Following the B7 design, AE released a new 4WD, the B84. The model has similar pivots and uses the same rear arm as the B7, with many parts interchangeable between the 2WD and 4WD. Parts like the internal diff gear, rear shock tower, rear arms, and rear hubs are shared. AE has seen great success on dirt tracks but hasn’t achieved the same results on carpet. Why might that be?
We haven’t had much time with the car yet. We got a couple of runs in on July 15, just a few days after receiving it and right before the Roar Carpet Nationals. Back in July, we didn’t have the parts needed to get it into proper race trim, which made it hard to gauge its pace. Instead, we focused on how the car felt and handled. Nationals was a learning experience in 4WD, but we took the data and used it to improve. Now, we’re looking to learn more about what to do when carpet racing.
The first thing we noticed was how rigid the front end was. I saw this while watching Brandon run the car. He mistimed a jump and had to plow the front on landing to get down as quickly as possible. The car’s reaction to this move really surprised me, I’d never seen anything like it from a 1/10 model. Luckily, I caught it on video. Brandon also pointed out how dramatically the car shifted its weight from front to rear when on the throttle.
While running the B7, I had a few thoughts. I figured the chassis jacking might come into play, so I focused on capturing video during long corners, knowing the car would perform well in hairpins.
At nationals, Brandon noted that the car was still transferring too much weight from front to back. He went through all the usual setup changes, removing all the caster from the front, adding as much anti-squat as possible—including a 1mm spacer under the D-Block—and standing the rear shock fully upright. This made some improvement, but not enough. We then began shifting weight in the car by moving the motor back. The car felt better, but still wasn’t quite where it needed to be.
Until now, I hadn’t really taken a close look at the car like I usually do. I made a point to do this a few weeks ago, and Brandon and I sat down to examine it. The first thing I noticed was how much higher the front hinge pin height was compared to the rear. Brandon mentioned that the B84 had more kickup than the B74.2, and I think that extra kick contributes to the higher front roll center. Removing all the caster or kick with the pills also adds to this because it raises the B-block pills to their highest position. I suspected the front suspension was extremely rigid, and it’s starting to make sense, high roll centers add stiffness to the suspension.
One drawback of narrowing the pivots is losing stability in the pitch axis while increasing stiffness in the roll axis. Shifting the motor and battery to the rear adds static weight there, so the car transfers less weight to the back since it’s already loaded. Adding weight to the front to dull steering feel follows the same logic. Like with the B7, the challenge is figuring out how to keep the car flat within this geometry.
On a small track with tight 180-degree hairpins, these effects are usually less noticeable. The same goes for running a stock 4WD buggy, as stock cars don’t shift weight like modified ones do. If long corners with clear entry, middle, and exit points are giving trouble, I’d start by lowering the roll centers, being pretty aggressive about it, especially in the front. I’d go lower until the front starts to push off-throttle from overload, then fine-tune the rear roll center to balance on-power steering and weight transfer. I’d also try a larger front tire, as it might help improve the front dog bone angle.
Regarding the chassis, this is a tricky situation. I’ve read plenty of posts and comments about it, and I’m not sure there’s much we can do with the current geometry. Narrowing the pivots and lengthening the arms has raised the roll centers, which stiffens the suspension. A stiffer suspension transfers more energy through all the components after the links. The instant center, where the roll is controlled, is now closer to the car, shortening the “lever” and making energy transfer quicker. This also makes it harder for the driver to control how fast the car rolls. With the chassis being so narrow, combined with the holes and countersinks as they are, it’s a perfect storm. I think the best approach is learning to tune within these parameters. Lowering the roll centers will allow more and slower roll, which will absorb some of the energy moving through the links and other components before it reaches the tire. This should help protect the chassis.
We plan to test these and other factors in the coming days and will share our findings afterward. These are my thoughts and ideas on improving the car’s drivability on high-grip carpet. One general observation is that lower grip requires a stiffer suspension, while higher grip allows for a softer setup. Controlling how much energy is transferred to the tires helps determine how much grip the car has and when it’s available.
Give it a try. We would love to hear your results!