yyhayyim

iTrader: (26)

I spoke w/ MaxAmps tech support a few days ago and they assured me that their Lipo packs can be safely charged at 2C rate as long as you charge them simultaniusly w/ a balancer...that's very nice news...they assured me that like the Orion packs, charging the Maxamps Lipos at 2C rate will not in any way harm the pack if you use a balancer at 2C rates...Anyone own a good maxamps pack they'd like to experiment with and give us some feedback on that?

John Stranahan

iTrader: (27)

About this T2 guide.

Well as usual anything that relates to weight transfer is backwards or just wrong.

When throttles is applied, there is faster weight transfer rearward. The reason the rear end becomes loose is the rear suspension becomes stiffer on power and there is more lateral weigtht transfer at the rear. Lateral weight transfer on an axle reduces traction. This is for our independent rear ends. This works very differently on my pan car.

The ability to handle bumps is better if you mean the car does not bottom, but instead it leaves the ground. The car will not accelerate while off the ground or with unloaded tires.

The roll center changes do not affect the amount of lateral weight transfer; they do speed the weight transfer up or slow it down. Faster with high roll center slower with low roll center.

I charge my maxamp packs at 2C sometimes. I usually turn the charge rate down for the finishing step.
John

mattnin

iTrader: (44)

"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

=============================

I used to do this type of thing racing oval adjusting camber and cross weight (wedge) but adjusting out 7 ounces might be a bit too much. I do have an idea for you though. Why not just run 2S1P A123 batteries and not 2S2P? I know, it is only about 2300mah, but isn't that enough for 5 min heats? The reduced space on the battery side might give you enough room to place the ESC there, not to mention cut the weight even further. It is worth looking at, right? I run Novak 5.5r and 6.5r in offroad vehicles and rarely put back in 1900mah in my lipos according to my Duratrax ICE charger.

Yes, the book I have is Miliken and Miliken, so if you have any questions, do not hesitate to ask.

However, I believe we can be in for a healthy debate. I am under the impression that anti-squat reduces weight transfer to the rear compared to a vehicle with no anti-squat because the ride height is increased under acceleration. We all know that when ride height is increased on one end, the other end experiences a loading increase. So, a car with greater than 100% anti-squat and with a front end that doesn't have excessive suspension travel will experience increased loading in comparison to a vehicle with exactly 100% (no anti-squat or squat) or less (squatting) anti-squat.

Also, you mentioned, "Lateral weight transfer on an axle reduces traction." According to my physics book, increased weight on the tire or any object increases the static frictional force, therefore increasing traction. And, according to M&M, this results in increased lateral/longitudinal force at the expense of tire deformation and tire load sensitivity. So, I understand this as although due to increased load and increased lateral/longitudinal force as a result, the tire reaches the kinetic friction limit sooner due to the increased tire load sensitivity. In layman's terms, put more load on your wheel and you will have more traction, take it away and you will have side bite. Am I wrong?

mattnin

iTrader: (44)

Here is a picture of my MF2 when I was doing oval and my scales setup.



Well, I hear the baby waking so it is feeding time, see ya!

John Stranahan

iTrader: (27)

Mattnin-Thanks for the post and pic. Nice balance setup.
Now might be a good time for this discussion.

Tire Traction vs Vertical Load
The friction that rubber tires produce on pavement is rather unique. Yes it increases with load, but it does not do so in a linear manner like the subjects in a first year Physics text. The more load on the rubber tire the less efficient it is at producing friction. There is a very nice graph in Miliken showing this relationship. The line goes up showing increased friction but then it curves with the concave side downward as load is increased, showing that it is less efficient at high loads.

Herb Adams in "Chassis Engineering" has a similar graph. The graph on the left below uses Herb Adams data. Note the curve downward showing an efficiency decrease as the load on the tire is increased.

Herb Adams discusses this further. Here is a quote from my book

"The cornering power of a tire can be thought of as the ratio of cornering traction developed by the tire, in pounds, over the weight that the tire has to push around the corner, in pounds. At the limit of traction, this ratio expresses the tires maximum cornering power in g’s, where one g is the acceleration of gravity. If a tire has a load of 500 lbs on it and it develops 700 lbs of cornering traction then the cornering power is 700lbs/500 lbs or 1.4. The tire can develop 1.4 g’s of cornering power. This is 1.4 time the acceleration of a falling object."

On the right is a graph of cornering power (g's) vs load for the same tire. Notice it drops as we increase load. So what happens on an axle when there is lateral weight transfer is the inside tire looses cornering power at a high rate, nearer the left side of this graph, and the outside tire gains cornering power at a low rate, nearer the right side of the graph. There is a net loss in traction.

The net result of all this is that lateral weight transfer is always bad on pavement. That's why guys want to put stuff low. It reduces lateral weight transfer. I'll discuss the antisquat some more in the next post.

John Stranahan

iTrader: (27)

More on Antisquat
If you read the full size car books you will find a little different story than we find with our RC cars. With a full size car either solid axle or independent suspension we are supposed to get the chassis lift you are talking about which should load both the rear tires up. This should increase rear cornering traction. But what we find on the independent 1/10 scale RC car is that cornering traction is reduced. This is known world wide. I had a thread on just this subject. For this reason I suspect there is no chassis lift. The reason I think is the suspension joints are primitive. The full size cars have lots of mass and low friction spherical joints on the rear suspesion. Our cars have simple pivots with considerable stiction. The rear gets stiffer on power, but there is no evidence of chassis lift; no extra rear cornering power is generated, it is actually lost.

When ride height is increased there would be more rear weight transfer. My pan car depends on this as does the 2 wheel drive buggy I spoke of. Longitudinal weight transfer depends only on g force center of gravity height and wheelbase. More below.

Now my home built 3 link rear suspension pan car is a completely different story. The leading links on the rear axle are half the length of the cars wheelbase, almost. which leads to low pivot friction. The rear has a lot of leverage on the pivot, so stiction is low. The car is a solid axle design. There is considerable extra forward bite added with antisquat. This is good evidence of rear chassis lift. The rear cornering traction is not decreased on power, this is good evidence of extra rear loading. You don't see any of this on our touring cars. The pic shows the long links on the 3 link pan car prototype.

I use way more than 2300 mA-h even on my 2 wheel drive pan car. I have averaged 38 amps sometimes (4100mA-h). This touring car is sure to suck even more power. Li Ion Nanophosphate packs are only a good substitute for 5 cell. I am running a six cell equivalent LiPo at the moment outdoors.

When I put my speed control up higher I made a compromise. I deciced, by experience, that I can drive a mod TC faster when it accelerates straight by having good side to side balance than I can with a badly unbalanced car that has all the weight low. There is no problem to be solved here.

John

mattnin

iTrader: (44)

What charger are you going to use for your A123 batteries? I am going to sell an ICE and try to either get the BC8 or Balancepro, although I am wary buying another charger because it seems like with the various LIFEPO4 batteries, they all require different CCCV cutoff voltages. I would really like to find a variable CCCV charger if one exists.

John Stranahan

iTrader: (27)

mattnin-Thanks for fixing the thread width. That quote box will do that but only occasionally now for some reason.

I plan to get this 123 charger but the test is a few months away. I think you need a 123 charger to get fast charge times. This one goes to 10 amps max. Seems to be nonadjustable.

http://www.horizonhobby.com/Products/Default.aspx?ProdID=AQR400088

Here is a .PDF FAQ on those batts. Charge time seems to be in the 15 to possibly 30 minute range for the 4600 mA-H pack.

http://www.horizonhobby.com/ProdInfo/Files/A123FAQs.pdf

Again the voltage is lower than a six cell pack. Should be a good replacement for a 5 cell pack.

John

elvo

The only way to find out for sure would be to mount diff thrust bearings on the hinge pins of a touring car


Personally, I think the independant suspension is much more of a factor than the friction.
On an independant suspension car (TC), anti-squat loads up the outside rear mostly = bigger load difference inside vs. outside tire = bad for cornering. (See magnificent graph)
On a beam axle car (John Stranahan Special pictured above), anti-squat loads up the rear, period. The rear axle in its entirety. Both rear wheels are loaded pretty evenly, and just plain more = more traction.


Anti-squat is always a bit unpredictable; it's always a matter of balance. Even more so in off-road! You balance loading up the rear versus stiffening up the rear suspension. One is good for traction, the other is bad. Bumps, friction, turn radius, amount of power available, friction graph,... they all influence the equation.

BTW, sometimes, in off-road, when running the hard spike type of tires, I get the impression that the traction graph curves UP instead of down, because of the spikes digging into the soil. Then anti-squat is even more messed up :-)

John Stranahan

iTrader: (27)

elvo-Thanks for the post. I put in the qualifyer, pavement, for just your spiked tire effect. The full size car books do notice that on loose dirt sometimes this lateral weight transfer is good. In all of my offroad racing I never experienced this and always set up the truck just like on pavement. My surfaces were never truly soft though. Most often a thin layer of loose dirt with hard pack underneath.
John

elvo

Try a buggy next time, truck tires are so huge that you can never get enough weight per spike for it to dig in properly.

John Stranahan

iTrader: (27)

I had two buggies. On 1/10 scale. One 1/8 scale. Never had the loose (fluffy) dirt to race them on. Even the topsoil track was hard packed under a loose layer. I have seen two full size trucks traction roll on Padre Island from this very effect, however, on very loose sand. They did not look quite as nice after the roll. It was easy to see out the front though, as there was no glass anymore.
John

J@UNE

iTrader: (1)

John,

Quick question, If I drop a camber link 2mm on both end, will that affect roll center or handling? Sorry I'm still a page behind

Thanks in advance.

Jon Gagnon

John Stranahan

iTrader: (27)

Jon - When you drop both ends of a camber link you are making opposite moves to the roll center. Lowering the outside lowers ther roll center. Lowering the inside raises the roll center. The handling may change a little as the moves will not exactly cancell each other. I put a little chart showing the effect on roll center and handling in a pic of a chart a few posts back. Better to do one end at a time and limit moves to no more than about .030 inch at a time. I mentioned this before that the chart applies only to front ball diff or front one-way equipped cars.
John

J@UNE

iTrader: (1)

Thanks a bunch