Tweety

Well... the problem with that assumption is that you haven't accounted for the fact that the bike might not actually pivot around the stand... I'm not sure where the midpoint is, but I'm pretty certain Honda didn't make it that easy for us...

Easiest way to measure rake is using a string with weight to get an absolute vertical plane and then measure out the angle of the fork...

Well, 102 mm vs 97 mm tells me without knowing the rake that you are high in front... since you have the rear up by (linkage * 6 mm) it should be less than stock if you had the fork legs at the equivalent height as stock...

nath981

[QUOTE=Tweety;201486]Well... the problem with that assumption is that you haven't accounted for the fact that the bike might not actually pivot around the stand... I'm not sure where the midpoint is, but I'm pretty certain Honda didn't make it that easy for us...

the sidestand is a couple inches ahead of the rear shock and a couple feet behind the front forks, i guess that's why.

the way I measured trail is by standing the bike upright on its own weight, clamped a small square to the rear of the fork to the floor. Then, stood a contractor's level perpendicular(bubble in the middle) with the back edge through the axle center and measured from the back edge to the square edge touching the floor.

in terms of the rake, the pic on wickipedia is unclear. It appears that the line goes from the front side of the fork to the rear side of the fork. It seems to me that the line should travel along the front side of the fork and then from the center line of the axle perpendicular up and measure that angle. I'll get another source to see before measuring.

thanks, Nathan

Hawkrider

Nathan,

Re: rake and trail, generally speaking, more trail means more stability. This also translates into more difficult turn-in.

Re: quoted post above, this does not sound good. It sounds to me like you have effectively jammed the needle into the base valve and possibly caused some damage. There is a specific procedure for setting the rebound rod so that you have full adjustment range and it does not bind on it's seat. If I'm correct then you will have essentially no rebound bleed and all flow is going through your shimstack. This will not make the bike unrideable because the rebound shimstack is very weak and you should have enough flow to keep you out of trouble.

I can fix most of the problems on your forks when you're ready. The holes in the carts I can fill with epoxy and I should be able to salvage the valves. As for the rebound rod I may have an idea but will depend on the skills of one of our forum members, truckinduc, to make you some new ends.

nath981

whoa! that sucks. I know nothing of that procedure. sounds like I need to go to medical school before operating on patients, huh! I'll pull the cap off and see if there's still an end on the rod.


thanks, Nathan

Tweety

The measurement for trail seems reasonably valid in that way... Should probably be centerline of the fork though to be 100% true if you are using centerline of the axle... And that should make your measurement longer... ie more difference to stock... To simplify you could do front and back edge and calculate the centerline...

Measuring the rake is actually simpler than one might think... It's the angle between an absolute vertical plane and the pivot angle of the fork... It's not anymore complicated than that... one line along the fork pivot (axle in the triple) and one straight down (hence my comment on string and weight) and the angle between them...

Stand the bike upright, hold the hanging string against the axle thingy and measure the angle... Done... Could probably simplify it by sticking something through the hollow in it to measure against...

Tweety

A more motorcycle oriented image...

Truckinduc

wait, what, hmm, who said my name? What do i need to do now? rebound rod end

Tweety

And looking at that I just realized that the measurement for the trail has to be re-done as well...

Hawkrider

Nathan,

Here's the procedure for setting the rebound rod correctly:

Okay, this can be done with the forks on the bike, but it's safer if you use a front and rear stand and take them to a workbench. Keep them upright so no oil drains out. The on-the-bike procedure is in smaller font.

- Back off the preload adjusters, noting the number of turns. This minimizes the spring preload inside the fork.

- Loosen the fork cap. It's 24mm and the fork cap is aluminum, so be careful not to bung it up. Sometimes a paper shop towel between the socket/wrench and the fork cap will prevent marring the finish. It's better to loosen them on the bike even if you're going to take the forks off. Also, you may need to loosen the upper triple clamp before you loosen the cap.

If you're doing this on the bike one fork assembled is enough to hold the bike up while you work on the other one. The front of the bike will drop quite a bit though when the first fork cap is unthreaded. Suggest you have a second person to ensure the bike does not fall over.

- Slide the male tube down enough to see the spacer, washers, and locknut.

On the bike, as the front of the bike drops it will expose the fork internals enough to loosen the locknut

- Loosen the locknut holding the fork cap to the damping rod. There are flats on the bottom of the fork cap to hold it steady. I believe both are 14mm.

- Unscrew the fork cap from the damping rod a few threads, not all the way off.

- Using a flat screwdriver, turn the rebound adjuster ONLY (not the fork cap) such that the bottom of the slot is even with the top of the preload adjuster AND the dot is aligned with the tick mark on the sticker. Now gently screw the fork cap onto the damping rod just enough so the rod end is seated in the valve. You should note some resistance. Ensure the dot and tick mark are still lined up.

- Now carefully tighten the locknut making sure to not turn the fork cap on the damping rod. Reinstall the fork cap. It doesn't need to be tight, just snug. A little oil on the o-ring would be good.

- Check the rebound adjuster to ensure you have at about 3 full turns from fully seated to fully backed off. This may differ since you have ground your rods. If you have limited adjustability loosen the locknut, back out the fork cap exactly 2 rotations and retighten the locknut. Check adjustable range again. Repeat if necessary.

- Reinstall fork on the bike.

- Tighten upper triple clamp

- Reset preload adjuster to previous setting.

FWIW, the service manual mentions nothing of this procedure.

nath981

thanks Tweety

trail 101.6 mm

rake 26 degrees


forks at OEM level, RT 85kg springs, no preload added

rear 6mmspacer, no preload added

thanks, Nathan

nath981

Greg,

as my grandsons often say, "Dude, you rock!"

You're right. I read the shop man as I put things back to make sure and I didn't see anything similar to this.

Tomorrow, I'll take the cap off and inspect the rod tip and see what we have. Let you know.

thanks for the help. nathan

nath981

I believe the procedure you described was exactly my problem. I followed it and it seems to be fine in the garage(too snowy to try). The adjuster is working fine now.

thanks much,

nathan

Hawkrider

Good to hear it worked out for ya!

RCVTR

This forum is so cool!!
Greg, I'm pretty sure you can't find the rebound damper needle adjustment anywhere else.

I set mine to the bottom of the 45 degree chamfer, then screwed the damper rod up until the needle lightly touched the seat. Same idea, but I had to pick something for a reference.

Tweety - Great diagrams. Better than any verbal description

Tweety

Yeah... well they are so good I realised I needed to check my own measurements and make sure I did it right...

And I had... but that was just a coincidence as I really hadn't understood one part myself completely until the diagram spelled it out for me...

gboezio

Well after reading this thread, I went for some measurements... my bike suspension geometry is so out of wack, 28.2° rake and 130 mm trail... I may buy some GSXR forks and CBR swingarm and have some fun. SAG is set to 20mm front, 10 mm back and I have the back 1" higher, I bought this bike new and did not crash in a way to bend anything, I'm wondering if I'm chasing a factory ghost, it never went well, freaking me out many times, turns like a semi truck and slaphappy at higher speed...
There may be something wrong with the rear shock, it does not compress even at the lowest preload setting, but a higher rear should reduce rake and trail.
I think I'm gonna drop the forks to try it out

RCVTR

1 inch high at the back is huge!
It may explain the slaphappy at high speeds part. Drooping the forks will make it worse.
It should turn in quickly, to the point of being twitchy. Have you checked your wheel alignment?

You said you haven't crashed hard enough to bend anything. The fork stanchion tubes are really soft on the VTR forks. It doesn't take much to tweak them.

Rear sag is 10mm, but laden sag is also 10mm? I'm not sure what you're saying.

Changing spring preload, does not change spring rate. the spring is only truly preloaded when the shock is fully extended. Once the shock comes off the stop, the spring length is the same for a given load. Preload only changes where the suspension sits in it's travel range, with that load.

RCVTR

Trail measurement is difficult to do accurately. But what you are trying to do by raising the rear is to increase the rake angle for lighter steering response. You do this at the expense of some trail, which starts to affect stability at high speeds. To get trail back, you would have to change triple clamp offset. If you also raise the forks, you have negated what you are trying to accomplish and set the bike at some unknown attitude. Maybe if you were looking for more ground clearance, it would make sense.

I would go back to the stock position, with the correct spring rate for the forks to get 25mm or so of laden sag, and see what that gets you, with +5mm at the rear.

nath981

I'm not the expert that others on this forum demonstrate, however I can give you my experience with the rear shock. 1) I raised the rear via 6mm spacer on top of the shock. This raised the *** end about 3/4" and it was necessary to shim the sidestand that amount.

The shock was dead for two reasons:a) the top bushing was bound(loosen bolt and lube with silicone spray and tighten; b) footpeg nut from bent-in shifter mount rubbing a groove in the swingarm(result of my last git-off I guess). Large wrecking bar brought it back. Now the shock works freely. If you can't move the rear up and down, don't ride the bike.

Note: I dropped the front end 11mm yesterday and this required the sidestand shim to be reduced to 11mm also.

thanks, Nathan

gboezio

The measured SAG is with me on the the bike in riding position, I have 6mm shims as well, it could be 3/4 inch to 1 inch, the rear swing arm never really compressed well unless I jump a sidewalk or get someone to sit on the back. I'll check the swingarm mounting pivot, there may be something there, but as RCVTR pointed, the spring rate seem to be abnormally high, it just wont compress, I had it with the bike brand new.
The rear shock shims and high side I had on the track was after some pretty nasty tankslappers.
The shim helped a lot on the handling, quicker turn in and never had another wobble, maybe I'm having so much trail and rake angle that I'm running into some other troubles.

I'll take a shot and print some lines on it.
Sorry to pollute this thread, but I felt ppl were already talking about trail and rake

RCVTR

It's all related, so no thread jack IMO.

Nathan brings up a good point. If you shim the shock, it changes the shock angle, which binds the shock bushings. Loosen the top and bottom shock mounting nuts ( the ones that go through the shock) and retighten.

Sounds like you may have other shock issues. I have a stock rear shock with 320 miles on it, if you need it.

Swingarm pivot is probably not bound up, unless something is really jacked.

Increasing rake angle decreases trail. Increasing triple clamp offset also decreases trail.

RCVTR

Ooops. I misstated that.

Decreasing the rake angle makes the forks steeper, which decreases trail. Shimming the shock decreases the rake angle.

nath981

I demo'd my bike and bought it back from the ins. co., so I checked it out pretty well as I repaired it; but I did miss some things in the rear esp. You can losen the swing arm bolt and tap it half way out, or all the way, with a similar sized rod. Check it and lube this and the other rear linkage to make sure they are freely operating.

Also you can check the alignment with simple measurements. I bought 1/4" square steel rod for $2 from a surplus store, cut it into 12" lengths. Grind the edges of the square rods to fit into the axle and swingarm-mount holes. This will allow you to place one rod into each hole(3 on each side) and measure fairly accurately, especially if the square rods are placed in straight and flat so that you can measures edge to edge. I now depend on this method for alignment when doing chain adjustment. You can use the string method to align front and back wheels(board alongside front wheel to account for width differential makes this easy to measure).

It's not laser accurate, but this method will keep you in the ballpark.

nathan

RCVTR

I have a good example of how the spring rate/preload/damping rate relationship works.

the RC51 I bought has an Ohlins shock on it. Most people bolt it in and run it. I wanted to know what the racers do, and it was time for an oil change and service.

I sent the shock to Pettersson Pro Suspension in Anaheim, CA for service and supplied my suited rider weight and told Stig Pettersson that it was to be a track bike runing slicks. I talked with him on the phone (great guy, by the way) and asked him what he would recommend.

He told me that the shock on the RC51 runs 25-30 mm of initial preload on the spring (initial preload is the amount of spring compression when the shock is fully extended). This high preload must be due to the fact that an RC51 needs a high spring force to get the sag numbers right, but if you use a high spring rate, the rear is too stiff.

He also asked me if the bike was harsh in the rear, because the spring rate was 10% too high for my rider weight. I told him that it was, based on the 40 minutes of putting around I did on it. Dean weighs about the same as I do, but somebody got the spring rate wrong for him. I thought he must have had too much compression damping. I dropped two steps in spring rate, from 10.0 kg/mm to 9.0. That's a big change!

So the next thing he said was that because of so much spring preload on an RC51, the standard factory rebound damping was insufficient to control it. Dean had told me that the bike was really sensitive to ride height in the rear and had stability problems under hard braking, if the rear was too high. I believe that too little rebound damping in the rear may have been a contributing factor. With a much lighter spring and restacked rebound valving, as well as a new needle and jet, it should be a different animal.

Talk about a thread, Jack.

RK1

Did you pull up the fork tubes when you installed the Race Tech springs? I think I remember you saying you cut the spacers to 75mm, which would leave you more OAL spring+spacer than stock. Add to that the fact that the heavier spring will raise the front anyhow, I'm wondering if you aren't way high in front.

I weigh same as you and same .85 Race Techs- cut the spacer to 70mm and raised tubes 11mm.

FWIW, I had been running 4mm worth of spacer over the shock and it felt really stiff. I just took out 2mm of it and the rear feels way more compliant. I don't really understand why that is but it seems to have made a real difference.

gboezio

Ok I have done measurements with static sag only, the bike was on the balance point with a 2x4 and some carboard under the stand, I did this quick lol
So far I can measure a static rake angle of 25.8° and a trail of 73mm quite different from my first sloppy measurements, I used an offset of 31mm
Front static SAG is 12mm, rider SAG 27mm
Rear static SAG is 5mm and rider SAG 22mm
Preload is set to min
I need to take a pic with me on the bike to see how it behaves, that small trail may explain the front end instability.

RCVTR

The margin of error in your measurements is probably +/- 5-7 mm an a couple-three degrees, with angular parallax. It's not easy to do accurately.

At least both ends are coming off the stops and seem to be moving. Sag should be in the ballpark. 6mm increase in the rear shock length is a lot. But a lot of people run that much and more. It should be better, now that you've put springs in and decreased the front sag and increased the spring rate a bit.

I think I would run it with the forks in their standard position and see how it goes.

You know that a highside and/or tankslapper can be caused in a perfectly set up bike by being too aggressive with the throttle, right? Just ask Troy Corser, or Jorge Lorenzo, or Noriyuki Haga. I saw Reuben Xaus do it on fresh tires, turn 2 at Laguna Seca on the warm up lap, in '02. Had to sit the race out

gboezio

I have been there, last warmup lap cost me my left side fairing.
I will move that exhaust out of the way and take off the rear shims, I will wait until I get that TIG welder and buy some cheap 304 SS and start having fun.
Then I will be able to lean the bike enough to justify using grippier tires.
I was thinking if that rake angle was of any use, I guess that the trail is the measurement to look at.
The rear shim was to get more lean clearance.
Removing the big leftover stock can will lighten up the back and decrease the riding SAG even more...

RCVTR

I'm with RK1. Your front sag with the adjusters all the way out is on the low side.

I think you should probably have more like 35 mm with the adjuster backed off. (am I right?)

It sounds like you should trim the preload spacer a bit, so you can run in the middle of the range and have some adjustability.

gboezio

They are pretty deep right now, I will try to go down to 35mm and remove the rear shock spacer, but the bike is now track only.
I'm looking to swap a few parts and will at least take care of the exhaust.