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PostPosted: Sun Jan 25, 2004 8:27 pm 
I need some ideas for the rear suspension of a new project bike I just started. I am planning on running forks and triple clamps from a 1979 CR 250. On the rear I was thinking of running 16.25 inch Works Performance shock and leaving the stock shock mounts where they are. Any ideas will be appreciated.

Derik


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PostPosted: Sun Jan 25, 2004 8:28 pm 
If you use the 76 frame with that project you stand a chance at really messing up the handling of the bike. the rake of the forks on a 79 is balanced for high speed stability and low speed turns. and at the rear with the 16in shocks in the stock location it may bind up the rear travel. But thats what projects are for. hope for the best.


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PostPosted: Sun Jan 25, 2004 8:29 pm 
Yea it's going to be a lot of trial and error. I appreciate all feedback. I am going to use the stock frame, and don't really want to cut or weld it if I can keep from doing so. My theory is that if the wheelbase is kept within an inch or so from stock and if the rear height and front height are similar to stock it should handle....or not!


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PostPosted: Sun Jan 25, 2004 8:29 pm 
I've got a 76 that I have done some suspension work to. I.m no frame expert but here are some ideas. I don't know what lenth the new forks you are using is but as a starting point measure the difference. Also compare the difference in offset with the trees.That is the distance the fork tubes are in front of the neck. Offset affects stability. Since the new front end is going to be substantially longer you can also slide the tubes up through the top tree to keep the bike level. If the bike is taller or shorter in the front it will also effectively change the trail characteristics.Higher or lower. If anything you want to decrease trail. Either keep the bike level or bring the back up a little. To the back. When you increase the shock legnth you will have a looser chain because the increased pivot angle from front of arm to back. Do not just tighten the chain because it will be to tight as you compress past the stock location. Get a spring loaded tensioner to replace the chain guide. Also if you can find an after market swingarm that is 1 inch longer that will increase the hieght of the rear to better match the front. Also according to some magazines testing this model bike, felt that this change would improve stability. Good luck and keep us informed. As you said it will be some trial and error. Blake


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PostPosted: Sun Jan 25, 2004 8:30 pm 
Thanks for the information Blake. You make good sense to me. I do have an extra Swing Arm, and am a good welder/fabricator, but wanted to keep the basic frame geometry the same as stock. I ran a DG shock lay down kit on my old elsinore back in the day and all it did was look cool. The bike handled poorly and was like a motorized pogo stick. I think if I slide the forks up and get the trail right, and add a good pair of slightly longer shocks it will be an improvement over stock. Let me know what you have done to your bike. Thanks

Derik


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PostPosted: Sun Jan 25, 2004 8:31 pm 
As I said, I am no frame expert but I have read a few things. First of all the mods I did on my 76 are suspension only so far. I will try to get some pictures of that 1 sent into the site. I found a NOS FMF fork kit on E-bay from a guy in CA named David Turk. Super nice guy that seems to come up with some hard to find MX stuff. The kit was a set of springs and a set of longer damper rods. This is what increased the travel from 7 to 8 inches. The instructions suggested sliding the tubes up through the trees as not to raise the height of the front end a therefore affect handling characteristics. With the rear I believe the stock shock length is 14 3/4" I put on I think 15 1/2" works shocks. That kept the front and rear the same with out sliding the fork tubes up. I had tried 16" shocks before and thought they were to long. But the rear is still more complicated than the front. There is a guy I believe his name is Richard that has written some stuff on the red racer message board on shock angle and swing arm geometry and shock fade. All interesting stuff. Now some other things, when honda came out with the 73-74 cr250s they were ahead of their time but a lot was changing in the motocross world. They had rear mounted shocks. 16" from the swingarm pivot. If you look at the stock manuals for the 74 and 76 cr250s they have the same front and rear travel 7 and 4.1 inches even though the 76 has forward mounted shocks. If you think about shock angle and you have a vertically mounted shock 4" of shock travel translates into 4" of rear wheel travel. That is how a 76 with a longer shock has no more rear end travel than a 74 with 13 1/2" shocks and rear mounts. By 76 the cr250 was outdated by design. Look at the RM 250 with laydown shocks. Honda didn't even make a cr250 in 77 because it was going to be a joke with out a complete redesign. Look what they had in 78, laydown shocks. But if you take a shock and simply lay it down it will also have different damping characteristics. That is probably why your modified bike handled poorly. The shock has to be designed for the angle it was meant for. If you look at Foxxes manual for their airshox the rebound and damping pressures go up as the angle increases. I actually have a C&J chromemoly arm for my 76 taht is 1 inch longer and has the stock 12" shock mounts. That really kicks the rear end up a lot. I am going to get it welded up with 14 and 16" mounts. That will increase the actual rear end travel and also bring the rear of the bike back down. Wow, I don't think I have ever typed so much. Hope it helps. Blake


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PostPosted: Sun Jan 25, 2004 8:31 pm 
Thanks for the input Blake. I do disagree with you on a few minor points. The 1976 did have 4.1 inches of "Shock Travel" but boasts a whopping 5.4 inches of "rear wheel" travel. The book doesn't differentiate freom the two. I have 1976 Cycle World thats helped a little. A 4.5 inch shock will have only 4.5 inches of travel only if it is mounted on or close to the axel. The closer it is to the pivot point of the swingarm the more travel you get. The 1976 Montesa Cappra and Bultaco Pursang both had vertical forward mounted shocks yet yielded over 8 inches of travel. So my basic theory was to get 16 inch shocks that had around 1.5 to 2.0 more inches of shock travel than stock which would give me roughly another 3-4 inches at the rear wheel. Chew on that and let me now. I am not trying to argue with you just sorting things out in my "design stage"

Thanks

Derik


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PostPosted: Sun Jan 25, 2004 8:32 pm 
Derik,
You are right on the shock position VS. wheel travel. A pair of 16.5" shocks should give you about 9" of rear wheel travel, but be careful of using the later model Elsinore forks. If a knowledgeable inspection tech sees them and knows what they are he may not allow you to run in the Historic class because the forks were from a later time period. Myself, I am using 77 Husky forks. As per the AHRMA rule book all modifications must be made in the intended time period (75-77)
I think the longer wheel base may cause a little difference in the turning charicteristics, but my skill level (as with a lot of people) is not high enough to notice much of a change. I honestly think a lot of us put too much stock in what the magazines said back in the day. If you rode different bikes on a daily basis then you would come to know what the differences are and notice the changes the different rakes and angkes make in the handling, but if you spend all your time on one bike you become accustomed to how it handles and can go faster and faster the more you get used to it. If you end up with a bike that is an absolute bear to turn then let me know because I think yours will be finished before mine will, :lol:


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