Old 10-30-2009, 07:18 PM   #40
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Track Report
Track, low grip. Temperatures mild. Sorex 36R with traction compound, Traction Action. Outdoor large asphalt track. Some damp parts.

Changes from Stock
lowered roll center- I noticed slightly more cornering grip, need to go one more step down with Matt Francis Height. I will probably sand my own down.

Removed kickup-Kickup is helpful with a standard diff and adds on power steering from roll stiffness softening (due to the kickup angle) and midcorner cornering traction (from more caster). It is not so much help with a locked diff. Without it, I noticed more on power steering. Enough to cause a few spin outs. I adjusted the upper shock angle to take just a little on power steering away.

Lengthened car by using rear block up front- Enough to require a retrim on one front wheel well. I was getting an occasional rear end wiggle on huge power out of the corner. This is gone. I am getting no shudder or wheel hop from removing that front inner hinge pin outward angle. Man that was an easy change using the rear block up front.

The session finished with a very good handling car. I'll see if I can get just a little more cornering grip.

Balancing Oversteer/Understeer
It is very helpful if the touring car has a neutral Oversteer/Understeer balance. This means you don't loose the back end (Oversteer) in a corner nor does the car push (Understeer) too much. When you achieve this you have the best cornering traction possible from the car.

We achieve this balance by taking into account an unusual property of rubber. Simple physics tells us that as the load increases on an object the friction increases proportionately. This is true of most materials except rubber. Rubber has the best grip when it is lightly loaded. Grip is increased with more load but not proportionally. See the graph on the left below. The line curves down.
In addition rubber has the best grip when it is sliding a little. The projections of rubber on the surface of the tire act like little fingers and walk across the surface of the road when you are in a controlled drift.

The second graph shows the cornering power in g's of a rubber tire on pavement as we increase the load. The term "Cornering power" is a construct of author Herb Adams in the book "chassis engineering". The data is from his book. Cornering power in g's (gravitational units) is the load on the tire divided by the cornering force (from graph 1). cornering power vs vertical load is graphed on the second graph below. Notice that the cornering Power decreases sharply as we increase the load on our test tire (move to the right on the graph). The tire becomes less efficient at cornering the higher the load on it.

Conclusion, light is good, heavy is bad and also the best cornering power would be obtained when the load is distributed evenly on the tires. No one tire is too far right on the second graph.

Weight Transfer
The properties of rubber tell us then to load the tires equally to get the best cornering traction. This is what most guys try to do with their RC cars. That is well and good until we hit the first corner. Then we get weight transfer. Centrifugal force acts on the center of gravity of the car which is some distance above the ground. Weight transfers from the inside wheels to the outside wheels. This had nothing to do with springs or shocks.

Imagine this case. I put a table with rubber feet on the concrete. I push on the top edge of the table. Weight transfers to the feet away from me. If I push hard enough and the feet have sufficient grip I can transfer all the weight from the inside to the outside, and, the table tilts (does a traction roll). The table has no shocks and springs. There is no table roll until the inside feet have lost 1/2 the weight of the table. I mention the table because guys get weight transfer and chassis roll confused. You get weight transfer regardless of chassis roll. Chassis roll is however caused by the same centrifugal force that causes weight transfer. See figure 3 on weight transfer.

We had our ideal case. Left and right tires are loaded equally. We hit the corner, weight transfers to the outside. Efficiency is lost on the outside tires but we have the same loss front and back. We have Oversteer/Understeer balance.

Well we were just lucky. In real life some weight transfers back to front while slowing for the corner. Let's say now that we end up with too much front grip. The car oversteers. How do we fix it.

What we do is increase roll stiffness up front. Since the car is stiffer in front more weight will transfer sides at the front; the front outside spring reaches steady state compression first. Our outside front will lose efficiency. This will kill some steering traction. We achieve Oversteer/Understeer balance. All is good and the car is very drivable again.

To kill some traction on one end of the car then, cause more weight transfer to happen there by stiffening that end up.

I'll throw a locked diff monkey wrench in all this later.
Attached Thumbnails
LOSI XXX-S, Tips and Tricks, Open Mod, etc-graph-cornering-traction-vs-load001.jpg   LOSI XXX-S, Tips and Tricks, Open Mod, etc-graph-cornering-gs-vs-vertical-load001.jpg   LOSI XXX-S, Tips and Tricks, Open Mod, etc-schematic-lateral-weight-transfer-resized.jpg  
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Old 10-30-2009, 07:28 PM   #41
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OMG John !! This is good stuff. I have to read it 3 or 4 times to get it but man is this an education. Thanks. Herb Adams....Thats a name I haven't heard in awhile. The Pontiac Trans Am would never be the car it was without him.
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Old 10-30-2009, 08:09 PM   #42
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Thanks-

Here are the new low roll center blocks. .150 shorter than stock. I used silver sharpie. Then I scribed a line .150 up. Use dividers or a hemaphrodite caliper, or height guage to make a line. I used the dremmel and a course drum to remove most of the plastic then a belt sander or sand flat by hand on a flat surface. They came out nice.

thanks C&B RC
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Old 10-30-2009, 08:17 PM   #43
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I'm just eating this stuff up!

If you need a little more wheelbase for the open asphalt ( I run tight carpet) move the arm spacers. This seems to really change the handling. Moving the front arms forward is suppose to smooth out steering and be the most concistent. moving the rear arms back will reduce traction.

I've been meaning to sand a set of blocks myself....cheaper & easier than finding a set of MF's
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Old 10-30-2009, 09:45 PM   #44
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Ya know it's gonna be a good thread when the hermaphrodite caliper makes an appearance!


Good stuff as always.
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Old 10-30-2009, 10:28 PM   #45
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Thanks guys-The low-low roll center blocks are installed. I had to make some mods up front. There is a lip on the edge of the frame that needs to be sanded off or it will limit down travel of the arm. The nub on the droop screw up front is completely removed. The droop tab must be narrowed so the bumper will go on and not restrict down travel of the arm. Some short droop screws need to be installed front and back. This is the same setup I used to run outdoors on the XXXS years ago.

Ride Height
You will find the traction about maximum with a 6 mm ride height when using these Matt Francis low roll center blocks. If you lower to 5 mm you will find yourself sliding again. What happens is when you lower the car the center of gravity is lowered, the roll center only slightly lowered, the torque arm between the two is thus shorter and the car acts stiffer. So lower is not always better on outdoor asphalt. I remember this from the previous car.

C&B RC- I'll take a look at those arm spacers and see if I can eliminate any more drive axle angle.
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