Schwinn Homegrown frame design

Report on Schwinn frame design by designer Stephen Levin. This type of testing was done on Project Underground frames to dial in the ride:

1995 SCHWINNS:

How Those Shaped Tubes Give You A Better Ride

INTRODUCTION:

Our 1995 bicycles are 100% NEW Schwinn.  Product managers and engineers worked together to design the line as a complete package, rather than as an assortment of bikes sharing a nameplate.  The results are shared performance features and benefits on all models.

In particular the design of the actual frames is emphasized, rather than just the material choice.  To us it's like cooking:  Good ingredients do not automatically ensure a good meal; the cook has to know how to use them correctly.  Yet in the bicycle industry, too often people see a frame as only being affected by its material and not its design (such as tube size, thickness, and shape.)  For example, aluminum is only one-third as stiff as steel, yet aluminum frames are typically thought of as being harsher than steel ones.  Thus the dominating ride characteristics of most aluminum frames are from the design (over-sized tubes) rather than the material itself.

Bikes can be too flexible, but unquestionably they can also be too stiff, resulting in harsh, uncomfortable rides and poor traction.  Generally, lateral stiffness (side to side) is good as it stops the rear triangle from flexing during cornering or hard pedaling.  In contrast, vertical compliance (up and down) is desirable as it softens the ride.  Traction is also improved as the frame tends to ride over bumps rather than bouncing over them.  Granted, vertical compliance isn't full suspension, but it does take the edge off big hits.

HOW DO WE DO IT?

Accomplishing this symbiotic relationship within one frameset isn't easy, but we do it in several ways on all of our 1995 Mountain Bikes.  First we carefully select tubes to provide enough stiffness without being overly massive.  We are so specific, that our S9Five series actually uses smaller diameter down tubes on the two smaller frame sizes.

Up front, we have horizontally ("sideways”) ovalized top tubes.  The ovals are subtle and you may not notice them until you feel the tube.  Yet this slight ovalization is enough to make  the tube about 30% stiffer side-to-side than up-and-down. 

In back, we have our unique patented EpicenterTM Seat Stays which provide three big benefits.  First, and most apparent, is the massive tire clearance.

Second is the improved braking power.  Most straight stays flex under braking causing the brakes to feel mushy.  The easy answer is to make the stays thicker in the brake area.  Unfortunately this adds weight and stiffens the stays vertically which makes for a harsher ride.

Nature shows the most efficient way to counteract a force is to push back against it.  Think of empty flat bed trucks or a suspension bridge.  A side view shows they aren't flat, but curve upwards to resist the load.  Our Epicenter Stays do the same, "curving" into the load to resist the braking forces.  Our brake bosses are not only on the center of curvature and but are also mounted on the center of the tubes.  This increases brake stiffness and reduces flex.  Even on our lightest weight frames we get brake stiffness rivaling steel frames with hefty seat stays.

Third, is (you guessed it) vertical compliance and lateral rigidity.  Typical mountain bike seat stays are relatively short and stiff.  They are very unforgiving as deflection comes from trying to directly compress the material.  By contrast, the "hoop" at the top of the Epicenter stays makes them naturally compliant like a spring.  The movement is slight, but it is enough to take the "edge" off the ride.

HOW DO WE KNOW IT WORKS?

We know it works based on rider feedback and formal lab testing.    Early in 1994 our racers rode aluminum frames with conventional rear triangles.  Then we switched them to bikes with Epicenter rears.  They noticed the difference right away.  Tim Gould finished second at the Grundig World Cup race at Mt. Snow in Vermont.  In his race report he said the Epicenter, "...seemed to give me superior braking power.  The downhill section was very bumpy and the new triangle seemed to give a more forgiving ride, yet was still responsive under acceleration."

But how to measure these improvements?  As we all know a good ride is very subjective and hard to quantify.  Schwinn has one of the most complete research labs in the industry, and so we put it to work.

As most people assume, we use our lab to test products to make sure they meet our high standards for strength and reliability.  This traditionally means destructive testing during the prototype phase to make sure they are ready for production.   In addition, however, we also perform "Load-Deflection" testing.  "Load-Deflection" means we apply a specific load and then measure exactly how much it deflects (bends, twists, etc.).  Our goal is not to break the  product, but to measure its stiffness and compliance.  We compare our measurements with rider feedback.  In this manner, we can dial in just the right "feel."

THE RESULTS PLEASE...

We performed numerous load-deflection tests on one of our high-end aluminum frames and compared it to a high-end steel frame of conventional design.  Both frames were 19" sizes.  We used a Schwinn "Project Underground" Easton-tubed aluminum frame.  This frame is very similar to our Homegrown USA made frames.

The competition used a conventional twin seat stay (non-wishbone) rear triangle.  This frame was our baseline, so for ease of comparison, its values were set to 100%.

1. Weight

The frames were weighed.  Low weights are desirable.

FRAME:              Weight             Baseline Weight

Schwinn               3.4 lb                         72%

Steel                    4.7 lb                       100%

2. Bottom Bracket Torsional Stiffness

This test clamps the head tube and dropouts, and pulls the bottom bracket outwards (parallel to the BB spindle).  The test measures the torsional stiffness of the bottom bracket.  High pedaling loads try to deflect the bottom bracket outwards.  Deflections at the top of seat tube and the bottom bracket are recorded.  As this relates to pedaling, greater stiffness values are desirable.

FRAME:                          BB Twist          Seat Tube Ltrl. Displ. 

Schwinn                             98%                            93%              

Steel                                100%                           100%              

3. Frame Torsional Rigidity

These are two related tests.  One clamps the head tube and twists a dummy axle in the dropouts.  The other does the reverse.  Deflections are recorded at the bottom bracket and at the top of the seat tube.  These tests measure the total frame torsional stiffness.  Torsional rigidity is a combination of lateral and vertical rigidities.  High torsional rigidity is desirable for the frame as a whole.  For the rear of the frame, however, it is necessary to break out the lateral and vertical rigidities separately.  This is done in tests 4 and 5.

Frame                              Torsional Frame Rigidity

Schwinn                                       124%  

Steel                                            100%

4. Rear Lateral Stiffness

This test clamps the head tube, supports the bottom bracket and pulls laterally (outwards) on the dropouts.  This test measures the stiffness of the rear stays (as the front part of the frame is supported).  As this relates to lateral flex (such as pedaling or cornering), greater stiffness values are desirable.

Frame                              Stay Ltrl. Displ.           

Schwinn                                   126%

Steel                                        100%

5. Rear Vertical Compliance

This test clamps the seat tube and bottom bracket, and a load is applied vertically up on the rear axle.  This test measures vertical stiffness of the rear triangle.  As this relates to comfort, more compliant (less stiff) values are desirable.

Frame                              Rear Vert. Displ.        

Schwinn                                     35%      

Steel                                        100%

6. Rear Brake Stiffness

This test screws a rod into the rear brake stud.  The rod is then pushed outwards causing the stud to deflect as it would when the brake is applied.  The less the stud deflects for a given load, the stiffer the brake is.  High stiffness is desirable.

Frame                              Brake Stiffness

Schwinn                                      92%     

Steel                                        100%      

CONCLUSIONS

The results confirm that the 1995 Schwinns feature a unique frame design which above all is efficient.  Our philosophy that how a frame material is used is just as important as what material is used has been proven true by both pro-racers and our research lab. 

The efficiency of our frame is demonstrated by its ability to match or surpass the stiffness of a high-end steel frame while weighing well over a pound less.  Specifically, the overall frame torsional rigidity and rear lateral stiffness are significantly greater than the steel frame, and the bottom bracket rigidity is approximately equal.

The success of our unique Epicenter design is clearly shown by its ability to simultaneously increase lateral stiffness, maintain brake stiffness and provide dramatically increased vertical compliance.  Conventional methods to improve lateral and brake stiffness also reduce vertical compliance.  The results show we have triple the compliance of the baseline frame.  The deflections are small, but are enough to remove the edge from big hits.

These results carry over to other Schwinn products including the mid-price S9Five aluminum bikes and the steel framed High Plains and Moab series.

In conclusion, these tests demostrate that the special tube shapes of the 1995 Schwinn's are highly effective at providing stiff yet comfortable frames.*

Report Prepared By:

Steve Levin,

Engineering Manager

* In slightly simpler terms: THESE FRAMES ROCK!!!