Post by pcnsd on Jun 20, 2008 12:04:47 GMT -5
Disclaimer:
This is an advanced frame modification; some planning and forethought are required. If you get it wrong, you and/or others may be injured or killed. Your property and/or the property of others may be damage or destroyed. I make no claim to the fitness of this modification for any application other than my own. The information provided is abbreviated. The opinions expressed are my own.
This is a popular modification for TT based dirt track bikes. I feel it all but eliminates front wheel push in a turn. Like an aftermarket carburetor and pipe go together to improve performance, so too a steering angle reduction and an extended swing-arm go together. If you choose to change your steering angle, consider also an extended swing-arm to receive the full benefit.
What will change?
Raise the front suspension, reduce wheelbase if not used with an extended swing-arm, move the center of gravity forward; usually achieves a 50/50 weight bias with 1” extended swing-arm, decrease the steering angle, reduce trail if used with the stock clamp and move the handlebars slightly forward.
Stock measurements for the SR:
Steering angle: 27.5 degrees, Fork offset: 45mm, Trail: 4.6 inches, Wheelbase: 55.1”* (* this number is from Yamaha and must be based on the shortest possible wheelbase, as measured, mine was 57.5” in about the center of adjustment. I am using a Profab 1” extended swing-arm)
Some numbers: (approximate but should be close enough.)
Changes to steering angle: .100”gap = 1 degree steering angle change. (Based on 6” from top of the oil tank to the bottom of the front box section minus a little, because a round tube doesn’t make a perfect hinge.)
Changes to trail based on stock clamps: (this is averaged based on the limited range available in the SR, 2 degrees maximum) 1 degree of steering angle reduction = .387” of trail reduction.
Changes to wheelbase if all else stays stock: 1 degree =. 546”wheelbase reduction
Tools and materials required:
1. TIG welder.
2. Copper coated ER70-S welding rods 3/32”.
3. Oxy-acetylene rig with a medium rose bud tip.
4. 1/16” or 3/32” sheet steel for reinforcement and the means to fabricate (1020, 8620 steel are good choices.)
5. A large sledgehammer. (6-8lbs will work)
6. A study bench and a way to clamp the frame to it.
7. The thinnest 4” cut off wheels you can find (.080” maximum), a 90-degree air driven die grinder and a mandrel to make them work together.
8. An air compressor.
9. Some skill and common sense.
A suggestion:
Have a window of change at least 1/4 degree wide that you will be happy with and shoot for the high side of that window. If you decide on a 2-degree change and over cut, your front wheel will one day bottom against your pipe or frame and you will go for an unhappy trip beyond the handlebars. At 25.5 degrees there is no space for a front fender and you need clamps with more offset. A note to motoX’rs, 26.5 degrees is the minimum steering angle with a 21” tire on the TT/XT and you will need leading axle forks and a different front fender.
The process:
1. Clean the oil residue from the oil tank, (Rinse with mineral spirits, then acetone, then hot soapy water, then denatured alcohol, then blow out with compressed air, until all scent is gone or use your own method to accomplish the same.)
2. Fully weld these two areas. I think one will pose a risk of failure once the frame is modified if not fully welded and the other has flex potential. These areas are the lower box section at the front of the frame, below the oil tank (Photo 1) and the frame connection at the rear of the oil tank (Photo 2). There are many other weld and gusset opportunities if you’re interested, but do these two at minimum.
3. Scribe the lower box section at about center. Photo 3.
4. Cut and gap the box section for the appropriate amount of steering angle change based on the numbers provided above using the die grinder and cut off wheels. For me this target was .125”, my actual as measured cut was .138”. Photo 4.
5. Securely clamp the frame to the bench in a position that allows you access to the front of the frame.
6. Heat the oil tank tube section directly above the cut/gap with your rose bud until bright orange (do not over heat.) You want to heat evenly the lower ¾ diameter of the tube in a band about 1” wide. The upper ¼ is the hinge point. The sides of the lower box section are your vertical alignment guides. If you do not heat evenly it may wander when you bump it and you will have to chase it back into position. The cut gap is your stop. Heat the oil tank tube evenly and a single bump with the sledgehammer will seat it to the cut gap stop.
7. Bump the steering head straight in with the sledgehammer to close the gap. One or two bumps should do it. The bottom of the oil tank tube will bow out slightly in the heat zone as the gap is closed. Photo 5.
8. Fully weld the now closed cut gap line. Photo 6.
9. Fabricate and weld gussets (I drill a small hole in one of the side plates so the expanding hot air does not blow out my welds and then weld that hole closed as the frame is starting to cool.) Photos 7 and 8
10. Done, but still mucking with the stabilizer angles. Check fender/tire to frame/pipe clearance with springs out of fork tubes and front end fully compressed. Don’t forget to check side-to-side clearances at the same time. None of this should be a problem if you stay away from the maximum. Photo 9.
This is an advanced frame modification; some planning and forethought are required. If you get it wrong, you and/or others may be injured or killed. Your property and/or the property of others may be damage or destroyed. I make no claim to the fitness of this modification for any application other than my own. The information provided is abbreviated. The opinions expressed are my own.
This is a popular modification for TT based dirt track bikes. I feel it all but eliminates front wheel push in a turn. Like an aftermarket carburetor and pipe go together to improve performance, so too a steering angle reduction and an extended swing-arm go together. If you choose to change your steering angle, consider also an extended swing-arm to receive the full benefit.
What will change?
Raise the front suspension, reduce wheelbase if not used with an extended swing-arm, move the center of gravity forward; usually achieves a 50/50 weight bias with 1” extended swing-arm, decrease the steering angle, reduce trail if used with the stock clamp and move the handlebars slightly forward.
Stock measurements for the SR:
Steering angle: 27.5 degrees, Fork offset: 45mm, Trail: 4.6 inches, Wheelbase: 55.1”* (* this number is from Yamaha and must be based on the shortest possible wheelbase, as measured, mine was 57.5” in about the center of adjustment. I am using a Profab 1” extended swing-arm)
Some numbers: (approximate but should be close enough.)
Changes to steering angle: .100”gap = 1 degree steering angle change. (Based on 6” from top of the oil tank to the bottom of the front box section minus a little, because a round tube doesn’t make a perfect hinge.)
Changes to trail based on stock clamps: (this is averaged based on the limited range available in the SR, 2 degrees maximum) 1 degree of steering angle reduction = .387” of trail reduction.
Changes to wheelbase if all else stays stock: 1 degree =. 546”wheelbase reduction
Tools and materials required:
1. TIG welder.
2. Copper coated ER70-S welding rods 3/32”.
3. Oxy-acetylene rig with a medium rose bud tip.
4. 1/16” or 3/32” sheet steel for reinforcement and the means to fabricate (1020, 8620 steel are good choices.)
5. A large sledgehammer. (6-8lbs will work)
6. A study bench and a way to clamp the frame to it.
7. The thinnest 4” cut off wheels you can find (.080” maximum), a 90-degree air driven die grinder and a mandrel to make them work together.
8. An air compressor.
9. Some skill and common sense.
A suggestion:
Have a window of change at least 1/4 degree wide that you will be happy with and shoot for the high side of that window. If you decide on a 2-degree change and over cut, your front wheel will one day bottom against your pipe or frame and you will go for an unhappy trip beyond the handlebars. At 25.5 degrees there is no space for a front fender and you need clamps with more offset. A note to motoX’rs, 26.5 degrees is the minimum steering angle with a 21” tire on the TT/XT and you will need leading axle forks and a different front fender.
The process:
1. Clean the oil residue from the oil tank, (Rinse with mineral spirits, then acetone, then hot soapy water, then denatured alcohol, then blow out with compressed air, until all scent is gone or use your own method to accomplish the same.)
2. Fully weld these two areas. I think one will pose a risk of failure once the frame is modified if not fully welded and the other has flex potential. These areas are the lower box section at the front of the frame, below the oil tank (Photo 1) and the frame connection at the rear of the oil tank (Photo 2). There are many other weld and gusset opportunities if you’re interested, but do these two at minimum.
3. Scribe the lower box section at about center. Photo 3.
4. Cut and gap the box section for the appropriate amount of steering angle change based on the numbers provided above using the die grinder and cut off wheels. For me this target was .125”, my actual as measured cut was .138”. Photo 4.
5. Securely clamp the frame to the bench in a position that allows you access to the front of the frame.
6. Heat the oil tank tube section directly above the cut/gap with your rose bud until bright orange (do not over heat.) You want to heat evenly the lower ¾ diameter of the tube in a band about 1” wide. The upper ¼ is the hinge point. The sides of the lower box section are your vertical alignment guides. If you do not heat evenly it may wander when you bump it and you will have to chase it back into position. The cut gap is your stop. Heat the oil tank tube evenly and a single bump with the sledgehammer will seat it to the cut gap stop.
7. Bump the steering head straight in with the sledgehammer to close the gap. One or two bumps should do it. The bottom of the oil tank tube will bow out slightly in the heat zone as the gap is closed. Photo 5.
8. Fully weld the now closed cut gap line. Photo 6.
9. Fabricate and weld gussets (I drill a small hole in one of the side plates so the expanding hot air does not blow out my welds and then weld that hole closed as the frame is starting to cool.) Photos 7 and 8
10. Done, but still mucking with the stabilizer angles. Check fender/tire to frame/pipe clearance with springs out of fork tubes and front end fully compressed. Don’t forget to check side-to-side clearances at the same time. None of this should be a problem if you stay away from the maximum. Photo 9.
. If you'd done the job correctly (IE cut out the head stock and re angled it in a jig then rewelded it) you would not have needed to add all that extra plate work.
