Lateral Stiffness

**Lateral stiffness.**In a full suspension design, the rear wheel moves up and down absorbing the bumps of the trail, keeping you in control and allowing you to ride faster than otherwise possible over bumpy terrain. While up and down movement of the rear wheel is highly desirable, side to side (lateral) motion certainly is not. A bike with poor lateral stiffness will be more difficult to control and far less predictable than a bike with good lateral stiffness characteristics. The more the bikes wheels are left pointing in any direction other than exactly where you want them to be, the less control and less speed. Forces wanting to push your rear wheel sideways are encountered every time your rear wheel strikes an object at an angle not directly head on. Rock gardens and tree roots crossing your favorite trail on a diagonal angle are notorious examples. Good riding conditions offering fantastic tyre traction can also allow a talented rider to put some serious sideways forces through the bike that can produce noticeable lateral flex in sub standard frames. Hard tail frames have a significant advantage over full suspension designs in the realm of lateral stiffness. The lack of any pivot points means there is no weak link in the chain for hard tails. With full suspension designs there are challenges that need to be met in order to produce a frame that has good suspension performance, yet admirable lateral stiffness also.

Pivot points are inherently a place where lateral stiffness is difficult to maintain. Any slop in the pivot point, no matter how minimal, will be felt at the rear wheel. Even with an extremely well built pivot point, the bearings themselves actually compress under load to an extent that can be both felt and measured. The leverage force acting against your main pivot point bearings can be calculated as the lateral force pushing against your rear wheel, times by the distance between your rear wheel axle and the main pivot point. It is the simple formula for calculating torque. Force x distance = torque. Double the distance means double the torque forces acting against your pivot points, and double the potential lateral movement at the rear wheel. The problem magnifies as the bearings at the pivot point wear and slop at the pivot point increases. To help over come these problems, designers have, over the years introduced larger and larger diameter bearing and bushings, particularly at the main pivot point location. Some riders upgrade to much more expensive ceramic bearings which do not compress as much under load. The downside is of course greater weight and expense in the pivot point.

We shall shortly look at three different suspension platforms and see why the VAST link system has an inbuilt advantage over the others in the realm of lateral stiffness.

For the purpose of these calculations we will assume that the lateral flex in the solid chainstays themselves to be zero. Why do we assume this? Modern carbon fiber can be made to such extraordinarily high modulus (stiffness) that it can actually become too much of a good thing. Even intermediate modulus carbon fiber is extremely stiff compared to other frame building materials. So stiff it is that modern carbon fiber hardtail and road bike frames are being made with thinner and thinner diameter seat-stays to try and allow even just the smallest amount of flex to help absorb the vibrations. Curved seat-stays are another technique used to try to incorporate a tiny amount of flex. If we use quality carbon to build large diameter chainstays, then the flex within the chainstay itself can be considered negligible.

We will also be calculating flex along the chain-stays only, not the seat-stays. This is because linkage driven single pivots, dual linkage designs, and the VAST link can all be made to have very similar linkage systems along the seat-stay line to the shock. Different rear axle systems can also effect lateral stiffness, but again it is an even playing field for all suspension platforms in this regard. The principle difference in lateral flex between the designs exists from the rear wheel axle along the chain-stays to the main pivot point. We will be focusing our calculations there.

__Single Pivot__

Let us imagine three riders racing along their favorite trail. The first rider is on a linkage driven single pivot design, the second is on a dual linkage design, while the third is riding VAST link. All bikes are made using

__exactly the same bearings__of the same diameter at their pivot points. Just ahead lies a mighty big tree root diagonally crossing the trail just meters before an abrupt turn. A sideways force will soon be exerted on the back wheel. With a standard

__single pivot__design the distance from the main pivot point to the rear axle is approximately 430mm. It differs between designs but that is a fairly normal figure for a 27.5 inch wheeled 120mm travel bike. The torque force felt at the main pivot location is: the lateral force multiplied by the distance, in this case 430mm.

Lets imagine the lateral force against the riders back wheel to be 100 units. So the torque force acting against the main pivot point is 100 units x 0.43m (chain-stay length). Forty three is hence the torque force. If we assume that ten units of force twists the pivot point 1 degree then we have 4.3 degrees of deflection. This number over the 430mm chain-stay length equals

**32mm**__of sideways rear wheel d__

__eflection__.

__Dual Linkage__

With designs that use

__dual linkages__on the chain-stays there are two pivot points to consider and the problem is potentially doubled. We will assume the dual linkage design has its main pivot point directly above the bottom bracket (to keep it even with the single pivot bike above) and that the second pivot point is 50mm further back. Now if the same 100 units of lateral force hits the dual linkage design, the main pivot point experiences a force identical to the single pivot bike above. Namely 4.3 degrees of deflection at the main pivot point. By the time we get to the second pivot point 50mm rearward, this 4.3 degrees has laterally shifted the second pivot point 3.75mm. The distance between the second pivot point and the rear wheel axle is 380mm. The torque force acting against the second linkage member pivot point is thus 0.38m x 100 units. Equals 38. The torque forces will thus cause 3.8 degrees twist at this bearings location. The combined angle change of both pivot points combined is now 4.3 plus 3.8 -equals 8.1 degrees. 8.1 degrees over the remaining distance to the bikes rear wheel axle equates to an additional 53.68mm of sideways deflection. Add to that the 3.75mm already deflected at the second pivot point and its a total of over

**57mm**__of sideways deflection__at the rear wheel.

__VAST Link__

Our third rider is on a

__VAST link__. The distance from the rear wheel axle to the main pivot point is just 160mm. The same 100 units of lateral force over just 0.16 meters equals 16 units of torque on the main pivot point which equates to just 1.6 degrees of angle twist. That amount of twist over 0.16 meters equals just

**4.47mm**__deflection at the rear wheel__. Its a massive advantage over all current designs. The VAST link, for the majority of the chain-stays length is in all respects enjoying all the lateral stiffness qualities of a hard tail bike. The distance from the main pivot point to the rear axle is, on the prototype, just 160mm. This is a whopping 62.8% decrease in distance (compared to a normal single pivot design) which results in a huge decrease in torque forces acting against the bearings and an enormous decrease in any lateral flex felt at the back wheel. Pivot point bearing life is proportionally increased.

The rider of the VAST link bike is at a huge advantage as he strikes the diagonal tree root and then hits the turn. His wheels are pointing where he needs them. The rider of the linkage driven single pivot bike hits the bump and his rear wheel deflects sideways a whopping 7 times further than the VAST link bike. He has less control after the bump and has to brake harder to regain control for the turn. Its even worse for the dual linkage bike rider whose rear wheel deflects almost 12 times further sideways than the rider on the VAST link bike. The loss of control is even more severe and greater amounts of speed are lost.

This large gain in lateral stiffness the VAST link achieves may offer the bike designer two enviable options. Stick with a conventional linkage system along the seat-stays and enjoy a frame of far superior lateral stiffness, or eliminate the seat-stay linkages entirely and rely purely on the main pivot points to handle the lateral flex (as in the prototype built). The benefit of this option is extremely light weight.

Either way, the VAST link design platform has a very large and very enviable advantage over other contemporary designs in the realm of lateral stiffness.