As we previously mentioned, the Sparco Rally xD team had a rear twist beam axle failure during the last event, Susquehannock Trail Rally in PA. Once they got the car back down to California, they dropped off the rear dampers to us for inspection/repair.
Like we stated before, we knew that the dampers had some damage and broke the piston shafts. Here’s a look at the carnage
Remote hose was ripped right off. The shell case took a few hard impacts causing some dents. Now onto the piston shafts
The above picture shows the top of the shell case where the ring nut seals off the damper. The piston shaft broke midway and the remaining pieces recessed into the damper with oil coming out this ring nut portion. The top portion of the piston shaft was still attached to the shock tower.
This pic below shows the piston shaft broken towards the top, snapping right off after the twist beam axle gave way.
But after a full teardown, we saw that the inner tube (where the piston is inside of) was still intact. The outer shell case took a hit, but the inner tube was unscathed during this violent axle breakage. Here’s a pic showing the dent
The second pic from the top shows the other indentation on the shell case.
On a mono tube damper, this wouldn’t fare so well with that type of dent. Since the piston would reside right against the inner lining of the shell case of a mono tube damper any imperfection to the case will make it either impossible to function properly, or to be field stripped, or both. This is what I meant as testament to the durability of the twin tube damper structure and this case shows it well!
If you’re wondering why we would make a twin tube damper for a rally car, there’s a bit more to it than just the damage that could be inflicted to it physically. Our HG twin tube structure isn’t like a normal damper.
In a standard twin tube damper, there is still an inner tube that contains the piston shaft and piston. This area is fully submersed in damper oil. From the bottom of the damper and through the base valve controlling oil flow into the outside of the inner tube (inside of the shell case) is where there is some oil and the nitrogen gas charge. This charge is important in sufficiently providing rebound force to the damper.
What we do with the HG is fill the inner and outer tube with damper oil.
But wait. If you fill a damper full of incompressible oil and no compressible gas like nitrogen, then technically the piston shaft will never recess into the damper and it wouldn’t function at all.
That is true. But this is also why we utilize an external reservoir to keep the nitrogen charge separate. With this, we can eliminate any chance of aeration (nitrogen gas mixing with damper oil, causing it to froth, decreasing damping effectiveness) while still being able to secure sufficient rebound. The twin tube structure also allows ample stroke length for a given shell case size compared to a similarly built mono tube damper.
So why not utilize this design into all rally suspension? Mono tube dampers still have many great benefits- they dissipate heat better; when inverted, they decrease unsprung weight (you can technically invert our HG damper, btw); they allow a larger piston size; overall damper oil volume can be much higher. They also allow for us to use our Group N.technologies, which aren’t readily applicable to twin tube dampers
http://www.tein.co.jp/e/products/gr-n_frs.html
http://www.tein.co.jp/e/products/gr-n.html
But mono tube rally dampers are a much higher investment as development costs are quite high. Also, they are physically much larger in length and body diameter. the pistons used in them are much larger too, which is key for precise damping control.
So in the end, the HG is still very capable and also very durable at a much more reasonable price!
Anyway, enough ranting. We went right to work getting it rebuilt with new shafts and internal seals. this time, we stuck with our shorter length reservoir hose, still giving the team the freedom to reposition the external tank for their needs.
Awwww. Looking as spiffy as it will ever be! Ready for more abuse!!!
Speaking of which, here’s a video the Rally xD team posted from their blog showing the twist beam axle breakage