I first really felt a problem when the seat on my Bacchetta Aero pivoted at the base when I tried to shift my butt slightly in the seat. This was at 53 mph on a descent in Oregon in 2003, on the Vine Ride Century. It felt like a low rear tire rolling over onto the sidewall. Scary. This was with the stock hardware stack up: 5mm (3mm hex lens head) bolt, thin fender washer, thin rubber washer, seat, thin rubber washer, seat plate, split ring washer, wing nut. In time I discovered that the only way to really tighten it enough, so the seat would not pivot at all, was to use pliers on the wing nuts. I could not on the drive side as my bolt was under under the idler brace. The 3mm bolt heads could not withstand enough torque to get the seat tight enough for me. Even so, after a few months I discovered that my seat has been moving slowly back, as the threads on the bolts had chewed up the carbon in both holes. I can't seem to tighten the bolts enough to keep the carbon in one place. Instead of holes I now had a slotted seat.
After several iterations, I settled on this, and this is what we do at the shop today. To be honest it looks a lot like what Optima does. Drill out the holes in the seat plate with a 1/4" bit so they will just accept a 6mm bolt (4mm hex flat head). We drill out the seat too with a 9/32 bit, to accommodate the head of the flat head bolt. We have a jig for this that allows us to drill the seat centered and straight every time. The jig has multiple drill guides that can be used depending on seat angle. You want the seat to be parallel to the plate at the point of attachment. We also have holes drilled some multiple of the seat plate pitch, plus half a pitch, apart. This means you are not limited to seat positions every 3/8", but 3/16" of an inch. This is good as currently the way to dial in your position on a Bacchetta with a seat plate is to get close with the fore aft location of the seat and use recline to get your leg extension just right. That, or slot the holes in the seat. The next jig I build might have holes that allow for 1/3 of the stock seat plate pitch seat position. So 1/8" between each seat locations.
We use a washer with an inside diameter just smaller than the flat head bolt. This washer is quite a bit thicker than the stock one, so it does not flare. It has a smaller outer diameter though. The bolt head and washer can pivot relative to each other which is nice too. Then comes a thin rubber washer, the seat, a thin rubber washer, 7 fender washers, the plate, a split ring washer and an extra long nut, not Nylock. Nylock bolts have less thread and are easier to strip. The fender washers are not needed on the older M5 seats, likely not on an Optima seat either, as they have enough space between the ribs for the seat plate. With the CCK or VK seat now found on most Bacchettas you need to elevate the seat with something incompressible so that the ribs don't hit the sides of the plate. An aluminum spacer would be ideal - the Bacchetta Hardshell Mounting Kit is perfect!
If you tighten the bolts enough, the rubber washer will ooze out the sides of the washer quite a bit. Also If you wheel your bike up to a wall, and put the wheels right in the v formed by the wall and the floor, and now hold the bars in one hand and the seat back in the other and twist, you will find the seat no longer moves independently of the frame tube. Your cornering confidence should improve. You should feel one with the bike. Shifting yourself in the seat should not be accompanied by the sensation of a low rear tire. Once the seat is no longer pivoting, the seat struts play no role in laterally bracing the seat. The odds are they will never crack. If you tighten the bolts enough, they will not come loose either.
There is a downside to all this, and this is Bacchetta's concern: you cannot go changing your seat angle without first loosening these bolts and you may even have to drill new holes in the seat if you change the seat angle a lot. You need to be bolting the part of the seat that is closest to parallel to the plate. You cannot pull the grenade pins and shove the seat down to fit the bike in a vehicle either. That's a great way to crack a seat too.
The upside is this: you may find some threads on using RANS struts or some custom brace in an attempt to stiffen up the seat laterally. The seat does not flex laterally, it only moves if the base bolts are not properly secured. Once you bolt your seat base down so it cannot pivot, none of these modification will have any affect on seat movement at all. The issue has already been addressed. The struts just keep the seat from landing on the rear tire once you sit on the bike.
We do use Nylock nuts on the bolts holding the seat struts to the rear dropouts on the frame. These are just to make sure the bolts never loosen, they do not have to withstand a lot of torque. Its our belief that although checking fittings every once in a while is a great habit to have, nothing should require frequent tightening.
(originally posted on bentupcycles.blogspot.com on 10/10/2012. Written by Jim Verheul, updated by Dana Lieberman)