Thanks to Paul Danik I am now the proud owner of a 1974 Jackpiner. Got it home Sun. unloaded it and went riding. As soon as that Jackpiner fired I instantly went from 50 years old to 18, what a wonderful drug these pentons are. This bike has multiple shock positions, moving the bottom mount forward or back and also laying the shock down. Any feedback on which positions might be better or problems with any of the positions. Thanks
richard j allison
Welcome to Piner land!!! If you are going to race it in AHRMA, and have standard length shocks, use the far rearward hole on the bottom and the upright top hole. This is the 5" travel hole. AHRMA should make you put in a plastic travel limiter to restrict the vertical wheel travel to 4". If you use one of the more forward holes which will get you more vertical wheel travel, you will need to use more plastic shims to get back to 4" and this means the shock will actually move less, so you have less control.
I have calculated the length of shock needed to get 4" of vertical wheel travel using the upright top hole and the hole at the axle and it is 15". This position is legal without plastic shims and gives a maximum of shock movement for more control. Using the hole over the axle also angle the shock backwards more than using any of the other holes and I think this makes it react better to abrupt changes. Don't forget to check the spring rate and make sure it is appropriate for your body weight and the angle of the shock. The more angled the shock, the heavier the spring needs to be.
If you are not going to race, ignor all the above and get the most travel you can.
Teddy
P.S. You probably should have posted this in the Penton Talk forum. TL
Thanks for the info, for some reason it will not allow me to log onto the Penton talk forum. If somebody knows what I'm doing wrong would appreciate it. not very computer literate Thanks
richard j allison
Ted & Teddy,
You must measure the travel of the rear axle to get the actual wheel travel no matter what length of shocks or how many shims you use:). Don't forget the triangulation angle involved. Just a heads up from someone who used to challenge AMA rule book with what it forgot to say on what I (we) could or couldn't do.
Just my take, and the information is probably worth what you paid for it[^].
Ron Carbaugh
Ted:Correct me if I am wrong. Isn't it actual axle travel that is measured? Since I have never built a bike strictly to adhere to ARHMA rules this is important to me! I have several aluminum swingarm set ups for early Pentons that I have measured this way and no one has questioned me.
Ron Carbaugh
Hey guys,
Thanks for reminding me that I forgot to also include the stroke length (exposed shock shaft length) of the 15" shock in my post. Sorry. I will measure it tonight and post it tomorrow.
I bought Ohlins classic shocks with the 15" overall length after my calculation showed that that length would keep the bike at the same elevation. Then at that new shock angle, to get the 4" of vertical axle movement, I calculated the shock travel to accomplish it. I made sure the Ohlins had more than this in exposed shaft length. Then I bought a cylinder of fairly stiff rubber and cut off a piece that was the right length to make the shock shaft's exposed length correct according to the stroke length calculation.
Hopefully I won't forget to look up the calculation and/or measure the shock tonight!!!!
Teddy
Of Course its all what you can get away with.[:p] AHRMA did not notice that a 6 days at Gainesville last week somehow had 3 shock holes in the swingarm showing, with the shocks actually mounted forward into a 4th hole;)
Ask Ron C. to send you a certain spy photo in action.[:p]
I believe I saw that bike.
Lew Mayer
Hey Iron Lew,
Send me an e-mail and I will dig up a few pics.
Tom B
[email protected]
Isn't this fun.[:p]
Ron Carbaugh
As I was leaving for work this morning, I remembered to go to the barn to get the exposed shock shaft length. The Ohlins came with about 4.375" exposed. I put rubber bumper material on the shaft to cut this down to around 3.5". In my haste, I could not find the sheet where I had done all the calculations, so the real exposed length may be plus or minus 1/8" different from 3.5".
With a vertical wheel travel of 4" and exposed shaft of 3.5", the shock is really getting a chance to do some work. Some of the shock mounting angles limit the exposed length to 2.5" for a 4" wheel travel and this means the shock has less time and motion to do it's job.
Yes Ron, this is fun!!!! See you in Texas!!!! Please look me up if I don't find you. I will have a big PENTON banner set up on my popup.
Teddy
Teddy,
Your smile has Penton all over it.[^]
Ron Carbaugh
G'day fellas, this thread is timely as I've just fitted a set of new Works Performance shocks to my 74 250. These are the shocks that WP build for the KTM to mount them in the forward position while keeping to the 4" travel rule. You can take a look at them here http://www.vmxktm.net/docs/74KTM250WP.html.
The exposed shock shaft measures 2", and after measuring with the springs off the rear wheel travel is 3 3/4" plus I'd guess another 1/4" for bumper compression. Best part is that in the lay-forward position the bike sits up like KTM's are supposed to.
Will be racing tomorrow so assuming I get through bike inspection (Oh my Lord, a KTM with lay-down shocks must be illegal [:0]) I'll let you know how it went. ;)
Cheers
VMX1963
Western Australia
//www.vmxktm.net
Thanks for the links to VMXKTM. WOW your bike is fantastic. Let us know how your races went over the weekend.
Tom B
Then there's another shock position that I had'nt thought about when building my Penton with this same frame/swingarm configuration. Wished I had.
Top mounting point being the bolt normally used when mounting the shocks vertical, or the bolt near the seat mounting screw.
And the bottom mounting point being the one furthest back on the swingarm, or just above and in front of the rear axle.
Next time I buy new race shocks, I'm going to change to this set-up. I want to be like Mike....Lenz that is.
Oh and also, if you mount the shocks vertical, or use one of the holes furthest forward on the swingarm. That places the load in the middle or weakest point of the swingarm. The swingarms have been known to twist or bend if ridden hard enough in that position.
Mine has the reinforcement plates welded to the inside of the exsisting swingarm gussets. That trick brought to us by Knute.
Bob
Rob W,
Seems like I remember some of those short travel swing arms (bent)[:o)]. What was so funny after a Moto I came in and was looking at how loose my chain was and never realized my swing arm was bent. Great memories, but not at the time[:p].
Ron Carbaugh
My jackpiner has the same gussets down the swingarm as the 250 of the aussie. Did they put additional gussets on these swingarms?
richard j allison
Heres what I figured out for max shaft travel to give you about 4 inches travel at the wheel in the different positions with the shock in the upright hole on top:
All the way back over the axle 3.4"
next hole forward 3.25"
next 3.1"
next 2.9"
next 2.75"
all the way forward 2.6"
I also use 15 inch shocks in the rearmost hole. If using the other holes you would need 13.5 to 14.25 inch shocks.