Designing a Motorcycle Rear Fork/Swingarm
Designing a Motorcycle Rear Fork/Swingarm
(OP)
Hello everyone,
I'm in the process of designing a swingarm for a 250 pound motorcycle. The kind of a swingarm is the traditional types found on cruisers. Wikipedia calls it:
Swinging fork - the original version consisting of a pair of parallel pipes holding the rear axle at one end and pivoting at the other. A pair of shock absorbers are mounted just before the rear axle and attached to the frame below the seat rail.
I'm lacking confidence in my FEA and having problems calculating the forces. Two things that I want to FEA on this swingarm are:
-when driving through potholes, I want to make sure that when the suspension travels in bump that it is strong enough.
To analyzed this, I assumed a weight bias of 0.7 and used a magnification factor of 2 for the amount of force that the mounting point of the rear axle will receive. Is 2 a reasonable factor to use for this application? I feel that it is a bit too low but have no idea how to calculate it.
250*0.7*2=350
Anyways, I fixed the pivot point of the swingarm and the two shock mounts on the swing arm. Then I applied the force of 350 on the axle mount.
-when driving through bumps where the tire contact patch is in contact with the road in a non-centered part of the wheel, I want to make sure that the swingarm is stiff enough when it is being twisted.
I modeled the axle in CAD and defined the contact between the axle and the swingarm. I then offset the force of 350 pounds from the middle of the tire by half the tire thickness.
I feel that I should have learned all this in school but I didn't. Thanks to everyone.
I'm in the process of designing a swingarm for a 250 pound motorcycle. The kind of a swingarm is the traditional types found on cruisers. Wikipedia calls it:
Swinging fork - the original version consisting of a pair of parallel pipes holding the rear axle at one end and pivoting at the other. A pair of shock absorbers are mounted just before the rear axle and attached to the frame below the seat rail.
I'm lacking confidence in my FEA and having problems calculating the forces. Two things that I want to FEA on this swingarm are:
-when driving through potholes, I want to make sure that when the suspension travels in bump that it is strong enough.
To analyzed this, I assumed a weight bias of 0.7 and used a magnification factor of 2 for the amount of force that the mounting point of the rear axle will receive. Is 2 a reasonable factor to use for this application? I feel that it is a bit too low but have no idea how to calculate it.
250*0.7*2=350
Anyways, I fixed the pivot point of the swingarm and the two shock mounts on the swing arm. Then I applied the force of 350 on the axle mount.
-when driving through bumps where the tire contact patch is in contact with the road in a non-centered part of the wheel, I want to make sure that the swingarm is stiff enough when it is being twisted.
I modeled the axle in CAD and defined the contact between the axle and the swingarm. I then offset the force of 350 pounds from the middle of the tire by half the tire thickness.
I feel that I should have learned all this in school but I didn't. Thanks to everyone.





RE: Designing a Motorcycle Rear Fork/Swingarm
Another thing is that when the bike is cornering, the force is the vector total of gravity plus the cornering G-force, and in sport bikes, the cornering G-force can be easily 1.0 and possibly 1.2-ish, and on top of *that* you should apply the bump load.
Yet another thing to consider is the force imposed by the final drive (chain / belt pull or torque reactions imposed by a shaft drive). The chain pull can be quite a bit higher than gravity under maximum acceleration. It helps that it is imposed along the length of the swingarm rather than at 90 degrees, but it's still there.
Sport bike swingarms are largely governed by stiffness considerations - just take a look at any late model swingarm and you can see this. No idea what the FEA would say, but it looks like they are designed to be extremely stiff in torsion and ordinary bending, and less stiff in side-to-side bending (going over a bump when leaned wayyy over requires the swingarm to bend sideways slightly). This is a tricky business, though. The swingarm length, pivot height, etc have an influence on the handling.
RE: Designing a Motorcycle Rear Fork/Swingarm
FYI: Vehicle suspensions are typically designed to withstand 3 (racecar) to 5 (street) times the normal tire weight for bump situations and, as Brian mentioned, this includes the passenger(s), cargo, fuel, etc...
RE: Designing a Motorcycle Rear Fork/Swingarm
RE: Designing a Motorcycle Rear Fork/Swingarm
For the swing arm, I had deformation of about 1/16" max but i don't think that matters too much. I think what matters more is the difference in deformation between the left and right leg of the swing arm. Am I on the right path?
The FOS of the swing arm is only 1.6, with the weak spot being at the hole for the axle. I modeled the FEA with the swing arm, bolts and axle parts for the mounting points and assigned contact between them. 1.6 seeems a bit too low to me since I was aiming for something closer to 2-3.
For the chassis, how much deformation is too much? Does 1/32" for the 5g bump test sound like a lot? What about 1/16"?
RE: Designing a Motorcycle Rear Fork/Swingarm
Cheers
Greg Locock
SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.
RE: Designing a Motorcycle Rear Fork/Swingarm
Regards
Pat
See FAQ731-376: Eng-Tips.com Forum Policies for tips on use of eng-tips by professional engineers &
http://eng-tips.com/market.cfm
for site rules
RE: Designing a Motorcycle Rear Fork/Swingarm
The chassis I'm designing is almost identical to the Norton Featherbed chassis due to packaging concerns. It was not surprising to see the head tube area being the weakest.
GregLocock,
Will the deflection be calculated from the dynamic loads then? In other words, if the dynamic loads show a maximum of 2g, would a static load of 2g calculate accurately the amount of deflection?
Or would it be twice the amount shown for dynamic load? In otherwords, if the dynamic load is 1g * mass, would I use 2g * mass then?
If you know the theory behind all this, it would help very much also. Thank you very much.
Horace
RE: Designing a Motorcycle Rear Fork/Swingarm
Cheers
Greg Locock
SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.
RE: Designing a Motorcycle Rear Fork/Swingarm
-when driving through potholes, I want to make sure that when the suspension travels in bump that it is strong enough.
To analyzed this, I assumed a weight bias of 0.7 and used a magnification factor of 2 for the amount of force that the mounting point of the rear axle will receive. Is 2 a reasonable factor to use for this application? I feel that it is a bit too low but have no idea how to calculate it.
<<<
Search this site for discussion on 'bump stops'. It's been covered. .. Or maybe I shouldn't have mentioned it.
>>>
If you know the theory behind all this, it would help very much also. Thank you very much.
<<<
The theory behind all this is that when it breaks, you will go to court and be skinned alive, figuratively. ... unless the plaintiff was a Hell's Angel, in which case it will be literally.
No offense is intended, but you seem to be in over your head, and deeper in it than you think you are.
Mike Halloran
Pembroke Pines, FL, USA