Project Rebellion MMx

[fredswain]

Feel free to skip ahead to the pics if tech info bores you. This is a background on what I've learned and has led me to this design and is going to be quite lengthy.

A couple of years ago I bought my first mid motor car, a CR2 which is a mid motor conversion for the XXX-Cr from Atomic Carbon. I have always believed that weight within the wheelbase is the superior way of doing things, even on a 2wd buggy. That car was very short with a 10-1/2" wheelbase. When I bought it, it had the standard XXX 3 gear setup with an additional idler gear between the pinion and the spur gear so the motor could rotate the correct direction. From a handling perspective on the street I noticed that I could slalom that car at a noticeably faster rate than my rear motor cars. There were some troubling characteristics of that car though. First when on power the car would wheelie at an alarmingly easy rate. I couldn't keep the front end down. I swear my rear motor cars would out accelerate it without lifting the wheels. During braking the car didn't stop as well. The rear wheels locked up easily. I attributed these traits to setup and the fact that concrete wasn't the best test for off road cars.

Off to the track. I took the car to the local clay track. People said that mid motor cars can't hook up well on clay. I thought that any track that was so hard that foams would nearly work should have plenty of traction for a mid motor car. Once again pulling wheelies was not problem. The issues all happened in corners. On hard braking for turn in, the rear wheels would lock up easily and the rear of the car would slide around and I'd spin out while the rear motor guys went happily by. I tried dialing the braking back heavily on the esc and by the time I got it where the wheels wouldn't lock, I had very little braking power left. The rear end would still come around easily going into turns though. The solution was to throttle it hard to shift weight back. However now the front wheels were unloaded as the car tried to backflip and it didn't steer well. Mid motor was definitely looking like a failure on clay. I thought about it for a while and determined that I was shifting far too much weight back and forth from the motor. With the 3 gear and external idler setup, my spur gear and slipper also rotated in the same direction as the motor and wheels. I decided to get a 4 gear setup to offset this. I bought a Gurnell 4 gear box and tried everything again. To my horror things were no better. I couldn't make this car work on anything by high grip. You could drive very creatively to get the car around but you were still slower than rear motor.

I was talking to Chazz Sinclair one day about my mid motor troubles and he said that my problem was my car and not mid motor or the 4 gear and that I should try the X6. So I did. He said their car had a suspension designed for mid motor while my car didn't. I didn't see how this would fix my problems but I gave it a go. The X6 was definitely better. It did have more rear grip in the corners but still suffered from excessive weight shift from the motor rotation being the same as the wheels. He explained that this is necessary to get forward grip in a mid motor car. I decided to try their old 3 gear setup. He tried hard to talk me out of it since the injection mold quality on that case was subpar. The gears were quite noisy in it. I learned something neat from this setup though. With the motor turning opposite of the rear wheels I no longer had the low braking power, the on power corner exit understeer, or the tendency to wheelie. It did however have a hard time initially getting going and putting the power down during acceleration. That was the trait the 4 gear was clearly trying to solve but to me it creates other issues that I'm not happy with. I ended up running 0 antisquat and things got a little better but still weren't good enough. On carpet the 3 gear setup would probably be awesome. I still had the rear end coming around easily during corner entry though. Mid motor wasn't working out all that well. Why though? It should.

I started playing around. I started weighing cars and found that most rear motor buggies have 67%-70% weight on the rear wheels whereas the mid motor cars had less. The CR2 had 63% and the X6 had 65%. The X6 was a little better in the corners with side bite but not as good as the rear motor cars. Could static weight be the key? I went back to my CR2 and decided to lengthen the wheelbase from 10-1/2" to 11". I bought a piece of carbon fiber and made my own chassis. I got rid of the saddle pack battery configuration and went to a stick pack across the back like the X6. I was not running any shorties at the time. When I weighed the car in this configuration I now had 68% weight on the rear wheels. Back to the track. The tendency for the rear end to lose grip and slide out during off power corner entry was gone! The side grip was where my rear motor buggies were. It was immediately faster around the track. The extreme weight shift from the 4 gear was still wreaking havoc on control though. Braking power did increase as well. It made sense. Add weight to the only wheels that are braking. I realized that the key to mid motor working everywhere, even on low grip tracks, has everything to do with static weight distribution while control has everything to do with artificial weight transfer.

While searching randomly online one evening I came across the Team Azarashi Greyseal conversion for the B44.1. It moved the motor rearwards and ran a shorty lipo opposite of the motor. Since this car has the motor mounted sideways in the chassis, it doesn't transfer weight artificially from rotational intertia. It's inertia can be countered with proper balancing of the spur, slipper, and driveshaft rotation which are opposite. Bingo! That was it. I don't want any artificial weight shift front to rear from the drivetrain if possible. Even a 3 gear setup will shift weight forwards a bit during acceleration and rearwards during deceleration so I didn't want that. Now how do I build that with a 2WD front end? It was within a couple of days that I saw pictures of the Team C TM2. That car had never caught my attention before for some reason but once I saw the pictures I realized that someone else had figured it out. They did the exact same layout that I was thinking about. A local driver here has one and I asked him about it. He stated that he can't get his to hookup on clay. Other people have said the same thing about it. Weird. Is mid motor really relegated to the realm of higher grip? I wondered what the weight distribution on that car was? The long wheelbase should help shift some of that weight rearwards but I didn't know and still don't have an answer.

After studying pictures and his car and comparing it to the pictures of the Azarashi Greyseal I noticed that the Team C car has the motor and battery farther forwards than the equivalent location on the Greyseal. What if I build a car with a B44 rear end with the motor flipped around and then placed the motor and battery as rearwards as possible like the Greyseal? Then potentially run the small B44 add on weight between the battery and motor if necessary. It is still within the wheelbase. The weight bias would be more rearwards than the TM2 and weight on the drive wheels is what matters not where the weight is located in relation to the wheels. That was the rear half of the car. The chassis would be made by me. What about the front end though?

I have a B4 front end on my X6! I should use it. I parted out that car and sold off everything except the B4 front end. I also sold the CR2 and used the money to buy a used B44.1 on ebay so now I had most of my car in front of me. The problem was that I needed to make a chassis with a front kick. No problem. I make a chassis and shock towers on my pin router. I'll just order up a blank with a 25 degree front kick. Oh wait. You can't get one with a 25 degree front kick here. Only 30. That was going to create other issues so that idea was out. How about buying the X-Factory nose from the original X6 which is now being used on the cubed? That gives you the kick and bolts onto a flat chassis. Sounds like a solution.

I was going to order it until a couple nights ago when I listened to the Cliff Lett podcast on radio impound. He was talking about the old World's Stealth car prototypes and how they were the prototypes that taught them equal arm lengths and where to mount the shocks and that the B3 and B4 were influenced by them. I have a complete B3 front end. Why did I never compare it to the B4? It turns out the arms are the exact same length pin to pin and the shocks mount in the exact same locations. The B44 rear arms are also the same pin to pin as the B3 and B4 rear arms which are the same length as the front arms. Perfect. It all matched and I already have the front kick plate for the B3. It is now going to be the front end while the B44 is the rear end. I am using the B4 steering hubs and carriers though instead of the B3 pieces.

Yesterday I started laying out the car. The first thing I made was the new top plate that locates the motor position. The original B44 top plates are not the correct length since I rotated the motor around. I used the originals as a template on the router and punched out this new plate of the proper length. I would have made the chassis yesterday too but I ran out of material from making shock towers so more is on order. I bolted it all together on top of the stock B44.1 chassis just to look at it. The chassis will not work and I need to build a new one but the layout is clearly evident at this point so kindly ignore the chassis. It won't even be shaped like this. I think this is going to work. The question is, after I get done building it and applying all that I have learned from my other mid motor cars to this one, will it handle as well as I think it will on loose tracks? Time will tell but if my experiences to this point have been accurate then I believe it should.

My main motivation for this project is the fact that I hate modern blue groove track design but we have so many of them here. I've always hesitated to race on them but may as well accept that they are here and where the racing is right now. Why not build a car to excel on it? My vintage cars don't do so well and are always breaking on them and the handling is less than spectacular. I love them on our dirt track though!

Since our clay track is humongous, this car is going to push the ROAR maximum wheelbase length. I am setting the overall wheelbase at 11-1/2". That's practically a limo but with our long fast straights and huge jumps I believe the added length will have a stability advantage over most of the track than a shorter wheelbase. We also have a new indoor carpet track about to open. It isn't as fast or large and I am going to do an alternative version of this car later specifically for it.

As it sits right now, the driveshaft from the slipper to the diff is the standard rear driveshaft of the B44. The front shaft is longer. On the alternate chassis layout I am going to use it instead which will shift the motor and battery forwards about 3/4". I am also going to shorten the wheelbase on that variant to 11". That car will have a much more central weight distribution for carpet and a much more rear biased one for clay. I actually suspect that although the long wheelbase variation should technically not take tight corners as well as a shorter version, that the weight within the wheelbase will more than aid in offsetting this compared to rear motor cars thanks to better balance. It should just be able to tackle the corners harder.

To me this is the future of off road racing. I think there is a mid motor setup for every track surface. It is just all about identifying the true problems and then dealing with them which is what I'm trying to do. I don't believe in the convertible car from rear to mid motor. What I see as the future is one car for 2wd and 4wd where the only thing you swap is the front ends. The chassis and rear half of the car could be shared. That would just be sound financial practice for a company too since you truly only have 1 car.

Since I don't agree with most people on practically anything and am a bit rebellious, I present to you my new car I am naming the Rebellion MMx.

I'll update it as I go.

[fredswain]

I now have both of the top decks done. The short one will go on the dirt spec version and the long one will go on the carpet spec version. The dirt spec car places the motor and a shorty lipo as far rearwards as possible. That version of the car will have a long 11-1/2" wheelbase. The carpet spec version moves the motor and shorty lipo forwards 3/4". In addition the wheelbase on that version car will be a shorter 11". I need to redo the longer plate. I didn't get the master part taped onto the material as well as I should have and it flexed around the spur gear holes resulting in the material being a bit too thin around them. I'll fix this in the next version. I consider the first parts out of G10 to be prototypes anyways. The final versions will be woven carbon fiber.

[fredswain]

This shot clearly shows the length difference. The carpet spec top plate is installed but the dirt spec driveshaft is shown. It is clearly too short. The normal front driveshaft from the B44 is to be used with this top plate. The driveshaft shown is the rear piece from the B44.

[dex210Nick]

Absolutely brilliant! I can't wait to see it all finished up. It's great to see the old b3 front end put to good use, too. Although in my experience, the old b/t3 front ends don't give you much in the way of adjustment. You only have 2 horizontally placed ball studs locations on the front tower which doesn't allow you to raise or lower your RC.

I raced my old t3 a couple times this summer after driving my mm4 dex210 for the winter and wished desperately I could convert it to mid motor. I may have to try something like this with my t3 now. curse you for planting ideas in my head!

[fredswain]

The B3 and B4 arms are the exact same length. The shocks even mount at the exact same locations. With the exception of the different front kicks between the cars and some minor details, AE has retained the same basic suspension geometry since 1997 and even it much of it was derived from the world's Stealth cars of 1989 and 1991. The rear arms and shock mount points of the B44 are even the same as the B4.

I am running the B4 steering hubs so I've got more adjustment than the lone spot on the B3. I've got 2 inboard and either 2 or 3 outer points depending on the hubs I run. You don't really need all that much adjustment anyways. Most cars have far more than anyone ever uses.