5th A-arm ?

[Pete]

Noticed that some chassis builders (Masters, Barry Wright) mount there 5th a-arm brace the the quick change and some (Rocket) mount theres to the right side of chassis. Does anyone the know what the gains or losses are one vs the other? Thanks for any info.

[dfhotlm33c]

Simply put:
lift bar mounted to QC: the bar moves as the rear end moves, meaning the spring and shock see different angles , and do not always operate in a linear fashion. This can also yield clearance issues, as the spring can end up contacting the driveshaft.

lift bar mounted to frame: Uses heims at the rear end and at the link connecting the bar to the frame. This allows the bar to stay in place as the rear end rotates through its motion, and allows the bar to operate in a linear fashion. Eliminates clearance issues with driveshaft.

hope that helps

[Matt49]

Simply put:
lift bar mounted to QC: the bar moves as the rear end moves, meaning the spring and shock see different angles , and do not always operate in a linear fashion. This can also yield clearance issues, as the spring can end up contacting the driveshaft.

lift bar mounted to frame: Uses heims at the rear end and at the link connecting the bar to the frame. This allows the bar to stay in place as the rear end rotates through its motion, and allows the bar to operate in a linear fashion. Eliminates clearance issues with driveshaft.

hope that helps

Bracing the lift bar (or 5th a-arm as the OP calls it) to the frame does not make the bar stay in place left to right. The length of the bar mounting it to the frame produces an arc in the movement. The lift bar still moves to the right under acceleration (5th coil compression) but it isn't directly correlated to rear steer like when bracing it back to the axle tube.
If one way were clearly better than the other, all the chassis builders would do it the same. It's one of those things that people just do differently and it apparently works okay both ways.
I don't want to get too much into the details but I'll ask anyone to consider this. Why would you necessarily want the lift bar to stay centered when the car is rarely flat and the rear axle rarely square in the car?


Obviously both ways of mounting work as cars with both systems win many races.

[let-r-eat]

Bracing at the frame has less unsprung weight? I see no disadvantage to either, really. Both have pluses and minuses. Kind of like the rods holding a lower control arm. You get some caster loss on bump and gain again on rebound due to the arc as mentioned. The length of the rod determines how much.Agree with 33 and Matt.

[Rayburnt1]

I asked Keith Masters about this about 10 years ago. He said they tested it both ways at a test session they were at and there was no difference in speed or driver feel, so they left it alone. Thats all I got.