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Old 03-15-2007, 02:42 PM   #46
R-dubb OP
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Quote:
Originally Posted by Poolside
Are these two angles equal?
- Jim
No, they are not the same.

While the OEM swing angle is about 2 degrees (I think), my angle is a result of setting the swing arm to 6 degrees off horizontal and meeting the top of the fork. It is currently close to zero, relative to the frame. While I like the idea of carrying more sag, it complicates the shock design (more sag = more travel) and, the real problem is a 31 degree rake seems excessive. When I cure the rake issue the swing arm angle relative to the frame will steepen. That is where the balance occurs, unless the fork were to get shorter.
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Old 03-15-2007, 02:54 PM   #47
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Quote:
Originally Posted by R-dubb
No, they are not the same.

While the OEM swing angle is about 2 degrees (I think), my angle is a result of setting the swing arm to 6 degrees off horizontal and meeting the top of the fork. It is currently close to zero, relative to the frame. While I like the idea of carrying more sag, it complicates the shock design (more sag = more travel) and, the real problem is a 31 degree rake seems excessive. When I cure the rake issue the swing arm angle relative to the frame will steepen. That is where the balance occurs, unless the fork were to get shorter.
Can you explain to me why the swingarm angle becoming steeper is a problem? (apart from shockmount design).
It just seems to me that rake/trail is so much more important, handleing-wise... and that rear shock mounting is almost simple in comparison, that you would be better to take liberties with the rear end to make the front correct.

Sorry to bother you with what may be trivial-- I doubt I can be very much help, but worst case, maybe in explaining it to me you will discover some insights. Plus I'm really interested.
thanks,

J
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Old 03-15-2007, 03:46 PM   #48
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Originally Posted by Stagehand
Sorry to bother you with what may be trivial--
J
Not trivial at all. This is the core issue for knowing what the trail figure will turn out to be when it is all said and done. Your challenge is very helpful!

Two things happen when you lift the frame back towards the proper 28 degree rake angle. First and most obvious, the seat gets taller and ground clearance grows, but since it also levels and weight shifts to the front, this could be a good thing! Secondly, the swing arm rakes more steeply. It maxes out at a specific angle relative to the frame. I need to measure that angle. When it is maxed, it physically hits the frame and the drive shaft angle (U-joint) is stretched beyond it's design range. The suspension wont work without sag. Normal sag is 20-25% of the total travel range. I'll make a new diagram tonight. In broad numbers, if the maximum angle is 9 degrees, I can't set ride height at more than about 7, in order to leave enough sag in the equation. Additionally, the more that U-joint is angled, the more it stresses. If 2 degrees is stock, I'm beginning to get uncomfortable at more than about 5. How's that for wishy washy?

Ricky's solution to this issue is to angle the engine within the frame. Thus, the steering head geometry is not effected when the drive line is tilted. That's too hard for this boy to implement. I'm after plug and play.
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Old 03-15-2007, 03:51 PM   #49
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Quote:
Originally Posted by R-dubb
No, they are not the same.

It is currently close to zero, relative to the frame. While I like the idea of carrying more sag, it complicates the shock design (more sag = more travel) and, the real problem is a 31 degree rake seems excessive.

When I cure the rake issue the swing arm angle relative to the frame will steepen. That is where the balance occurs, unless the fork were to get shorter.
Got it.

Let's say you're going to select the rear spring to get as much sag as is practical with the available shock stroke and suspension travel. With no concern for the front suspension.

"When I cure the rake issue . . " Now this is interesting, will you alter the steering head angle on the frame? It does seem reasonable and straightforward enough. I think it's maybe easier than tilting the motor in the frame.

I think it makes sense because it allows you freedom to set up both ends with as little compromise as possible. With not much sag in the rear suspension, traction is sporadic offroad. And 30 of rake in the front is not so good either.

But I don't know if you are thinking of reasonable mods to the frame, let me guess that you are.

If you stand the fork up (decrease rake) the bike gets taller in front alright. But the forks can be progressively raised in the clamps as rake is reduced. Or, if raising the forks isn't possible, you can select a shorter spring to set a lower ride height while keeping the sag in front. (that is a simplification)

I hear the cries of reduced front suspension travel. The travel reduction will be offset though. The fork is already at 31 rake, and because of that the actual vertical wheel travel is only 86% of the fork travel.

So . . . are you cutting the frame Rick? It looks plain enough.

- Jim

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Old 03-15-2007, 04:41 PM   #50
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No, I refuse to cut the steering head off the bike. Too much effort, too risky. This is not new territory. It's called reverse engineering. Been done before, I'm just playing catch up and filling my soul with aquired knowledge.

Cure the rake issue means get it back somewhere closer to 28 degrees without a seat that I can't put my leg over and a driveshaft that won't turn. Dropping the clamps 10-20mm is very much in the cards. Doesn't cost much

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Old 03-15-2007, 05:00 PM   #51
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Quote:
Originally Posted by R-dubb
I'm after plug and play.
Ok, thanks-- thats what I was thinking/hoping- just wanted to be sure.
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Old 03-15-2007, 09:48 PM   #52
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Quote:
Originally Posted by R-dubb
No, I refuse to cut the steering head off the bike. Too much effort, too risky. This is not new territory. It's called reverse engineering. Been done before, I'm just playing catch up and filling my soul with aquired knowledge.

Cure the rake issue means get it back somewhere closer to 28 degrees without a seat that I can't put my leg over and a driveshaft that won't turn. Dropping the clamps 10-20mm is very much in the cards. Doesn't cost much
Oh, I didn't know that others have done these modifications to the stock frame and all.

They did it without taking a tiny section out of the frame to change the head angle?

How did they mount the swingarm?

How did they deal with the right-side-biased CG?

And was any adjustment made to the wheel track? If the rear does not exactly follow the front that is an Ok condition, but the triple clamps will turned a little relative to the headstock when the bike is travelling in a straight line. Typically that condition is resolved by a slight relocation (a slight lean to the left or right, depending) of the steering head.

Moving the head around is at least as easy as welding gussets. And welding gussets will move things around pretty well by itself.

- Jim

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Old 03-15-2007, 10:14 PM   #53
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Quote:
Originally Posted by Poolside


Oh, I didn't know that others have done these modifications to the stock frame and all.

They did it without taking a tiny section out of the frame to change the head angle?

How did they mount the swingarm?

How did they deal with the right-side-biased CG?

And was any adjustment made to the wheel track? If the rear does not exactly follow the front that is an Ok condition, but the triple clamps will turned a little relative to the headstock when the bike is travelling in a straight line. Typically that condition is resolved by a slight relocation (a slight lean to the left or right, depending) of the steering head.

Moving the head around is at least as easy as welding gussets. And welding gussets will move things around pretty well by itself.

- Jim
Jim,

Read the BMW dirtbike thread. Look at the pictures. This is well traveled ground. I'm just sorting data. Most of the devilish details have been developed by those who make a living at it. Not in books or spread all over the internet. It's just not easy to come by. We'll get to the same place others have been. We'll do it by analyzing how it works, thereby making it work right in the process.

No change to the head angle. I'm sure it's been done, but generally, no. My understanding is they mount the oilhead swingarm as far to the right as possible and the wheels probably track with an offset. BTW, I'm not attempting a swingarm transplant on my two copies. One will have an extended mono. The other will have two sides.

Gussets are not going to do any major contortion routine on the frame. With a decent wire fed welder, mild steel, that frame won't even feel it. There is so much mass, I just don't see a lot of distortion in the cards. We'll find out. The amount of welding required is pretty minimal. More importantly, JP knows his shit. Let's go back to better program.
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Old 03-15-2007, 10:17 PM   #54
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Oh, I didn't know that others have done these modifications to the stock frame and all.

Yes, Most of the conversions, including HPN, do not change the stock frame geometry.


They did it without taking a tiny section out of the frame to change the head angle?

All that I have seen remain the same.

How did they mount the swingarm?

If not using a 247 para lever or regular old style S/A, they mostly relace the offset of the rear wheel and stiil except some misalignment front to rear.

How did they deal with the right-side-biased CG?

Not a major factor in their case.

And was any adjustment made to the wheel track? If the rear does not exactly follow the front that is an Ok condition, but the triple clamps will turned a little relative to the headstock when the bike is travelling in a straight line. Typically that condition is resolved by a slight relocation (a slight lean to the left or right, depending) of the steering head.

Bingo, These conversions are primarily for offroad use. Not such a big deal.

Something else most of these have in common, besides a rather high seat height is a lengthened swingarm, usually by 100mm. This raises the rear without creating such a large angle on the driveshaft. The down side is, it makes for a very long wheelbase.

Another common thing to do is add spacers to the dampening rods, about 50mm this shortens the fork height and reduces travel to around 9.5 inches. Big deal.
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Old 03-15-2007, 10:30 PM   #55
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Where was I?

Here is the latest compromise. 8 1/2 degrees of swingarm travel, with sag factored in. I measured my frame. The arm will move 15 degrees before it locks out. That lowest 6 1/2 degrees of range equates to 50mm of vertical travel. If the shock tops out at 40mm, that will be enough sag. Backing into the steering head from that assumption, leaves 29 1/2 degrees of rake. I'm OK with that. I then reduced the head offset to 43mm, just to make sure that trail comes in a little on the high side, leaving room to drop the triple if that becomes desirable. Oh ground clearance grows by 72mm. That also seems about right.

Anyone see something, I'm missing?
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Old 03-16-2007, 05:07 PM   #56
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Quote:
Originally Posted by R-dubb
Here is the latest compromise.
8 1/2 degrees of swingarm travel, with sag factored in.
The arm will move 15 degrees before it locks out.
That lowest 6 1/2 degrees of range equates to 50mm of vertical travel.
If the shock tops out at 40mm, that will be enough sag.
I don't understand how you are applying the terms here Rick.

Can you put some dimensions on the drawing to make it easier for me?

Maybe something like this. It shows the wheels in the sag or 'normally laiden' position. And the rebound and compression strokes.

- Jim

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Old 03-17-2007, 03:41 AM   #57
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Hi guys

This fascinating thread has got me thinking and I'm a little mystified so I hope you don't mind me posing a question with my first post.

The 15 degree angle 'before it locks out' I assume is below horizontal - this doesn't seem much??. This photo isn't the most extreme I've seen but measures about 20 degrees off the photo. I understand that the sharper the angle the more the stress on the UJ but in order to get sufficient rear wheel travel, even with an extended arm it seems you have to give the joint a hard life.
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Old 03-17-2007, 09:20 AM   #58
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Quote:
Originally Posted by Magnon
Hi guys

This fascinating thread has got me thinking and I'm a little mystified so I hope you don't mind me posing a question with my first post.

The 15 degree angle 'before it locks out' I assume is below horizontal - this doesn't seem much??. This photo isn't the most extreme I've seen but measures about 20 degrees off the photo. I understand that the sharper the angle the more the stress on the UJ but in order to get sufficient rear wheel travel, even with an extended arm it seems you have to give the joint a hard life.
R-dubb,
I am not sure what you are saying causes the travel to "Lock Out" either.
As far as I can tell the first thing to make contact is the muffler/rear peg mount. If that's what you are talking about, Cut it. The drive angle will be pretty severe no matter what you do. But all those that have been done before move more than what you are saying.
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Old 03-17-2007, 09:37 AM   #59
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Hey guys,

I was a little mystified by this finding myself. I've got an R80 frame and swing arm on my bench right now. I'll recheck the measurements, post pictures and provide the info Jim drew on the diagram tonight. No time today. It's opening day for Little League!!

later...
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Old 03-17-2007, 03:28 PM   #60
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That's what makes this forum so great. Having a built in sniff test when your own is so full of shit you can't tell the difference.

I assumed the bottom frame rail rides level at a balanced condition. NO..... bad assumption. Here what I did to sort things out

First step. Set the frame angle so the steering head will be 28 degrees off vertical. I think that is a number we can agree on, right? To do this I strap the frame and rotate untill the steering head angle measures 28 degree off plumb with a level.





As you can see the top frame tube (and bottom rails) are sloped. I get 7-1/2 degrees off horizontal. Too bad I can hold the camera straight and balance the level at the same time...



Now, measure the swing ar at full tilt lock-out.



22-1/2 degrees!! See I'm not crazy, thats 15+ 7-1/2. So yes we have more travel potential than I indicated. Way more. My bad.

Here's what the swing arm looks like relative to a level table. (another not so straight picture)



Here's what prevents further rotation. BTW, a few moths ago I saw Sasha's old GS and the lower swing arm bell (paralever) was slightly crushed! Gee, I wonder how that happened?



I'll come back with a few of Jim's measurements later. Time for more baseball.


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