Rear shock length to order? Analyzing Geometry?

[CivilRock]

1982 GS1100 Frame
1985 GS1150 Forks
1992 GSXR1100 Swingarm (mono mount cut off, twin mounts welded on)

I want to upgrade my rear shocks, and my question stems from the length choice I have for replacements. I'm looking at the YSS Eco line, and the C-302 Twin Gas Shock Absorber for $399. They come in lengths from 300 to 340mm. Honestly, I like the looks of a Reservoir/Piggyback shock, but don't really need a super high performance set for the riding I do with this bike.

What I'd like to do is measure the bike as scientifically as possible then plug that data into some sweet programming and have it spit out what I should change.

Am I dreaming?

Normally I just make slight or drastic adjustments like any other road racer would do to try and make things better. But on this bike, there's no real baseline, and I don't know if I'm even in the ballpark. And before I spend $400 on shocks, it would be sweet to get a length that would improve the bike's handling, or at least be appropriate.

Thanks for your input. Also, is there a better shock for the money than YSS?

-Kevin

 

[posplayr]

I like the height adjustment of the SU-145 Ohlins. 1" in the rear in not excessive. I think I get 30-50 mm extra height.

* SU-145 Ohlins (stock length + 30-50mm)

 

[CivilRock]

TEC Shocks

TEC Shocks

Grand Rouge sent me information about TEC Bike Parts and their shocks for a very reasonable $150 ($129 + $21). I'm still trying to get in touch with Dave, US distributor, and sent an email to George in the UK. Don't know much more than the price and the pictures and two rave reviews.

It looks like they're a pretty new company, less than a year old, and already expanding the retail side of things in the UK. TEC Bike Parts is a UK company, and the shocks are outsourced to Taiwan. They do look a whole lot like the RFY units on eBay for $100. It's hard to find some quality analysis, and the quick judgement "China = Crap" flourishes on internet forums.

There's a fairly long installation guide here on a Triumph: http://youtu.be/zrn-O-B_BTI
The best quote was at 5:40-
"Remember, this thing is a dog pile Triumph Scrambler. It's 53 horsepower (of mayhem?). It's not really going to need Ohlins is it? That's like putting Nike Trainers on your Granny. It doesn't really make her run any faster does it? Although they look cool." :clap:

Grand Rouge also pointed me in the direction of Chris Livengood http://chrislivengood.net/wp/?cat=61 who actually bought several of these cheap shock brands, took them apart, tested them, compared them to top of the line (Ohlins/Penske) and found several startling discoveries. This is just the information that I love.

This guy Chris has looked at the RFY 1.0 and 2.0 (2.0 being what he calls Model 2 with adjustable rebound) and the TEC shocks, but hasn't taken the TEC units apart yet. There's some discussion about the functionality of the 2.0 reservoir and if the TEC is any better than an RFY shock. I'm going to wait to see what more I can learn about these. I'm the kind of guy that would love to buy shocks for $100, tear them down, put the right oil in them, set them up correctly and have a great shock for cheap.

I especially enjoyed the Model 2 review and the comments section: http://chrislivengood.net/wp/?p=1304

-Kevin

Here's the TEC shock:

Here's the RFY 1.0 apart:

Here's the RFY 2.0 w/ adj reb:

Here's the TEC on GR's beautiful one owner '83 GS1100E:

 

[posplayr]

That is very interesting on those shocks and especially the adjustable ones with inoperable bladder. It dosent take long to figure out that if someone (e.g. Ohlins) is sell a pair of shocks for $1500 then the Chinese will try and knock it off for 1/10 of the price. If they (Chinese) are not there yet then they are very close and will be soon.

There were piggyback shocks for mopeds on ebay a while back (a few years ago)and they has serious structural problems with the lower clevis. It would break. These seem much closer to being a great alternative. All of the traditional shock manufacturers are gonna be doing some belt tightening.,

 

[Guest]

...What I'd like to do is measure the bike as scientifically as possible then plug that data into some sweet programming and have it spit out what I should change.

Am I dreaming? ...

Yup! You're dreaming. You're looking for an engineering equation. Look through a few engineering books and try to get a feeling for a few of the equations. Fudge factors are common, because guided experience works best in most circumstances.

Factory teams make changes based on seat of the pants evaluations. Tbey have the best engineers for suspensions.

 

[CivilRock]

Yup! You're dreaming. You're looking for an engineering equation. Look through a few engineering books and try to get a feeling for a few of the equations. Fudge factors are common, because guided experience works best in most circumstances.

Factory teams make changes based on seat of the pants evaluations. They have the best engineers for suspensions.

I agree that fine tuning is mostly trial and error. If I had a bike that was designed by Suzuki, I would believe that my tinkering would be "fine tuning". But I have 3 different bikes in one. I have no idea if the end result is close to the average of those three bikes, or if it adds up to some funky geometry.

Check out Tony Foale's full software:
http://www.tonyfoale.com/suskin/MC_setup.htm

And here's a bunch of free calculators:
http://www.tonyfoale.com/software.htm

Here's what I believe; I think my trail is too long. This is just a guess. It's probably a good thing for drag racing, but not very good for riding around town. My bike really wants to go straight, and it takes a lot of input to make it lean over and turn. Either a lot of steering, or a lot of my butt off to the side.

I can't (or won't) change my frame rake or triple offset, but I can make minor adjustments of my fork height in the triples (easy), the rear axle in the swingarm (chain length), and the ride height in the rear (shock length - with aftermarket shocks).

My first goal is to somehow measure my bike with some degree of accuracy. Not sure how to do that yet. I'll plug those values into Tony's software and see what I've got. Then I plan on comparing those measurements to different factory bike specs and see where they fall. If there's an easy adjustment within my abilities, I'm going to make it.

Maybe I am dreaming, but I'm going to see if I can put some science to it.

-Kevin

 

[posplayr]

I agree that fine tuning is mostly trial and error. If I had a bike that was designed by Suzuki, I would believe that my tinkering would be "fine tuning". But I have 3 different bikes in one. I have no idea if the end result is close to the average of those three bikes, or if it adds up to some funky geometry.

Check out Tony Foale's full software:
http://www.tonyfoale.com/suskin/MC_setup.htm

And here's a bunch of free calculators:
http://www.tonyfoale.com/software.htm

Here's what I believe; I think my trail is too long. This is just a guess. It's probably a good thing for drag racing, but not very good for riding around town. My bike really wants to go straight, and it takes a lot of input to make it lean over and turn. Either a lot of steering, or a lot of my butt off to the side.

I can't (or won't) change my frame rake or triple offset, but I can make minor adjustments of my fork height in the triples (easy), the rear axle in the swingarm (chain length), and the ride height in the rear (shock length - with aftermarket shocks).

My first goal is to somehow measure my bike with some degree of accuracy. Not sure how to do that yet. I'll plug those values into Tony's software and see what I've got. Then I plan on comparing those measurements to different factory bike specs and see where they fall. If there's an easy adjustment within my abilities, I'm going to make it.

Maybe I am dreaming, but I'm going to see if I can put some science to it.

-Kevin

I gave you a starting point in that spreadsheet (I think I sent), it includes a trail calculation and various adjustments to the EZ/ED parameters, but interested to see if there is something new to be calculated.

 

[CivilRock]

I gave you a starting point in that spreadsheet (I think I sent), it includes a trail calculation and various adjustments to the EZ/ED parameters, but interested to see if there is something new to be calculated.

Yes, you did. Sorry, I didn't really look at it past the ground clearance section.

I'm reading about how to accurately measure bikes now. (Without hiring a professional) I think I'm going to try it with a tape measure, and also with a digital camera and SketchUp.

Do you know of some secret and easy method that I'm not thinking about (or finding on the internets)?

 

[posplayr]

Yes, you did. Sorry, I didn't really look at it past the ground clearance section.

I'm reading about how to accurately measure bikes now. (Without hiring a professional) I think I'm going to try it with a tape measure, and also with a digital camera and SketchUp.

Do you know of some secret and easy method that I'm not thinking about (or finding on the internets)?

My secret method was to do deltas off the factory specs which inherently limits the errors I can introduce. I did check to make sure that my formulas reproduced the factory spec trail. There was a trail tab if you look. I think the spreadsheet values are linked to the main one.

I'm not sure what other parameters you might be after. As I recall, trail and belly clearance were the only things I was after.

 

[RichDesmond]

Shock length is arguably the single most important parameter when setting up a bike. It affects SA angle, rake, trail, CoG (both height and F/R distribution), ground clearance, wheelbase, etc...
Because it impacts so many things, and in interrelated ways, it's very hard to say "This is the magic number!!", especially just working from a calculator. You really need experience with that particular bike to know what works and what doesn't.

When we're setting up a track bike for the first time, with no experience on that model, SOP is to start increasing shock length until either the front end start to want to tuck and/or loses feel (too little trail) or the rear end wants to step out on corner exit (too much SA angle). That procedure works if the stock setup has too much trail and too little SA angle, which is true most of the time.

 

[CivilRock]

So I did some measurements as best I could with what I have, dug up the law of cosines, and used Tony Foale's steering geometry calculator and came up with the following:

Wheelbase: 1568.5 mm (61.75")
Head angle: 60.9 (caster: 29.1)
Trail: 125.8 mm (4.95")
Shock: 324 mm (12.75")
Front: 254 lbs.
Rear: 261 lbs.
Total: 515 lbs. (full tank of fuel, read to ride)

I found some data for Yamaha Models for 2006 as a comparison:
(Wheelbase/caster/trail)
R1 1415 24 97 Super Sport
FZ1 1460 25 109 Sport
FJR 1550 26 109 Sport Touring
XV 1715 31 152 Cruiser

Here's some info that I stole from a Ducati forum as a guideline:
For every 1mm increase in rear ride height:
Trail decreases 0.4 mm
Wheelbase decreases 0.2 mm
Height of the bike’s center of gravity increases 0.8 mm
Percent of the bike’s weight on the front wheel increases 0.03 percent

For every 1mm that you raise the forks in the triple clamps (lowering the front end):
Trail decreases 0.2mm
Wheelbase decreases 0.5mm
Height of the bike’s center of gravity decreases 0.4mm
Percent of the bike’s weight on the front wheel increases 0.06 percent

From that same site the Ducati Monster is 24 deg and 94mm trail.

I don't think I'm too far off because the measurements I came up with are in line with how the bike feels. I'd like to decrease the trail by about 17 mm and get it in line with a sport and ST bikes at 109mm. If I went 20 mm taller in the rear, and raised the forks 10 mm in the triples that should net me (8+2) 10 mm less trail at 116. Not quite 109 mm, but I'm not sure how much longer shocks I should get, and how far I can move the 1150 fork tubes.

All this math is just an exercise in trying to put some science behind what I know to be the real test. But it makes me feel better if the math backs up what I'm feeling "seat of the pants".

If you can read all this, digest it, and give me some feedback, I'd love to hear it.

-Kevin

 

[CivilRock]

I like the height adjustment of the SU-145 Ohlins. 1" in the rear in not excessive. I think I get 30-50 mm extra height.

After all that number crunching, the first response was a suggestion of 30-50 mm extra rear height by posplayr. That's just what I need.

-Kevin

 

[posplayr]

So I did some measurements as best I could with what I have, dug up the law of cosines, and used Tony Foale's steering geometry calculator and came up with the following:

Wheelbase: 1568.5 mm (61.75") Stock was 59.4
Head angle: 60.9 (caster: 29.1) (Stock is 62 deg)
Trail: 125.8 mm (4.95") (stock is 116 mm)
Shock: 324 mm (12.75") (stock they are 300mm eye to eye IIRC)
Front: 254 lbs.
Rear: 261 lbs.
Total: 515 lbs. (full tank of fuel, read to ride)

I found some data for Yamaha Models for 2006 as a comparison:
(Wheelbase/caster/trail)
R1 1415 24 97 Super Sport
FZ1 1460 25 109 Sport
FJR 1550 26 109 Sport Touring
XV 1715 31 152 Cruiser

Here's some info that I stole from a Ducati forum as a guideline:
For every 1mm increase in rear ride height:
Trail decreases 0.4 mm (This type of sensitivity analysis should be easy to determine once you have a spreadsheet or other model)
Wheelbase decreases 0.2 mm
Height of the bike’s center of gravity increases 0.8 mm
Percent of the bike’s weight on the front wheel increases 0.03 percent

For every 1mm that you raise the forks in the triple clamps (lowering the front end):
Trail decreases 0.2mm
Wheelbase decreases 0.5mm
Height of the bike’s center of gravity decreases 0.4mm
Percent of the bike’s weight on the front wheel increases 0.06 percent

From that same site the Ducati Monster is 24 deg and 94mm trail.

I don't think I'm too far off because the measurements I came up with are in line with how the bike feels. I'd like to decrease the trail by about 17 mm and get it in line with a sport and ST bikes at 109mm. If I went 20 mm taller in the rear, and raised the forks 10 mm in the triples that should net me (8+2) 10 mm less trail at 116. Not quite 109 mm, but I'm not sure how much longer shocks I should get, and how far I can move the 1150 fork tubes.

As i mentioned above your bike stock from the factory was 116 mm trail. I figure my GS1100ED with 88 1100GSXR forks went to 119.6 mm with all of my mods.

According to the numbers I have in the spreadsheet if your go to 17" wheels and USD GSXR 750 forks then you can drop down to 111 mm. The issue will be ground clearance.

All this math is just an exercise in trying to put some science behind what I know to be the real test. But it makes me feel better if the math backs up what I'm feeling "seat of the pants".

If you can read all this, digest it, and give me some feedback, I'd love to hear it.

-Kevin

BTW you will need a more modern fork to lower the triple on the fork. A cartridge type fork deals with the road better and you don't need as much travel. The old forks will bottom out sooner and/or you will hit your pipes.

 

[Guest]

When I look at RFY shocks on Ebay I can't tell which is Model 1 & which are 2's. The 2's look like the one to avoid according to
http://chrislivengood.net/wp/?cat=61
so I only want Model 1.
Is there a way to tell the difference? Which ones should I order to fit my gs 1100's?

 

[CivilRock]

I went for an extended ride today to pick up my Series R/R from Polaris, and my connector lead from Triumph. Before I went out I checked my tire pressure. This is my 2nd 30+ mile test since I swapped the motor, and for some reason I didn't check the tires for the first ride. Front was at 22, rear 17 psi. I remembered that I had lowered the rear to 20 for drag racing, and never aired it back up.

So dumb. I spent an hour measuring the bike, and 2-3 hours looking up stuff and analyzing numbers. Plus who knows how much time reading and trying to educate myself on motorcycle geometry.

I aired up both front and back to 30psi and it's like a new machine!!! :-\\\ We used to run 28-29 psi on Pirellis road racing, and I think they're running 24-25 now. Even though the tires say 32-36. So I settled on 30 psi.

It still doesn't turn in like a sportbike, but sheesh is that wallowing feeling all but gone. I still want some fancy reservoir shocks on the rear.

I'm a perfect example of "analysis paralysis". I don't even know if that's the right phrase, but when I was a kid I completely took apart a lawnmower one time only to realize in the end that it was out of gas. I feel the same way right now.

Keep It Simple Sweetheart.

I heard a secret formula for a sweet setup is SV forks with F2 tubes. Apparently my 1150 forks are dogpile in the best condition, and it's way cheaper/easier to just swap in newer technology to begin with, and go from there.
I just spent so much time finding the right rotors, calipers, wheels, spacers, bushings, etc. to make the 1150 front end work; I have to decide how much this means to me.

-Kevin

 

[CivilRock]

When I look at RFY shocks on Ebay I can't tell which is Model 1 & which are 2's. The 2's look like the one to avoid according to
http://chrislivengood.net/wp/?cat=61
so I only want Model 1.
Is there a way to tell the difference? Which ones should I order to fit my gs 1100's?

If you go to Chris Livengood's review of the Model 2, read the comments at the bottom. http://chrislivengood.net/wp/?p=1304#comments There's a guy named Eric that got word from a reseller that RFY has addressed the missing port issue with the Rebound adjustable version 2.0. They can be identified by the amazingly awful color scheme seen in the pic below. The tell tale sign of the Model 2 is the presence of the rebound adjustment clicker just above the clevis.

If I were to buy these, I would fully disassemble them, check for obvious flaws, assembly errors, and missing components. I've read several reviews on them and come to the conclusion that they are a good unit if you're lucky. The quality control is apparently non-existent. Look for casting flaws, machining errors, assembly missteps, and even missing ports. Plan on replacing the fluid and getting them charged correctly with Nitrogen.

-Kevin

 

[CivilRock]

posplayr,
I really appreciate your numbers.

I think were my situation might be different is with the GSXR1100 swingarm I have on my bike. It puts my rear axle about 4" back from the stock GS point.

As I've learned, change in wheelbase alone will change the trail and rake numbers. I think the slight change in what I see from stock reflects that longer swingarm.

On another note: I noticed at the drag strip a lot of the guys were using straps to hold down the front ends on their street bikes. It makes sense right?
Backstory: So I raced Supermoto for many years and we all had "holeshot" devices borrowed from the MX industry. It's a little clip on your fork protector that holds down the forks until you hit the brakes (or slam down a wheelie). I used one, and it made a huge difference in my starts. We had a race with a really long start and several people had violent crashes at the end of the straight before the braking zone, and we finally figured out that the holeshot device was creating a very unstable geometry at speed. Now that I have a grip on the idea of "Trail". I believe they were basically eliminating the bike's trail and tank slapping themselves to the ground at 70+ mph.

Would these straps create the same dangerous lack of trail on an R1? Maybe not. The MX holeshot device would take out 8-10 inches of travel on a dirt bike, and only 2-2.5 on a street bike. Just my guess.

-Kevin

 

[posplayr]

posplayr,
I really appreciate your numbers.

I think were my situation might be different is with the GSXR1100 swingarm I have on my bike. It puts my rear axle about 4" back from the stock GS point.

As I've learned, change in wheelbase alone will change the trail and rake numbers. I think the slight change in what I see from stock reflects that longer swingarm.

On another note: I noticed at the drag strip a lot of the guys were using straps to hold down the front ends on their street bikes. It makes sense right?
Backstory: So I raced Supermoto for many years and we all had "holeshot" devices borrowed from the MX industry. It's a little clip on your fork protector that holds down the forks until you hit the brakes (or slam down a wheelie). I used one, and it made a huge difference in my starts. We had a race with a really long start and several people had violent crashes at the end of the straight before the braking zone, and we finally figured out that the holeshot device was creating a very unstable geometry at speed. Now that I have a grip on the idea of "Trail". I believe they were basically eliminating the bike's trail and tank slapping themselves to the ground at 70+ mph.

Would these straps create the same dangerous lack of trail on an R1? Maybe not. The MX holeshot device would take out 8-10 inches of travel on a dirt bike, and only 2-2.5 on a street bike. Just my guess.

-Kevin

If you go into the spreadsheet on the Trail tab, there is a column for wheelbase. Make your adjustment based on increase in swing arm length and you will see the "Rake" angle change based on wheelbase and "rear Lift" (i.e. shock extension from stock). The Rake angle is added to the "Head" angle to compute trail.

Nominal wheel base does not even enter into the equation unless you couple it with a rise in the rear or rear shock extension. Even then it is the shock extension that dominates in the change of trail.

I updated the spreadsheet a little to see where you are (I'll forward it).

To achieve this:

I'd like to decrease the trail by about 17 mm and get it in line with a sport and ST bikes at 109mm.

You are close as is unless you really have the front forks at one extreme or another. According to my calculations using the 1150 triple clamp, 17" wheel with 120/70-17 and the other nominal dimensions from the GS1100EZ you have 113mm of trail.

If you raise the tail 0.75" that will give you 109mm of trail :-\\\

Jim

 

[CivilRock]

Thanks. I'm glad you sent the Excel sheet because I wasn't figuring it out.

Jim, you know... sometimes I just have to learn the hard way.

I have about 0.5 mm of fork tube showing on my triple.

I'm going to make sure my new rear shocks can adjust ride height at least .75" (20mm)

I can't seem to get any response from the TEC people. Might have to try something else.

I'm amazed at the formulas behind that little Excel sheet you wrote.

-kevin

 

[posplayr]

Thanks. I'm glad you sent the Excel sheet because I wasn't figuring it out.

Jim, you know... sometimes I just have to learn the hard way.

I have about 0.5 mm of fork tube showing on my triple.

I'm going to make sure my new rear shocks can adjust ride height at least .75" (20mm)

I can't seem to get any response from the TEC people. Might have to try something else.

I'm amazed at the formulas behind that little Excel sheet you wrote.

-kevin

0.75 is the seat height change not the shock change.

Adding 1" to the shock gets you 0.87" at the rear, so look for at least an 1" shock length increase. The Ohlins is 30-50mm


While, I'm far from an Excel guru, back in the late 80's I worked at a company that forbade system engineers from "programming", (i.e, in Fortran, C or Basic) all of which I was fluent in along with some others. So for about 6 years everything had to be done in Lotus 123 (to keep us focused). I'll just make mention we did some pretty interesting stuff for not having a real programming language and learned a few tricks along the way. It was a small company building commercial multi mode air to ground missiles from scratch.