Dogbones - raising rear end

[threejagsteve]

Pursuing this notion of making a pair of shorter dogbones, I measured the distance my centerstand holds the back wheel off the ground (32 mm).

Now, this tire is nearly bald in the middle, so if I add a few mm to allow for a tire that actually has tread, the most I can raise the rear and still have a few mm between the tire and the ground on the centerstand is about 1 in.

The wheelbase is just under 59 in., but the rotational axis of this 'system' is (effectively, anyway) the top of the steering head. For the purpose of this calculation, I estimated the horizontal (x-axis) distance between the top of the steering head and the front axle to be 10". An inch or two either way isn't going to make a significant difference anyway, and I didn't feel like going downstairs to measure it.

Unless I erred in my trigonometric calculations, a 1" rear lift will deliver about 1.1 degrees less rake. So now I wonder - is this small of a change even going to be noticeable?  Will it be worth the bother?

TIA and cheers,
threejagsteve

[rktmanfj]

Pursuing this notion of making a pair of shorter dogbones, I measured the distance my centerstand holds the back wheel off the ground (32 mm).

Now, this tire is nearly bald in the middle, so if I add a few mm to allow for a tire that actually has tread, the most I can raise the rear and still have a few mm between the tire and the ground on the centerstand is about 1 in.

The wheelbase is just under 59 in., but the rotational axis of this 'system' is (effectively, anyway) the top of the steering head. For the purpose of this calculation, I estimated the horizontal (x-axis) distance between the top of the steering head and the front axle to be 10". An inch or two either way isn't going to make a significant difference anyway, and I didn't feel like going downstairs to measure it.

Unless I erred in my trigonometric calculations, a 1" rear lift will deliver about 1.1 degrees less rake. So now I wonder - is this small of a change even going to be noticeable?  Will it be worth the bother?

TIA and cheers,
threejagsteve

I just removed the 'bones, shimmed the r/w where I wanted it to be, then made the new 'bones to fit.  HTH

Randy T
Indy

[Pat Conlon]

Steve, you may want to stiffen up the preload on your soft oem back shock to get the best turn in feedback from the shorter dog bones.  Go ahead and do the bones, it's easy, you can get the steel stock at Lowe's/Home Depot. Cut the strap iron to length and drill the holes. Easy peesey. I did have to special order a metric drill bit for the holes (sorry, I can't remember the size) Hint: when you drill the holes, clamp both pieces together in a vise and drill both pieces at the same time. That way your holes will line up dead nuts with both pieces. *A important point*

Don't over think the mod, it's sooo easy, try it, me thinks you'll like it...Also, no worries about sacrificing high speed stability. I've had both my FJ's to 140++ on open desert roads and they're rock stable, nary a twitch.
Gotta blow the carbon out once and awhile don't 'cha know, flow some gas through those main jets, etc, etc..

BTW, I am also 6'3", it's nice to have a comfortable ride like the FJ huh? I spent a weekend on a GSXR, nice bike, easy to go waaaaay beyond my capabilities, and I needed a chiropractor on Monday....Not so on my FJ...

Enjoy!  Pat

Unless I erred in my trigonometric calculations, a 1" rear lift will deliver about 1.1 degrees less rake. So now I wonder - is this small of a change even going to be noticeable?  Will it be worth the bother?

threejagsteve

[racerman_27410]

Unless I erred in my trigonometric calculations, a 1" rear lift will deliver about 1.1 degrees less rake. So now I wonder - is this small of a change even going to be noticeable?  Will it be worth the bother?

TIA and cheers,
threejagsteve

there have been so many FJ'ers do this mod that the evidence is way more than hearsay.... the mod works.


even as much as i have raised the ride height on my FJ there is no hint of instability at any speed.



KOokaloo!

[Arnie]

Steve,

Its surprising how little a change is necessary for it to be noticable.  You should make only a single change at a time so you'll know which changes make what feel different.

Cheers,
Arnie

[the fan]

Unless I erred in my trigonometric calculations, a 1" rear lift will deliver about 1.1 degrees less rake. So now I wonder - is this small of a change even going to be noticeable?  Will it be worth the bother?

TIA and cheers,
threejagsteve

The rake number is deceiving. Its the decrease in trail that allows for a more nimble chassis. Also you might want to keep in mind that the FJ has a rake of 27.5 degrees and a trail of 112mm stock. A 95 YZF600r has a rake of 25 degrees and 97mm of trail, The 05 R6 has 24.5 degrees and 94mm. see a trend here...

If you reduce the rake of your FJ to 26.4 degrees and the trail to appx 100-105mm (too lazy to measure) the end result will be a much quicker handling machine but not so much so that you lose a significant amount of stability.

The biggest stumbling block I see with the FJ is the spring rates that Yamaha specified for the machine. Installing fork springs is a great mod and easy to do, but unless you raise the tail you will tend to run wide on corner exits due to the overly soft shock spring. This is still a problem when the tail is raised, but is less so due to the quicker geometry. The problem is that you need special tools to safely swap out the stock shock spring.

I will measure a stock spring tonight and see if a replacement is available or even cost effective. If I remember correctly the Penske shock is shipped from Traxxion with a 900lb (16.0kg/mm) so this should be a good match for the 1.0kg/mm fork springs. If I can locate a spring I can change them out (for a very modest fee) in a few minutes if someone were to ship the shock to me. The FJ shock is not serviceable nor is it even close to the performance level of a Penske, but a spring swap would go a long way toward improving the handling of the bike.

[racerman_27410]

Bill,

by raising the rear ride height (via dogbone swap) you are also changing the angle at which the linkage works against the shock spring.   in my experience raising the rea ride height also makes the shock spring feel stiffer due to the difference in leverage..... with my extremely raised ride height we actually went with a way softer spring than what Max thought we were going to have to use..


Kookaloo!

[threejagsteve]

Thanks for all the useful comments - ok, ok, I'm sold!

Yes, Pat, I'd already figured on doing both at the same time for the very reason you mentioned - a vise and a drill press, and a nice, sharp bit! (12 mm, I think somebody mentioned earlier; I'll double-check when I get the old ones off, though.)

the fan, I agree about the stock shock - I've had it on the highest preload setting since Day 1. If the special tools you're talking about are spring compressors, I have a set I got for changing the struts on my car. I doubt they'd handle much more than a couple inches at 900 lb/in., but if that's all the compression that's needed, I could accomplish the swap myself. But where to find an appropriate spring?

Cheers, and thanks again to all!

[ssuv93]

Hello, there is a rear spring from Progressive Suspension for $80 and free shipping on ebay.  It is new.

  http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&item=270430551377&viewitem=&sspagename=STRK%3AMEWAX%3AIT
         
                     Murray

[threejagsteve]

Thanks for thinking of me, Murray!

Checked it out - Sad to say, it only fits up to an '87 (I have a '91)... But maybe somebody else can use it!

Cheers,
threejagsteve

[waricle]

Just what the doctor ordered!
I will be removing the rear wheel to paint it so I will raise the rear while I'm at it!!

[Arnie]

3 Jag -
The free length of the 'up to' 87 spring is only 6.5mm shorter than the free length of the later bikes.  The late model bikes were also much softer sprung than the early bikes, which were still too soft.
To fit this onto your late model FJ all you'd need to do is add a 6.5mm spacer between the spring and the seat.

Cheers,
Arnie

[the fan]

The ID appears to be different between the early and late springs. the late model bikes use a 58mm ID spring while the earlier models have a 62mm ID (numbers appx, based on measuring 2 shocks I have disassembled on my workbench)

What numbers do you have for the free length and I.D. (curious)

[threejagsteve]

Regarding the Progressive spring on eBay, I later noticed that the same seller also has a complete Progressive shock-and-spring unit, also for '87-and-before FJs. The spring by itself looks identical to the spring in the complete unit, so it's also possible that the spring offered by itself is actually a replacement spring for the Progressive shock unit, and might be different from one that'd fit a stock shock.

[Arnie]

The ID appears to be different between the early and late springs. the late model bikes use a 58mm ID spring while the earlier models have a 62mm ID (numbers appx, based on measuring 2 shocks I have disassembled on my workbench)

What numbers do you have for the free length and I.D. (curious)

From the Clymer manual: free length of early springs is 174.5mm and later models 181mm.  I'll check and report back later about ID from the various springs and shocks I have.

Cheers,
Arnie