I need stock XS frame/suspension specs

[scott s]

I've Googled and searched til I'm blue in the face and I still can't find all the information I need.

Can someone give me the stock specs of a '75 XS650B?

Here's what I know:

Neck rake (steering head): 27 degrees
Offset: 1.938"
Fork Length: 29.5"
Wheelbase: 56.5"

What I need clarification on:

Trail: I've seen two numbers for this; 3.9" and 4.47". Which is it?
Tree rake: I've seen "zero" used, but when looking at the triples on my bike, there sure seems to be and offset. Or am I misunderstanding tree rake?

 

[scott s]

OK, here's what I've found so far:

Wheelbase: 56.5" (1,435mm)
Weight: 467lbs
Tire size: 26.1"
Neck Rake: 27 degrees
Fork length: 29.5"

Tree rake: Now I'm confused on this. I find measurements of 0.0", but there's obviously and offset on the triple tree; in relation to the steering stem and the centerline of the hole the forks pass through. I find an "OFFSET" number of 1.938". Are they referring to the tree rake?

Trail: Again, I find two numbers.... 3.9" and 4.47".

I found this diagram, too.

 

[scott s]

Wait...I think I get it.
The triple tree has no rake of it's on. The forks run on a plane parallel to the steering stem. The offset is 1.938", meaning they sit that much "forward" of the centerline of the steering stem.
The rake is 27 degrees and comes completely from the angle of the steering head on the frame. The triples don't add/subtract any additional rake.

 

[59Tebo]

Scott,
Okay, you're getting it. There's a lot to wrap your head around when it comes to suspension geometry. And I almost flunked geometry... You didn't say where you're going with your project, but there's more variables involved. The answers can be found here, written by people a lot smarter than me. Things like the difference between true and false trail, like if you extend (or shorten) your forks, you're changing both the rake and trail figures. Even changing tire sizes can affect these variables. Take your time and do your homework, 'cause the more homework you do, the fewer surprises you get. And the hurrier you go, the behinder you get!

 

[650Skull]

Usually to make a bike steer into corners quicker the the rear of the bike is raised, The front can be lowered as well. The weight is shifted forward onto the front wheel making it more responsive to turns.

Dropping the front or raising the rear will change the trail, the rake comparison between the frame and forks is the same but it diminishes or lengthens the trail when relating it to the ground.
Maybe these pics will help. Gordon Scott used these as an example when he was building his sidecar.

This is not something i have got into but i think i am right in suggesting, the forks your using will decrease the rake and trail so the rear shocks should remain at their standard height for better handling.

As you have said the stem and forks are on the same plane, The off set, (just measured mine), just under 50mm or 2". The leading link pic shows the effect, if the plane was different on the triple tree

 

[XSLeo]

In my Factory manuals from 74 up the bikes trail is 4.53 inches.
Leo

 

[chizler62]

In my Factory manuals from 74 up the bikes trail is 4.53 inches.
Leo

I tossed this number around the only way that it is a real number is if given offset 49.23mm, head tube angle 27, and front wheel diameter of .........28.49"

I have a couple in the shop I just measured. 350/19" They're worn but avg out at 26.25" if I plug in my wheels I get 102.18mm or 4.02"
The math is much deeper but everyone should be able to follow this.

Wheel diameter= 26.25"
Fork Offset= 49.23mm
Head Tube Angle=27°

Trail= Radius*Sin90-head tube angle- Fork Offset
Trail= 333.5mm*Sin90-63°)-49.23mm
Trail= 102.18mm or 4.02"

You'd need a wheel assembly height of 28.49" to get the trail numbers listed in manual.

Trail= 361.87mm*Sin90-63)-49.23mm
Trail= 115.06mm or 4.53"

Using my wheel Radius.
Fork Offset= Radius * Sin90- Head Tube Angle) -Trail
Fork Offset= 335.5mm*Sin90-63°) - 102.18mm
Fork Offset= 49.23mm

Using manuals wheel Radius.
Fork Offset= 361.87mm*Sin90-63°) - 115.06mm
Fork Offset= 49.23mm

I came to the conclusion that they, yamaha, didn't include the tire's contact patch. This proves just that-

Trail= Trail- contact patch*Sin63°)
Trail= 115.06mm*Sin63°)
Trail= 102.52mm

or

Trail= Trail+contact patch/2/Sin63°)
Trail= 102.52mm/Sin63°)
Trail=115.06mm or 4.53"

if you view the above diagram @650Skull post. Image a flat spot on the bottom of the tire where it meets the ground. Divide that in half and draw a line perpendicular to the steering axis. Thats actually the trail drawn and what designers and engineers use.

Don't be confused in thinking you can just lower your forks. If you put a 1" spacer in you head tube angle will change.

New Head Tube Angle= Tan`{WB*Sin63°)÷(WB*Cos63°)-amount lowered}
New Head Tube Angle= Tan`{1435.1mm*Sin63°)÷(1435.1mm*63°) - 25.4mm}
New Head Tube Angle= 63.88°

That's given some knows as rear trail, contact patches, axle to crown race etc

Well kids if you have any questions ask. My planes about to land for the Talladega NASCAR race!

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[chizler62]

What tire are we using? And what's the diameter?

Reason I ask is I see a big changes with the front center of the 71 vs 83 and it obvious three were changes etc

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[chizler62]

I doubled the wheel measurements and I get 26.300"

Anyone else?

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[TwoManyXS1Bs]

This is likely going to be one of those "reverse engineering" areas.

My 70-73 manuals show trail numbers varying in the 99-101mm arena.

Ramble mode engaged:

In 1970, we didn't have 99¢ hand-held calculators. The bikes originated in Japan, in metric. The American marketing branches chose what features to flaunt and publish. Translation and publishing errors were plentiful. Undocumented/secret changes occurred. The market moves rapidly, not enough time to dwell on nit-pik details. The horsepower wars were on, and a lot of interesting numbers were bandied about. Close-guarded secrets of upcoming models.

Handling, hi-speed wobbles, and public perception became issues with the new super bikes. Rumors abounded, need to put out these fires. Magazines received hefty support from advertisers, adding to the slants. For a while, trail numbers were an oft-asked question, almost up there with horsepower numbers. A field rife with marketing and used-car salesmanship.

A lot of these bikes weren't expected to last more than 5 years. Why waste time on last year's model when the money is in next year's model. After all, these things were "made in Japan", a long standing derogatory term, that has been replaced by the modern "made in China". Mass consumerism and "throw-away" economy was the battle cry.

We threw away so much period Honda, Triumph, Harley parts and documentation. Makes you want to find the old trash dumps and dig them up.

Demanding detailed specifics of that old stuff, relevant to today's technology, would be like asking the Wright brothers for a Nastran Finite Element Analysis data dump.

Ramble mode off.

The efforts of members of this and other forums are largely responsible for clarifying the old mysteries.
An ongoing effort...

 

[chizler62]

That makes sense. It appears they changed the front center on the 74? Assuming it was a touch squirrelly at speed with short front center, distance from swing arm pivot to front axle, on the 70-73.
Wobble?

Interestingly when entering sag into the mix. It looks like they used 140lb rider. Insert an average rider today at 220lbs and the wheelbase changes CV, head tube steepens, basically making it further from a solid baseline.

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[TwoManyXS1Bs]

A lot of the early hype was with 80 lb oriental jockeys. Stuff that made it here, with 170 lb magazine writers doing evaluations, got different reports.

The early CB-750 had a wobble reputation, under 4" trail. Magically became 4.25" with the stroke of the pen. Firefighting tactics...

 

[gggGary]

No measurements, just seat of the pants, but we took a 73 and 79 standard out Sunday put on 100 miles swapping back and forth between them. The 79 has most of the common handling "fixes", steering bearings, swing arm bushings, stock fork springs with MikesXS emulators, a 2.15 19 front rim and new stock length redwing shocks, The 73 is all stock and original, at 14K miles no chassis maintenance done. Tires were all recent, decent or better. The difference in handling was not big. Based on 30 years ago XS1B experience I was leery of the 73 but had no wobble issues, damper knob was not tight. I purposely tried to upset it by wiggling the bars at speed but it quickly self damped any oscillation. Both bikes have a slight loose feel, like the front and rear are connected through a "weak" head stock, a bit of a whippy feel in transitions. Nothing that worries but I'd say not "modern" chassis stiffness.

 

[TwoManyXS1Bs]

...The math is much deeper but everyone should be able to follow this.

Wheel diameter= 26.25"
Fork Offset= 49.23mm
Head Tube Angle=27°

Trail= Radius*Sin90-head tube angle- Fork Offset
Trail= 333.5mm*Sin90-63°)-49.23mm
Trail= 102.18mm or 4.02"

You'd need a wheel assembly height of 28.49" to get the trail numbers listed in manual.

Well kids if you have any questions ask.

OK, let's see if we can clear this up a little.

...Trail: I've seen two numbers for this; 3.9" and 4.47". Which is it?

In my Factory manuals from 74 up the bikes trail is 4.53 inches.

The manual's report of 3.9" is a roundup from the XS1/XS1B manual's 98mm report, which would be 3.858".
The manual's report of 3.9" is a roundoff from the TX650A manual's 101mm report, which would be 3.976".
XSleo's 74-up report of 4.53" matches the XS650B/H/J manual's 115mm report, which would be 4.528".

I'd expect that the smaller numbers are True Trail, larger numbers being False Trail.
Not surprising, given the other publishing errors we've seen.

Using:

A = Rake/caster angle (head tube angle) (in this case 27° from vertical)
O = Offset (in this case 1.938")
R = Radius, the vertical plumb-bob distance of the axle center to the ground (Tires squish, best not use diameter)
T = Trail (also called false trail, as measured on the ground)
Tt = True Trail (the lever arm from center of tire contact patch to steering axis centerline)

T = R×tan(A) - O/cos(A)
Tt = T×cos(A)
Also
Tt = R×sin(A) - O

For a range of R values (assuming that Rake/caster Angle remains at 27°)
Radius - Trail - True Trail
12.70" - 4.30" - 3.83"
12.80" - 4.35" - 3.87"
12.90" - 4.40" - 3.92"
13.00" - 4.45" - 3.96"
13.05" - 4.47" - 3.98" <=== as reported by scott s
13.10" - 4.50" - 4.01"
13.15" - 4.53" - 4.03" <=== as reported by XSleo
13.20" - 4.55" - 4.05"
13.30" - 4.60" - 4.10"

Kinda splitting hairs here.

The engineering guys use the True Trail.
The marketing guys (for the public's easy-to-measure on-the-ground) used false Trail...

 

[chizler62]

LOL yeah, my post was thinking out loud on a red eye flight.

As you refer it as true trail. frame builders, those that actual build to customer needs and not just copy, call it mechanical trail.

I guess for practical use its ok.

Measuring from center line of axle to ground doesn't include contact patch length. It is the length of the contract patch, pressure of contact patch, tire deflection, tire force curves and area of patch that formulate into a wheel diameter that you can then plug into that which will result in both front mechanical trail and rear ground trail.
It's that which has the lever affect over the ground.

rear ground trail= Wheelbase + mechanical trail/Sin(Head Tube Angle)

Yeah, its splitting hairs. You go out in your garage and measure front wheel diameter at 26.25" plug that in and get your number. Or take that along with lots of other info and figure mechanical trail at 55mph with a 220lbs rider. The two aren't that far off....but

I can assure you when delivering a custom bike customers LOVE that extra mile even though they haven't a clue.

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[TwoManyXS1Bs]

Here's my setup:

A = 45°
O = 1.938"
R = 12.5"
T = 9.76"
Tt = 6.90"

Just where I like it...

 

[zxpete]

this thread is very helpful thanks to all those adding their research !!