Rear arm length

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Old 08-28-2015, 10:51 AM
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Default Rear arm length

I was wondering what the effects are of having longer rear arms (MP9, D815) vs a short one (MBX7, RC8B3). I'm assuming is has something to do with roll resistance... Need some techy guys here
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Old 08-28-2015, 11:55 AM
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Originally Posted by redbones View Post
I was wondering what the effects are of having longer rear arms (MP9, D815) vs a short one (MBX7, RC8B3). I'm assuming is has something to do with roll resistance... Need some techy guys here
Its not totally in the arm length. You have the hinge point of where the rear arm attaches to the chassis, how and where the hub carrier attachest to the arm and the length of the hub itself.

If you start moving pivot points around, you start changing roll centers, travel speeds and arcs...

Most buggies and trucks today are very similar in their demensions. Some , like you listed, use shorter or longer arms and compensate by using a longer or shorter hub and hub carrier.

A longer arm has to move at a faster speed to cover the same distance a shorter on does. But it allows the suspension to be used more finely thru the travel range. A shorter one doe the opposite. If you were to use a Kyosho shock ( set up with the kit settings) and attach it to a Losi, it would feel very stiff and over dampened. If you used a Losi on a Kyosho, it would feel soft and under dampened. Even though both shocks are close to each other in size and length, they are designed to work with their different vehicles because of the different designs.

Make sense...
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Old 08-28-2015, 03:31 PM
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Hope I'm not giving false information but it my brain doesn't mess with me again it should be:
-Short arm gives more roll because the roll center height becomes lower as it rolls and vice versa for long arm. That's why the more grip the surface has, the longer the arm should be.
Jerm, the Motion Ratio (that's like spring leverage) can be the same between a short and long arm, of course it will feel different because as you said the shorter arm has to travel faster and therefore will cause more pack at the shock with same oil and piston as the long arm.
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Old 08-28-2015, 06:12 PM
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Originally Posted by 30Tooth View Post
Hope I'm not giving false information but it my brain doesn't mess with me again it should be:
-Short arm gives more roll because the roll center height becomes lower as it rolls and vice versa for long arm. That's why the more grip the surface has, the longer the arm should be.
Jerm, the Motion Ratio (that's like spring leverage) can be the same between a short and long arm, of course it will feel different because as you said the shorter arm has to travel faster and therefore will cause more pack at the shock with same oil and piston as the long arm.
Wasn't talking about motion ratio at all. That brings up another whole topic of tuning.

The different cars are designed to work with their specific components. Set 5 buggies up next to each other and I bet all of them will be within a couple MM of each other in rear track width. Yes some arms are different length. Again, to achieve the same track width, they have wider hubs and carriers. So the overall length from inner pivot point to wheel center is about equal.
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Old 08-28-2015, 11:00 PM
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Originally Posted by 30Tooth View Post
Hope I'm not giving false information but it my brain doesn't mess with me again it should be:
-Short arm gives more roll because the roll center height becomes lower as it rolls and vice versa for long arm. That's why the more grip the surface has, the longer the arm should be.
Jerm, the Motion Ratio (that's like spring leverage) can be the same between a short and long arm, of course it will feel different because as you said the shorter arm has to travel faster and therefore will cause more pack at the shock with same oil and piston as the long arm.
You're right on the money. At least this is what I have always been taught.
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Old 08-29-2015, 03:28 AM
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Originally Posted by Jerm13 View Post
Wasn't talking about motion ratio at all. That brings up another whole topic of tuning.

The different cars are designed to work with their specific components. Set 5 buggies up next to each other and I bet all of them will be within a couple MM of each other in rear track width. Yes some arms are different length. Again, to achieve the same track width, they have wider hubs and carriers. So the overall length from inner pivot point to wheel center is about equal.
I think we are talking about different things, Motion Ratio is the ratio between arm length and shock mounting hole, you can have the same MR between a short and long arm.
Rear width is regulated by IFMAR rules to be less than 310mm, they all are as wide as they can be as it helps cornering power. What you are talking about is wheel offset, doesn't change Motion Ratio but helps short arms retain working range.

Also want to add that short arms move roll center more than long arms when working, promoting a twitchy car. Here's a thread that is more in-depth.

Last edited by 30Tooth; 08-29-2015 at 04:15 AM.
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Old 08-29-2015, 06:50 AM
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Originally Posted by 30Tooth View Post
I think we are talking about different things, Motion Ratio is the ratio between arm length and shock mounting hole, you can have the same MR between a short and long arm.
Rear width is regulated by IFMAR rules to be less than 310mm, they all are as wide as they can be as it helps cornering power. What you are talking about is wheel offset, doesn't change Motion Ratio but helps short arms retain working range.

Also want to add that short arms move roll center more than long arms when working, promoting a twitchy car. Here's a thread that is more in-depth.
I didn't even bring up motion ratio. That was you. Again, I was only taking about lengths. Shock mounting location, angle, size, shape blah blah blah is something totally different. And Wheel offset is another whole story as well.

So IFMAR say 310mm. Buggy A has a 250 mm arm, buggy B has a 200mm arm and buggy C has a 300mm arm. But all 3 have the same track width. How is this possible???? Its cause the mounting point of the rear arm, hub and carrier are different sizes. That's what I was saying. Nothing about shock mounting.

Now I did explain why one car's shocks shouldn't be used on another car. But again, it wasn't because of the mounting location, it was because of the internals...
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Old 08-29-2015, 08:10 AM
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Originally Posted by Jerm13 View Post
I didn't even bring up motion ratio. That was you. Again, I was only taking about lengths. Shock mounting location, angle, size, shape blah blah blah is something totally different. And Wheel offset is another whole story as well.

So IFMAR say 310mm. Buggy A has a 250 mm arm, buggy B has a 200mm arm and buggy C has a 300mm arm. But all 3 have the same track width. How is this possible???? Its cause the mounting point of the rear arm, hub and carrier are different sizes. That's what I was saying. Nothing about shock mounting.

Now I did explain why one car's shocks shouldn't be used on another car. But again, it wasn't because of the mounting location, it was because of the internals...
I bring out Motion Ratio because that's one of the most felt difference between short and long arms and can't be dissociated from the discussion. Can't replicate a short arm shock setup with a long arm and vice versa. I'm not saying you are wrong, just clarifying.

Now that's a bad example, the MBX7/RC8B3/XB8/TLR 8 are wide rear inner plates with low/zero offset hub carriers, unlike THE Car which is a high offset hub carrier medium width rear plates. You have both and they all fit short rear arm/legal width, the difference is where the lost distance to long arm is re-gained.
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Old 08-29-2015, 04:00 PM
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I don't like using technical terms for this subject because it's simply not relevant to most people. Here are my findings in layman's terms:

-Variables: Rear arm length and hub offset. Constant: Arm holder width.

A shorter arm with more hub offset will have more forward traction and the rear will rotate more in corners, making the car feel more aggressive. This is more suited for technical tracks, and the car is normally faster this way. A longer arm with less hub offset will feel a little calmer and more stable and will normally reward smoother drivers. On the JQ car we have this option, and we've found that the short arm is the best to base setups around.

-Variables: Rear arm length and arm holder width. Constant: Hub offset.

A shorter arm with a wider rear arm holder will make the car feel more aggressive, and vise versa will make the car have more traction and feel more stable. Mugen went from the latter to the former from the MBX6 to MBX7, and some drivers went back to the 6 rear end to get more rear grip.
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Old 10-15-2015, 12:44 AM
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More understandable language for mortals!
Still trying to understand how to achieve the described effect of the changes made. :-)

- What is "hub offset"? How do you achieve that?
- "Wider rear arm holder". Do you adjust this at the suspension blocks, where the hinge-pins of the lower arms are mounted?
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Old 10-15-2015, 11:04 PM
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Originally Posted by Mister-E View Post
More understandable language for mortals!
Still trying to understand how to achieve the described effect of the changes made. :-)

- What is "hub offset"? How do you achieve that?
- "Wider rear arm holder". Do you adjust this at the suspension blocks, where the hinge-pins of the lower arms are mounted?
Hub offset is the horizontal distance between the wheel hex and the outer hinge pin. It can be adjusted on the JQ White Edition and Xray '13 as well as the old Tekno car, but their short arm position is to extreme to be useful. Most cars don't have this option.

The only cars that have this rear arm holder option are the Mugen MBX7 (by using the MBX6 rear end) and the JQ, by using their old rear end. It's not something that is normally adjusted, but I simply responded to the question being asked. I believe the original poster was moreso asking about the differences between the cars, but obviously there's much more to each car's handling characteristics then rear end geometry. Hope this helps.
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Old 10-16-2015, 01:44 AM
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Originally Posted by redbones View Post
I was wondering what the effects are of having longer rear arms (MP9, D815) vs a short one (MBX7, RC8B3). I'm assuming is has something to do with roll resistance... Need some techy guys here
Funny you should say that.

In fact the MBX7 rear arms are longer than the MP9 rear arms.
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Old 10-16-2015, 02:23 AM
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Originally Posted by TRF415boy View Post
Funny you should say that.

In fact the MBX7 rear arms are longer than the MP9 rear arms.
That was unexpected!
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Old 10-16-2015, 03:42 AM
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Originally Posted by 30Tooth View Post
That was unexpected!
MP9 rear arm with insert to the outside (longest setting) 99mm
MBX7 rear arm 101mm

The MP9 has more rear grip because of the distance between pivots (and subsequently to get the same track width, the hubs have more offset), also the rear camber change is vastly different.
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Old 10-16-2015, 07:49 AM
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Originally Posted by TRF415boy View Post
MP9 rear arm with insert to the outside (longest setting) 99mm
MBX7 rear arm 101mm

The MP9 has more rear grip because of the distance between pivots (and subsequently to get the same track width, the hubs have more offset), also the rear camber change is vastly different.
Good to know the length, I expected the MP9 arms to be equal if not a bit longer.
Sorry didn't understand the part, does the MP9 has more traction because of more hub offset, closer pivots or camber change curve? Or all together?
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