Transaxle Tradeoffs

Tradeoffs: Sequential vs. H Pattern Shifting

The PBS off road transaxle is offered either as a Sequential unit or as a conventional “H” pattern shifted gearbox.

The sequential shift system is based on a mechanical ratchet similar to the system used on most motorcycles. The four speed shifter has five positions, neutral, 1st, 2nd, 3rd, and 4th. These must be selected in sequence. A pull on the gear lever will go up one gear and a push will go down one gear.

This system can usually be shifted a little faster than an “H” pattern shifter, particularly compared to a 2nd/3rd shift with a typical 4 speed. On the PBS gearbox, reverse is selected by shifting to neutral and then actuating a separate lever to engage reverse.

The internal sequential shift mechanism allows the gears to be stacked differently than an “H” pattern system if desired. Thus in the PBS gearbox, we have stacked them as 1st, 3rd, 4th, and 2nd. This allows 1st and 2nd gear to be located next to bearings on the lay-shaft and pinion shaft rather than near the middle of these shafts as in a typical “H” pattern gearbox. Since 2nd gear is normally subject to higher loading than 3rd and 4th gear this improves the effective strength of 2nd gear. This results because the shafts are retained from spreading apart as much as they do when 2nd is farther from the bearing.

Also, the sequential shift mechanism is easily actuated by a paddle operated air shift system. Optional gear change indicator displays and sensors are readily available for the sequential shifter.

The down side of the sequential shifter is the fact that neutral cannot be selected directly from any position except 1st gear. Thus, if you stop the vehicle in a higher gear, you must sequentially shift it down rather than just selecting the desired gear or neutral directly as you can with an “H” pattern shifter. When the vehicle is not moving the shifting can be difficult if the dogs don’t line up. Reverse is selected with a separate lever with either the PBS Sequential or “H” pattern transaxle after selecting neutral.

The other advantage of the “H” shift patternis that you can tell what gear you are in by the position of the shift lever. The “H” pattern is a little more cost effective also because it has fewer and less complex parts. The PBS “H” pattern incorporates the PBS cable shift system which has been developed over a period of many years and is used on a great many type “1” and 2D transmissions.

Tradeoffs: Straight Cut vs. Helical Gear

Racing transmissions normally use straight cut gears. These gears don’t produce end thrust which reduces bearing loads, friction and heat in the gearbox. The down side is that they have a distinctive sound. Street applications invariably use helical gears to reduce noise. Obviously both types are widely used with generally acceptable results.

Tradeoffs: Four Speed vs. Five or Six Speed

The number of gear ratios needed in a transaxle is a function of the intended speed range of the vehicle and the engine power curve characteristics. As the number of gears is increased the gears either need to be reduced in width or the transmission must be lengthened and the bearings generally spread further apart. Thus the fewer speeds the more robust the transmission will be for a given size unit.

Naturally, the cost of the transaxle and maintenance costs will also be lower with fewer speeds. In addition, the shift mechanism is generally less complex and more reliable with fewer speeds.

Options

LS1 Bell housing with Camero Hydraulic throwout bearing, Gear change sensors and remote display, Paddle shifter, Air or Nitrogen.

The following are available from Outfront Motorsports: Subaru bell housing for hydraulic throw out bearing and Subaru bell housing for VW type throw out assembly.