Project Rebellion MMx (RC10B3/4/44.1.2 is far too long!)

[fredswain]

Fred, love the idea, but a question. Why move the motor that far forward at all? Think of the TC3/4 and original BJ4. The spur gear was just outside the rear diff housing & motor immediately afterwards. Doesn't that layout give you the best reward weight bias?

If this works, be prepared for many copies

I generally don't agree with most people hence the name "Rebellion" for this project. Many people state that more forward weight bias leads to more steering. The opposite is actually true all things being equal. I thought that I may as well try a more forward weight bias for carpet while I'm at it. In reality I will probably try a half a dozen different variations before I settle on one design. I'll probably try the long wheelbase but with the forward motor, the short wheelbase with the rear motor, and then different wheelbases in between them with different motor locations. I figured that I may as well try exact opposites. I'm a tinkerer. I love trying things and learning from it. My suspension tuning method is quite straight forwards but very different than most other people's as well. It's that rebellion thing. There was a thread about it on rctech a while ago and many people starting doing it the same way. I'm applying everything I have learned into this car without the bias of having designed an rc car before. My profession is/was mechanical designer so that's what I'm doing here. It's just another project to reach an end goal by discovering and dealing with different issues in the process. I'd rather be proactive in design rather than completely retroactive solely in setup in this regards. Wait until you hear my ideas regarding trailing arms! I've got some interesting thoughts there as well.

This all may fail spectacularly. We'll see. I'm going to try everything I can think of in the process though. At the very least I'll learn something and by keeping an online record of my thought process and experiments, others will learn from it too. Even if that means they'll learn what not to do.

If this all works maybe I'll have to start a company Rebellion RC Products? That's a nice name!

[fredswain]

What material are you designing your templates out of?

Templates? Templates? We don't need no stinking templates!

I made these by using the stock B44 top plates. I made half of the part that I needed and then either moved or substituted the other top plate to finish the piece. I did need to do a little bit of hand filing to make the edges smooth.

Any parts that I intend to keep producing will have a full template machined out of 1/4" aluminum.

[JK Racing]

Sounds good, I will keep watching

Trailing arms - awesome. I love my 5 link JRX.

[fredswain]

I think I know how to get a car to lean into a corner with trailing arms but it's just a hypothesis right now. I'll test that later when I redesign the Bullet trailing arms. I wonder how well trailing arms would work on this car? Hmmm....

[Jay Dub]

Love the project! I too have come to many of the same conclusions about mid motor 2wd cars, and currently am working on a nearly identicle project. I however am using the B4 front end, and TC3/4 or Bj4 style rear end. When it is finished I will let everyone kow how it works. -Jeff

[fredswain]

I'll end up building a version with the B4 front end at some point. The decision to use the B3 only came about because I didn't need to buy 1 little part to adapt the B4 front. After it drives and I understand the handling characteristics, then I'll change it.

[JK Racing]

AE part #7892 may be an option instead of the X-Factory part

[fredswain]

Swept arm design has a bunch of issues on high bite

I don't believe that. Swept arms are necessary with front end kick. The more front end kick you have, the more the arms need to be swept back. Try this and you'll see why. Remove the wheels from one of your cars. Prop the car up on top of some wheels that are on their sides or on something that holds the car and arms completely off the ground. Now you need to find a way to get your front and rear arms completely level from inner to outer kingpin. This is a bit confusing on arms like the B44 and many other modern cars that aren't straight but the key is that if you drew an imaginary line through the inner and outer kingpins that it would be parallel with the ground. If you can do this, then measure the distance that the front and rear axles are off of the ground. You want these to be the same. With a car with a 30 degree kick, if you had a straight arm and the steering was centered on it, the front axle would be far higher than the rear from the ground. The solution is to sweep the arms back until they align again.

Take a look at the B4. It has a 25 degree front end kick. The B3 has a 30. The B3 has arms that are swept back a little further as a result and it has everything to do with the front kick. The B4 however allows you to space the front steering forwards or rearwards. However if you look at what Associated recommends on location you'll immediately see why they do. The state that with inline steering that you use the spacer in front with the steering blocks further rearwards. However with trailing steering they suggest to use the spacer in back with the steering hubs further forwards. This is because with trailing steering the axles are farther backwards and with a front kick this means lower as well. By moving their position based on whether or not it is inline vs trailing steering they are keeping the axle height relative to the ground the same. It is this reason why I disagree with people that suggest that spacing it is purely a tuning aid. The Durango 210 is very guilty of this. I look it from a very logical standpoint that you want the front and rear of a car to be as close to the same as possible. Since I am running a B3 front end, I'll never run trailing steering.

Losi figured this out with the JRX2. When the original car came out the front end kick was 20 degrees. Later on they came out with the Proformance kit which resulted in the JRX-Pro. They still had the same front arms though and this was a mistake since the front now had a 30 degree kick. They corrected that error with the Pro-SE which had rearward swept arms. Since the rear hubs on the SE had optional kingpin holes, high and low, if you really wanted balance you'd pick which front arms you were going to use based on which rear hole location you wanted. Of course no one ever made this connection but they did at least learn about the necessary front sweep.

My original reasoning for using the B3 front end as already stated had everything to do with the fact that I already have the nose while I don't have a nose for the B4 front end which I also have except for that part. I was originally going to make a chassis with a built in kick. I didn't realize that the B3 front had a 30 degree kick while the B4 only a 25 until your post. I looked at it again. Now I can actually do a full chassis version with the front end later if I want. Suddenly I've got a car that has more options yet to try. I'll be able to try the design with a 30 degree front kick and correct geometry for that kick as well as one with a 25 degree kick with options for inline and trailing steering with correct geometry so this car is getting even more fun. I have decided to focus solely on the rearward motor position and all of it's iterations before I complete the forward version. I have a hunch that no matter what I'm going to favor it rearwards.

What is far more likely and makes tons of sense to me is that a car with greater front kick does worse on high bite than one with less front kick. Conversely one with more kick, to a point of course, would do better on rougher surfaces than one with less kick. I don't believe the problem has anything to do with the arm rake itself though.

[Charlie don't surf]

I guess I was wondering more why you didn't use the GT2 nose, allowing the drop in of the B4 front end, with the best possible steering geometry and matching hinge pin and camberlink locations, given the technology that's riding in the rear half.

Back to the swept arms though, the B4-.2 uses a straight arm, with a 4mm spacer and trailing arms, which give you an effective sweep in the arm, without the dive issues related to swept arms. If you look at a B3 under straight line braking, the nose dives (x) degrees under braking....but, while its transitioning weight to the front end downward, its also throwing the car forward since, the kickup and the sweep allow the arms easy movement rearward, wasting time and energy under acceleration after braking for the car to move rearward, before moving forward. To add to this, going from a braking to left or right transition allows the outside front arm to stay in a rearward position, since its mechanical leverage is decreased (do a push up, with your arms square with your chest....now do one with your arms 8 inches further to your waist). These two issues alone cause your total chassis to take longer to balance after transitions, which means longer before being back on the throttle, and wasted energy and momentum. This is also present on jump faces, and downsides....same fundamentals.

[fredswain]

I think that has more to do with with difference in front end kick.

[Charlie don't surf]

I think that has more to do with with difference in front end kick.

5 degrees is 3mm lower/higher in the front of the rake, but a B3 arm sweeps what, 10..12mm rearward? Kickup is 100% part of the equation, but not that much.

I'd just hate to see your project fall short of your goals.

[THEYTOOKMYTHUMB]

[fredswain]

I think that has more to do with with difference in front end kick.

5 degrees is 3mm lower/higher in the front of the rake, but a B3 arm sweeps what, 10..12mm rearward? Kickup is 100% part of the equation, but not that much.

I'd just hate to see your project fall short of your goals.

Simple geometry points out why that's true.

I'm not sure how it can fall short of my goals when I'm trying multiple setups. Remember I do have a B4 front end and will try it at some point. There isn't a car out there from anyone that meets my goals so the worst thing that can happen is that it'll be no better than the standard.

[Charlie don't surf]

Good luck with your endeavors

[JHarris]

Have you ever actually raced? I'm curious what your real world experience is. Not what you read on some forum or what looks good on paper, but actually putting a car on a clay track with 9 other cars and a lap counting system. I would even settle for some kind of verifiable testing at a real off road track. I've seen you spew your theories here and rctech, and I just can't seem to find any experience to support anything you say! Humor me please!!!

Oh, your assessment of Duratrax wheel on rctech was great. The problem is that its not supported by any racer feedback. I have Duratrax wheels that are over 3 years old with at least 100 runs (on a real race track) that haven't yellowed or cracked. The durability issues you posted are realized by guys using them for their bashers, not in race trim.