Relative Seat Angle/ Bottom Bracket Study

By Steve Robson

After working and riding on the various bike that I have built, I have felt that it is time to do a study on a number of bikes that I have built. I will be looking at the area of design regarding the relationship of the bottom bracket to seat angle and how it affects the rider. I will look at four of my bikes and give a seat of the pants feeling on how I feel about how it affects the comfort level when the bike is used.

The 206 SWB seat set-up drawing.

The first bike that Iím looking is my 206 SWB recumbent bicycle. The seat angle was set a greater angle back then on my first FWD bike. Although I did not thing about the time, I was to find out the importance of the working seat angle/ bottom bracket position. On the first drawing, it shows the relative working angles of the seat and bottom bracket location. The angle between the seat back and center of the bottom bracket is 139 degrees. The seat angle is about 44 degrees off the verical line. I get from it a stretched out feeling with this arrangement. In some ways, this angle is more like a communter bike that uses a high rise handbar. Although the frame structure has some affect on the climbing ablities, is a bit more difficult to get good leverage when sitting back at full incline of the seat. I need to to get out of the seat and pull my body into a tighter tuck while pulling on the handbars to get additional power output in a climb. The bottom bracket is placed so the chain line is in itís best line. When compared to some of my newer recumbents, this is more of a cruiser in feel.

The Strath Cruiser seat set-up drawing.

The next recumbent bike to be looked at is the Strath Cruiser. It uses the same seat angle as the SWB recumbent that I built. The major difference is the working angle of the bottom bracket. The working angle is 122 degrees. The one feature of the design is the high BB in relation to the seat. Since the bottom bracket is higher then the seat, when you push the pedal forward there is a pressure on the bottom section of the seat. This causes my butt to become numb after riding it for a bit. It feels tighter in turns of the seated postion when compared to SWB recumbent. Since there is a pressure point on the seat, this cuts off blood flow to the legs and this causes the feet to get numb as well over a period of time. I don't think that the raised bottom bracket is good design feature, at least for me. One note on this bike. Since it was built for a slightly larger person, bottom bracket is a little farther out for my size when compared to the other bikes that have been built for myself. I got the bike back after some small problems developed with the person so I now make use of it.

The Naboo CLWB seat set-up drawing.

The CLWB recumbent that I built is one of the best riding bikes I have built. Although the seat is much higher then the other two bikes that have been reviewed, the seat to bottom bracket angle workes out to be inbetween the two of these onther bikes. The working angle is 128 degrees. This works to be a very comfortable postion. Although the seat is high, it works out to be a great commuter bike. There is equal pressure on both the seat base and seat back. It makes this bike one of my favities to use. Since a shorter crank arm is used on the bike, the bottom bracket is a little farther out from the seat when compared to the other bikes. The leg reach is still the same as the other personal bikes that I have made. The seat angle is set at 60 degrees which more upright then the other bikes but it still is angled back then on some of the ones I have seen.

The Low Racer seat set-up drawing.

The last bike that I will look at is the Low Racer recumbent built in the spring of 2000. Although the design of the bike is far different from the CLWB bike, it also shares the same seat to bottom bracket working angle. The angle is 129 degrees. The only difference is that the seat angle is set at 45 degrees and the bottom bracket is set 3 inches above the seat location. Since you set "in" the bike, you feel much more a part of the bike when compared to any of the other bikes I have built. When you pedal this one, the main pressure point is now on the bottom section of the seat back, not the seat base. The result is that my bum is much more comfortable on longer rides. Although it can go numb, the chances of this happening are far less then on the LWB bike. I can get the best power output with this working angle for the BB to seat set-up. The only real limitation on this bike the low gear ratios that limit what could be a good high end speed.

I have concluded that small differences in the set-up of the recumbent bike make for big differences. Although there is only a 10 degree angle difference between the 206 SWB and the low racer, I feel far better on the low racer in terns of getting power output. Although the low racer is a heavier bike then the 206 SWB recumbent, I have powered it up some hills with a better feeling of leverage regardless of of the fact the is uses only a 6 speed gearing set-up vs. the 18 speed gear on the 206 SWB bike. The ideal set-up is a personal one and you will have to experiment yourself to see what works for you. This means testing a number of bikes to see which design will work best for you.

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