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RC10B4/T4 Forum

Old 05-21-2009, 02:45 PM
  #7786  
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Originally Posted by richard.bratton View Post
This is correct... Adding washers will make the angle between the A-arm and the camber link greater. This pushes the roll center further below ground and makes the distance between the center of gravity (CG) and roll center greater. This distance increase equates to a larger moment arm allowing corning forces to increase around the roll center. As a result, initial roll of the vehicle is greater as the resistance to rolling over is less. The attached picture is an simplification of how to calculate roll center. Basically you draw an imaginary line extending the camber link and A-arm until they intersect and another line from this intersection, to the center of the tire contact patch until this line intersects with the same line on the opposite side of the vehicle. This intersection is the instantaneous roll center. The roll center will change as suspension movement occurs. The a-arm/camber link intersection could be on either side of the tire and is obviously setup dependent. Regardless of the side, you calculate roll center using this same method.
Sorry, but that's not the correct way to find roll center. You're tracing the lines the wrong way and in your setup the roll center is crazy high, not low. Everything else you said is right though, just try a picture like this:
http://www.mitchellsoftware.com/ForceB1.jpg
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Old 05-21-2009, 05:28 PM
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Wow thanks for trying to explain guys. I don't know why it seems to loose me when ever truck geometry is explained. Im not following the second drawing is their a description of the lines (what they are represeinting)?
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Old 05-21-2009, 05:53 PM
  #7788  
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Originally Posted by oldandnew View Post
Wow thanks for trying to explain guys. I don't know why it seems to loose me when ever truck geometry is explained. Im not following the second drawing is their a description of the lines (what they are represeinting)?

You would really need a PHD to understand all those suspension geometry thing

The best approach is to spend a lot of time on the track to test the changes. Sometimes, you will get opposite effect from theory because no changes is 100% isolated.
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Old 05-21-2009, 06:34 PM
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Originally Posted by PartTime View Post
I didnt think this was gona be an easy question. lol. DK
Dont worry, youre not alone in asking this - roll centres are probably the most mis-understood thing in setup, even in full-size racing circles.

The important thing to know is what the roll centre actually means. When you are cornering, the side force from the tyre is directed to the chassis via the suspension links (the 'geometry'). Basically, the roll centre is the point in space at which the tyre force 'points to'. If the roll centre is high, when you turn, the side force from the outer wheel will point upwards and this actually pushes the chassis up, reducing roll angle. If the roll centre is below ground, the side force will push the chassis downwards, increasing roll angle. When the roll centre is at ground level, there is no up or down force applied to teh chassis from the geometry.

So in mid corner, a low roll centre acts like having a softer spring at that end of the car (ie more grip at that end) while a higher roll centre acts like a siffer spring at that end (ie less grip at that end). But thats only part of the story.

What makes roll centres so important is that they act immeduiately when you turn. When you first turn the wheel, the front tyres develop a slip angle and build up grip very quickly. This means that the roll centre force acts almost immediately. The chassis then starts to yaw, which creates a slip at the rear, so the rear tyre force builds up an instant later, meaning the front roll centre acts first then the rear roll centre. The chassis is starting to roll, so the dampers are starting to work, but dampers provide more force as they move faster, so they have a delayed action from the roll centres. Finally, the springs start to have an effect as their force is proportional to displacement, so it takes time for them to build up a significant force (you dont have instant displacement). As you exit the corner, these things happen somewhat in reverse. (this is obviously a simplification, but should give you the general idea).

So basically the roll centre allows you to tune the way that the vertical tyre force builds up through the turn. A high roll centre makes the force build up quickly, which can be good for low grip tracks where you need the tyre to bite; a low roll centre makes the force buildup more progressive, which is more useful for high grip tracks where you need the car to be a bit less responsive.

If we look at a high front roll centre, when we first turn the wheel, the outer front tyre will have a very fast build up of vertical force (it is trying to push the chassis up which in turn plants the tyre into the ground). This can make the front of the car feel very responsive, but once the car is in the middle of the corner, it will add some weight transfer at the front and take away a bit of front grip. A low front roll centre will tend to make the car less responsive to initial steering inputs (as the rate of force buildup is slower) but in mid corner provides more front grip.

Both the height of the ballstud and the length of the arm are used to adjust roll centre. If we raise the inner ballstud, we lower the roll centre (see diagrams below) and vice versa. The length of the upper arm affects how the roll centre changes as the suspension compresses / extends. (As the suspension moves, the roll centre moves up and down as well - it is an unavoidable fact of independent suspensions.) A longer upper arm keeps the roll centre lower as the suspension compresses, while a short upper arm keeps the roll centre higher as the suspension compresses. As Wild Cherry pointed out, the ballstud height is less sensitive than the length of the arm, but it varies a little from setup to setup. It also acts in a slightly different part of the corner.

To answer your question about the rear - adding a washer to the rear ballstud will lower the roll centre, which will give more rear traction. This is most noticable when applying power to the car as you are exiting the corner.

Sorry for the long post, but its not an easy subject to give a 5 line answer to!


Originally Posted by Krio View Post
Sorry, but that's not the correct way to find roll center. You're tracing the lines the wrong way and in your setup the roll center is crazy high, not low. Everything else you said is right though, just try a picture like this:
http://www.mitchellsoftware.com/ForceB1.jpg
actually, richards diagram is correct - it just shows a geometry that is not usually shown for full size cars (ie the upper links pointing upwards at the inner end of the car). Having this layout gives a very low roll centre as shown, but can also make the camber become positive in compression. This is a big no-no for full size cars, but our tyres are much less sensitive to camber and much more sensitive to roll centre. unning 2 or more washers on the front B4 ballstud gives you a similar geometry.

Ray

Last edited by ray_munday; 05-21-2009 at 11:21 PM.
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Old 05-21-2009, 08:27 PM
  #7790  
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Ft stud
more spacers = smoother

Rear stud
more spacers = more side traction

All that roll center stuff just confuses me......
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Old 05-21-2009, 09:23 PM
  #7791  
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Great information Ray. I'm with Cherry on this one. These in depth details sometimes make my eyes go crossed and I forget where I am for a few minutes.
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Old 05-22-2009, 07:30 AM
  #7792  
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Wow great description. Thanks I think I need to keep it easy for now. So if I might ask again? If you add washers to the bottom of the inner ball studs (the ones for castor in the front of the car). You lower the roll center witch allows the front end to slide a little bit at the start of a turn. And if you do the same in the rear of the car will actually adds grip (just the opposite as the front) and make it resist sliding more. Thanks for dealing with the old and stupid.
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Old 05-22-2009, 07:43 AM
  #7793  
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What would be involved in converting a T4 into a B4? Besides new tires and stickers?
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Old 05-22-2009, 07:53 AM
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Ok, the easiest way to get roll centers straight in my head is to think of them as camber gain. Add a washer and you get less camber gain. With that in mind, more camber gain should give less side bite and more forward traction. Less camber gain should give more side bite and less forward.

I race on carpet and clay. The funny thing is when Im on carpet and want to take some steering out I add a washer to the front. But when on clay and want to settle down the front I take a washer out. Ugggg. Starting to think that when on carpet I feel the camber gain before the chassis rolls over and on clay I only feel it when its leaned over.
Clear as mud??

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Old 05-22-2009, 07:53 AM
  #7795  
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Originally Posted by Fishn or Racin View Post
What would be involved in converting a T4 into a B4? Besides new tires and stickers?
Well, to be honest, it would only cost a little more to get another B4 but here I go.

Chassis
front a arms
rear a arms
front shock tower
rear shock tower
front shocks
wheels
tires
turnbuckles
body

This is just about everything, I recken that this will all cost about 160-180$. Like i said, better to buy a new one. Or you can get a roller on this site for about 130$. Most people will trade your t4 bor b4 usaully.
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Old 05-23-2009, 02:22 AM
  #7796  
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Ok i'v been stuffing around with the b4 tring to learn somthing about its suspension.

Any tuning tips for small jumps doubles and triples.

What will stop the nose diving? (I'v got no drag break)

What will reduce the chances of crashing if i dont make a double and land on top of the 2nd jump?

I have just moved the shocks to the inner hole on the front shock tower, i needed to add some preload to keep the front level. anyway now it jumps alot higher. but on big jumps to nose drops.

I seem to crash less on doubles or triples now but that may be besause its jumping higher now.
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Old 05-23-2009, 05:03 AM
  #7797  
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Originally Posted by mtsb4 View Post
Ok i'v been stuffing around with the b4 tring to learn somthing about its suspension.

Any tuning tips for small jumps doubles and triples.

What will stop the nose diving? (I'v got no drag break)

What will reduce the chances of crashing if i dont make a double and land on top of the 2nd jump?

I have just moved the shocks to the inner hole on the front shock tower, i needed to add some preload to keep the front level. anyway now it jumps alot higher. but on big jumps to nose drops.

I seem to crash less on doubles or triples now but that may be besause its jumping higher now.

as long as you have the standard setup on the car and the ride height set correctly then the only thing you need to do is practice practice and practice

you will learn that you need to have the car squared up in relation to the face and how to approach it with the correct throttle.

one good way of learning is watch the sx bikes on tv and listen to the engine as they approach the jumps you can hear them blipping and adjusting the bike and its the same theory for rc cars. blip the throttle mid air get the nose up and tap the brakes to bring it down.

for your triple you will be more consistent if you double single first off. get that right 50 out of 50 times and then start the triple. they are usually more critical because you need to punch it harder and land the down ramp to make I presume a corner not far after....

have fun
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Old 05-23-2009, 07:05 AM
  #7798  
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Thanks guys for alot of great info (thats alot of typing). I am off to track to try it out. Its the weekend... lets race...
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Old 05-23-2009, 08:43 AM
  #7799  
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once you learn your basic up and down pitching for jumps. you can also learn to pitch the car sideways to help set up for corners right after the landing by turning the wheel on the face of the jump right before you leave the face of the jump.
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Old 05-23-2009, 01:17 PM
  #7800  
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Anyone know how much weight the pros are adding to a t4 when using a lipo? I am going to be running a 6000mah lipo and need to know how much weight to start with.
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