The Rear Weight Bias Chassis

I'll be the first to admit, I was not a believer of rear weight bias RWD setups. I didn't like the idea of them. I didn't think that the performance gains over a conventional setup would be great enough to warrant the switch. I firmly believed that whatever a rear weight biased setup could do, I could tune a conventional setup to do. Afro has been running a diffuser mounted battery for about a year and a half now, maybe longer, I can't remember, but he slowly started converting members to start running their batters on their diffusers as well ever since. I was stubborn though. I didn't want to conform, and so I continued tuning my conventional setup to keep up with them. Slowly I started falling behind, no matter how well I drove, I was slower. So, during the process of building my second Doripake, I figured I may as well build it using a diffuser mounted battery, and see what all the hype was about...

It wasn't until I ran my newly finished Doripake that it clicked. This is quite possibly the ultimate setup when it comes to sheer performance, at least for our driving style and for our track layout. It was a pipe dream trying to tune my convention setup Doripake to keep up with this new breed of animal. I was like a pre-historic dinosaur in comparison, and I'm not putting that lightly... the level of performance you can gain out of your car by running a rear mounted battery on a RWD chassis is insane. At least at our track.

My initial opinions about rear weight setups was that it was going to have way too much understeer, that the rear end would snap way too hard, that it would have bad stability, and that it would be hard to catch the rear. I started out with a few tuning sessions in my driveway, about 30 minutes total, and quickly realized that everything I thought about a rear weight bias setup was wrong. I quickly came to the conclusion that I 'have' to try my car out at our local track, and get a real feel for just how good this chassis is in comparison to my other chassis. I already knew it was going to be better... but just how much better?

My rear weight bias chassis has minimal amounts of understeer, and while I admit that there is a little bit of understeer, being aggressive on the throttle makes it nearly non existent. The rear end is also firmly planted. The amount of rear end grip available while drifting is insane. You can throw your car into corners as hard as you like, and it feels like the tail will never come past the front end. I can't even begin to express how aggressive you can be with a rear weight bias setup, it's just crazy to me. You can put the pedal to the metal around most of the track, lifting occasionally to control your line, all while still maintaining the ability to run the walls. Even if I got it wrong, and was too aggressive, I could typically let off the throttle and the rear end would come back into line instead of going past the front end, allowing me to continue on drifting, laying into the power as went along.

So, basically what I learned during my visit to the track is that no matter how much I tuned my conventional setup, I was going to need to add weight to the rear diffuser in order to make it compete with this new breed of animal. Adding a ton of extra weights to a chassis is something I'm just not a fan of, but there's just no way around it, a rear weight bias chassis just offers so much grip and stability in comparison. In the exact same situation if I am pushing the absolute limits of my chassis running a conventional layout, my rear weight bias chassis is in a very comfortable zone, with performance to spare. What I've been getting at this whole time is that the rear weight bias layout currently reigns supreme in every way possible at our track, where we run a style very similar to Formula Drift. More power, more speed, more angle.

This setup might not work out as well for you as it did for me, but I firmly believe that if you're after absolute performance with your RWD chassis, a rear weight bias chassis is the way to go. I'll be going over some of my actual setup now, as I want to benchmark it, but keep in mind that what works for me isn't necessarily going to work for you. This is more so for me to be able to look back and see what my settings were, as I continue trying to develop my tune. Track surface, track style, driving style, there are just so many variables that come into play when tuning a chassis, but if you can walk away some points that help you out with your own rear weight bias setup, that's great. My setup has worked for me on asphalt, and a very slippery tile surface (very similar to polished concrete) so far. Now, let's get into the setup...

Front:
Tires: HPI Racing A-Drift
Offset: 7mm - Effective 8mm using MST brake disk wheel hubs.
Camber: -13*
Toe: 0*-0.5* (approximately)
Caster: 12* (approximately)
Ride Height: 7.5mm
Spring: Mikuni 34mm Medium Hard (PINK)
Bumper Weight: Overdose Bumper Support + 10g weights on lower wide position body post holes
Dampers: Yokomo BD7 Damper Set
Damper Piston: Yokomo 3 Hole 1.1mm Tapered Pistons
Shock Oil: 20wt Team Associated
Upper Shock Position: Lower Row, Second from the Outside
Lower Shock Position: 2nd Inner Most (WUN Type-3 Y-Arms)
Diff: 50g Weight in Front Diff Case
Other: When running a flat front tire setup, make sure at full lock, the lead wheel sits flat.

Rear:
Tires: HPI Racing T-Drift
Offset: 3mm - Effective 4mm using MST brake disk wheel hubs
Camber: 0*
Toe: 0*
Ride Height: 6.5mm
Spring: Mikuni 34mm Soft (BLUE)
Dampers: Yokomo BD7 Damper Set
Damper Piston: Yokomo 3 Hole 1.1mm Tapered Pistons
Shock Oil: 10wt Team Associated
Upper Shock Position: Lower Row, Second from the Inside
Lower Shock Position: Second Outter Most Position (Vacula Suspension Arm Set)
Diff: Yokomo Solid Spool + Stock Plastic Gear Set
Other: Oversliders Rear Full Size Lipo Mount

Drive Train:
Spur: 78t 48p
Pinion: 25t 48p
Final Drive Ratio: 7.5
Other: N/A

If you have anything you'd like to add, questions, or if you want to share your thoughts on your own attempts at building a rear weight bias chassis, feel free to leave a comment.

Damaged Goods

I'd just finished powering out of a hairpin and as I lined myself up to the car in font of me, I started closing in on his door. The tail of the car went flying out, clipping the wall, and at that point I started continuously ripping the e-brake while slamming on the throttle as the gap between me and the car in front was reduced to nothing. That's when all hell broke loose. It sounded like someone had just released the Kraken. I didn't give a shit though, I carried on laying into the power, and e-braking as I controlled the proximity between myself and the car in front of me... but it was only a matter of time, and eventually it was game over. The gears gave out, and I lost power. But little did I know, it wasn't my driving that destroyed the gears, it was an oversight while building the diff that caused internal combustion to occur...

As you could see, a screw backed out, and chewed the shit out of everything. A rookie move, I forgot to add loctite to an all important screw. You can see from the rear solid axle, that the bevel gear was in bad shape. The teeth were missing large chunks as the screw bounced around inside the gear case, ripping into everything it touched.

You can see how badly the smaller bevel gear was damaged as well, but I didn't know exactly how bad it was until I removed it. Although, the amount of dust sitting in the gear case from the graphite gears chewing themselves up should have been a clear indicator of how bad the damage was. 

First thing was first. I cleaned out all the graphite dust using an old paint brush. I didn't want any of it to remain in the gear case and cause my new gear set to wear prematurely.

As I mentioned earlier, it wasn't until I removed the bevel gear that I realized how badly it had been chewed up. There were huge chunks missing from the teeth... if you could even call them teeth anymore. RIP.

After I had everything removed, I directed my attention to replacing the ring gear on the rear solid axle. I pulled off the old one, and replaced it with a new set of plastic gears. This time, making sure to loctite everything.

After I finished rebuilding the solid axle, I prepared the smaller bevel gear for replacement. I really let that old one down. Got damn!

When I re-installed the small bevel gear, I made absolutely certain to thread lock the screw that holds it in place. The last thing I need is another parts failure to put an end to my day. Luckily this time around I had my back up chassis on hand to take over driving duties for the rest of the night. Next time I might not be so lucky though.

With the damaged parts replaced, I put everything back together. While I always take my time when building something, and have a certain OCD to the way go about things, this just goes to show how easy it is to forget something so basic... luckily this time around the damage was minimal, and the fix was straight forward. Now that the beast is up and running, I can feel it feigning for it's next battle... this time around though, hopefully I don't let it down.