Will do. Thanks


Its a ball diff and yes could use a rebuild, but I got two gear diff I want to try. Is 10k ideal to start?

I've only run ball diffs on my SC10. I've got a gear diff that should be showing up this evening. From what I have seen you are probably going to be closer to 5-7K. 10K would probably feel quite a bit like you are talking about depending on the grip level.

grip was up the wazzo. track was sealed with wood glue so mad traction, we ran slicks or bald m3-4 tires!! but now that the rain and cold weather is here its going to be teared down and ran wet. it was not being ran wet due to high temps during summer (90-110*) so watering was useless.

If you don't have a chart for your pistons, this may help.

http://www.rctech.net/forum/6513442-post20.html

Fred I have loved your posts on this thread. You are certainly unconventional and usually spot on; however, wouldn't putting thicker oil in the dif of a 2WD car give you more steering (at least that is my experience with 4WD cars)? Your hose test has certainly given just about all of us something to think about because I don't know anyone who has been doing that to the rear shocks. While I like the factory set ups as a reasonable place to START, especially if I know the track that setup is for, I've only seen a handful of setup sheets that didn't use the same piston in both the front and rear shocks. BTW I found an equation in Tune to Win by Carroll Smith for calculating the Wheel rate (page 66). First the motion ratio is defined as the wheel travel divided by the spring travel (both of which can be measured on an RC car). The equation is wheel rate = Spring rate/(Motion ratio^2) (^2 is Excel speak for squared). After looking at the wheel rate equation, your spring balancing approach looks like you are in effect balancing the wheel rates. Using the equation and a little algebra, if you know the wheel rate at the rear of the car, and the motion ratio of the front, you can calculate the spring rated needed for the front in order to have a balanced F/R wheel rate. Haven't tried it, but it should eliminate some of the trial and error.

If you ran a completely locked diff as in no diff at all, you'd have serious understeer since the car would want to go straight. This is an extreme of course but tightening the diff in essence takes you closer and closer to this point.

Thanks for the equation,I have a question. RC shocks aren't vertical like in 1/1 cars are,does that equation still applies to RC?

Don't think of shock orientation in relation to the ground but rather the arms. No shock is always at a straight 90 degree relationship to the arm. It constantly changes. I personally try to set mine up so the orientation is the same front to rear. Using my 210 as an example, I have the front shocks at the inner lower position and the upper outer while the rear is on the lower outer but the top is on the next to outer upper. I try to set mine up so that when the suspension is fully compressed, the orientation of the shock to the arms is as close to 90 degrees as possible as seen if you drew a line between the inner and outer hinge pins. The Durango gets close to this but not quite. I do this because the spring gets the most leverage on the arm as it approaches full compression which is where I believe it is needed the most. If we stood the shock up more so that the arms were 90 degrees to the shock at ride height, we'd get a more even amount of leverage on the arm from the spring throughout the suspension movement with the most leverage being at static ride height. Hopefully I'm being clear about this.

At the beginning of the thread I mentioned that you should move the locations of the shocks until you found balance. Moving the bottom of the shocks outwards gives more leverage on the springs and moving the tops fine tunes it with a smaller effect. That was when I was using standard bore shocks with limited spring rates and that was certainly a better way than just going off of a random tuning guide such as Hudy that suggests you add spring rate to decrease steering, etc. That's terrible advice btw. Now that I am using big bores and with the large selection of spring rates, it is not much easier to place the shocks where I want them to be and to then match the spring rate around their location to achieve balance. Same end goal, different way of doing it. A better way.

What happens if I get to a point where I can't quite balance the chosen shock locations with the available spring rates? Since there are so many you should be able to get close however this is where I consider added weight to be a fine tuning tool rather than a standard tuning method. If I get to the point where the front rate is slightly high, I'll add a small amount of weight up front to balance it. In the case of my 210, this is as easy as the bolt on weight location under the servo. I never add unnecessary weight without a purpose. Adding weight "to increase steering" is also bad advice and actually counter to what shifting weight forwards does.

This answer was quite a bit more than you were looking for.

There is a traction breaking point that determines if you will push or hook with a tight diff, If traction is uber high (carpet) If you tighten the diff it will push badly, But If you are on Med/low grip surface you will notice a tight diff will rotate the car around (have you ever driven a posi posi rear end in a 1:1 in a wet parking lot) b/c of tire slip

I was thinking that you would get less leverage on the shock. My reasoning is that if I had a, lets say, five foot long board that was hinged at one end and placed a ten pound weight at the other end and tried to lift the board up then it would be very difficult but as I slid the weight closer to the hinge pin it would become easier because of the increased leverage (?).

I'm not real sure he this relates to having the shock more or less vertical to the arm?

As for your comment, moving the bottom inwards or outwards on the arm does change the leverage. That's why you change spring rates to adjust. Once you understand that a stiffer spring is needed on a shock mounted closer in to maintain the same wheel rate as a shock that is mounted further out, actual shock location really becomes irrelevant as long as you have the spring rate you need. We really don't need long rear shocks with shorter front shocks. If we mounted the rear shocks at the same location relative to the hinge pins as the fronts do, we could have the same shorter shocks at the rear. We'd just need a stiffer spring to compensate. This would actually be a preferable scenario since we could get the rear center of gravity down and get the rear shock tower out from in front of the rear wing.

I was just questioning where you said " Moving the bottom of the shocks outboard gives more leverage on the spring .... "

Hideeho
You have the right idea with the board, but your moving the wrong part.

5' board hinged on 1 end with a 10# weight fixed to the other end & a rope tied to it to lift it

There are 2 things you can look at with this.

1st pulling the board straight up with the rope tied 6" from the hinge vs tied 6" from the weight. It will take more effort to lift the weight on the hinge end.

2nd tie the rope 6" from the weight but pull it from 6" above the hinge vs pulling it from directly above the knot. It will take more effort to lift the weight pulling from 6" above the hinge than from directly above the knot.

This is a direct comparison to the way a shock works except in reverse. Instead of a weight we are trying to pull up we have a wheel we are trying to push down. Instead of a rope pulling up we have a pring pushing down. The principles of the forces involved are the same.

Funny that Fred and I were both thinking of the same live axel example and apparently came to different conclusions based on our observations at the tracks we run. I was thinking of the inside rear tire slippage (because of the shorter inside turning radius) with a live axel losing traction in a turn, especially on tight turns on low grip dirt surfaces (my off road experience). I have limited experience with dirt off road and oil-filled diffs, but the elite 1/12th scale on roaders I know spend about the same amount of time building their ball diffs as most of us spend building our entire car. I try to build the rear diff as free as possible to help keep the back end planted in a turn and make the car steer with other adjustments. Just the way I do it from years of oval and on-road racing. A few weeks ago I was in Virginia and my B4.2 with an oil filled gear dif (I know most people run a ball diff but adjusting the slipper clutch and the ball diff is one adjustment too many for this old racer) was pushing like a pig on a very (almost uber like) high grip track at Debbie's RC World. I changed a lot of things that week but I never thought of putting heaver oil in the diff - we are all slaves to our past!

Once again I focused on the wrong suspension part, I forgot the semi circular motion the arm(even a live axle,McPherson...)does results in a progressive shock,rendering all shock action progressive. That's why the RC10B3 tuning manual mentions shock progressiveness change with different shock tower holes. I see the whole picture now.
This goes in the direction I was thinking lately,means to control roll resistance at different suspension loads. There's a bumpy sweeper in my home track, if I tune for the rest 95% of the track I lost too much time there and vice-versa. Learned to break a problem into smaller ones so I thought about increasing rebound and/or tapered pistons but they affect the whole suspension stroke and progressive springs aren't available in a wide variety. So now that I can tune shock progressiveness I can tune a shock for 95% of the track where the suspension is not as loaded as in the sweeper and then tune more stiffness has the shock compresses. Brilliant stuff and thanks it's clear and 100% understood.

This can come in handy