OT: Changing capacity of a motorcycle spring - Page 2

# Thread: OT: Changing capacity of a motorcycle spring

1. Titanium
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Originally Posted by 9100
Its a dual rate spring. ................
With that in mind it would seem like I want to weaken the spring on the coils that are on the right (closest together) on the spring below so the spring will compress onto them and stop, leaving the rest of the spring to work as designed.

2014 HONDA CTX700 CTX 700 REAR BACK SHOCK ABSORBER | eBay

Steve

2. Stainless
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Originally Posted by Jim Williams
There is no practical way to calculate the rate of the resulting odd shaped spring, so you would have to complete the task empirically.
If you calculate the I (2nd moment of area) of the round section, then if you take a consistant amount of material off, by either reducing the diameter or putting say a flat on the outside, then recalculating the I will tell you reasonably closely the new rate.

3. Originally Posted by SteveF
With that in mind it would seem like I want to weaken the spring on the coils that are on the right (closest together) on the spring below so the spring will compress onto them and stop, leaving the rest of the spring to work as designed.

2014 HONDA CTX700 CTX 700 REAR BACK SHOCK ABSORBER | eBay

Steve
Sounds reasonable to me. Soften the everyday rumbles but keep most of the maximum bump range. Remember to taper off the flat where it ends so you don't produce a stress riser.

Personally, I ride a hardtail Indian with a sprung Messenger solo saddle.

Bill

4. Titanium
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Originally Posted by 9100
.........

Personally, I ride a hardtail Indian with a sprung Messenger solo saddle.

Bill
LOL - then your back is in FAR better shape than mine.

Steve

5. Plastic
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Get in touch with Cannon Racecraft in Oklahoma. They will make you what ever you want. Last time i got a custom spring from them it was just over \$100.00

6. Titanium
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Part of the reason for wanting to modify the spring is that the ride stiffness of this bike is noted among many owners. As I said initially Penske will make one with a custom spring and a fully adjustable shock that makes the ride vastly better according to those who forked over the \$950 for it. Very few want to take that route simply due to cost. While \$100 isn't much, I'm trying to see what improvements can be made for the least cost and then will post that info to that forum for those who want to pursue it. I'll probably get a custom spring for my bike as part of the experiments.

Steve

7. Originally Posted by SteveF
LOL - then your back is in FAR better shape than mine.

Steve
At times I have had serious back trouble but the ride didn't seem to bother it. A lot of the reason is that the seat is shaped to match the human gluteus maximus, not some idiot's idea of style. It has the original foam rubber pad that has holes in its underside to adjust the compressibility of different zones for max comfort. When I got it, about 1970, the leather cover was bad and I recovered it and sat on it in Canada, most of the states East of the Mississippi, from Brownsville the length of Mexico to Panajachel, Guatemala and back. It has been on three different bikes. I recovered it again a few years ago and expect it to outlast me.

Part of the incentive for riding Indians was the upright seating position, foot boards that I can change position on and the Bowden wire throttle that stays wherever I set it so I can let go and flex my hand once in a while. I hate the modern bikes that have stubby handlebars and foot pegs so high that your legs are crimped up like a jockey's. The old man I used to travel with said a lot of modern riders look like a monkey fucking a football.

Bill

8. Stainless
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Originally Posted by 9100
...The old man I used to travel with said a lot of modern riders look like a monkey fucking a football...
...from behind...

Oh, and my vote is for a new spring--you can get exactly what you want and know that it's right. In spite of what
has been said once you start messing with a spring you're in unexplored territory...

9. Stainless
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Originally Posted by LKeithR
... once you start messing with a spring you're in unexplored territory...
Marc mostly has his act together ... more or less

10. Cast Iron
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A bit of looking and i found this info:

Suspension adjustment? - Honda CTX700 Forum

11. Hot Rolled
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Most modern shock absorbers besides the cheapest utility grade ones have multiple stage damping usually three.What you initially feel is the hydraulic shock resistance to a bump and if not for different stages and unloading valves if a bump is hit fast enough you would feel like you had no suspension at all.

Hydraulic fluid moving through valves is very speed sensitive but is controllable by unloading in stages to prevent momentary lock up.

If you have a shock with no fluid the ride would feel very smooth but would oscillate to being uncontrollable.

Externally adjustable shocks are the best way to tune your ride but are expensive compared to non adjustable ones.In other words changing first stage valving would show more results than spring rate changes for a smoother ride.

12. From a book on chassis design- the purpose of springs and dampers is not to give a smooth ride but to push the tire back in contact with the road as quickly as possible. The damper absorbs the energy put in the spring by the bump and limits the amount the tire bounces off the pavement. Improvement in the ride is secondary.

Bill

13. Hot Rolled
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Quite the opposite Bill.Kind of reminds me of when I sold an elderly gent a new set of shock absorbers (some 50 years ago) for Before this Lincoln.After installing he came back and said they were all defective as they made his car vibrate.Trying to explain to him that his out of balance tires were the problem seemed futile,as the old worn shocks were covering up the problem.

The unsprung to sprung weight of the Lincoln is quite high unlike a motorcycle where it is quite lower so the damping effects are more noticeable.

When a tire hits a bump it deflects to its maximum without very much damping while doing so it accelerates the unsprung weight up towards the sprung weight, the spring absorbs this energy and starts to move the sprung weight which has a lot more inertia than the unsprung weight so there is a time delay before sprung weight moves.When it does that is when the rider feels the impact.With no damping it will not be very harsh at all and varying spring rate small amounts will be hardly noticeable.

Now add a hydraulic shock but block all the valving so that it will not compress.When you hit a bump the spring will not compress and absorb any energy only the tires will.The whole vehicle becomes unsprung weight and the rider will feel the shock almost instantly.

Somewhere between full hydraulic lock and none at all will be a tolerable level of shock that will give a acceptable ride.

The only problem is that value constantly changes depending upon speed and magnitude of impact.That's where variable rate damping comes into play.

If you are able to remove the shock(obviously not on a spring /shock combo) or drain the oil out ride it both ways and you will feel the difference

I believe that the development of motocross shocks(starting in the mid 60's) and off road vehicle suspensions is what started serious improvement in shock absorbers. Before then Monroe offered non gas different jounce/rebound ratios on their race shocks and Koni offered externally adjustable jounce/rebound and Delco had a low pressure gas bag in their shocks.The motocross shocks offered all three components(added high pressure gas).

14. Hot Rolled
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I ment to add that on impact the spring and shock both absorb energy simultaneously,on rebound the shock absorbs the stored spring energy.The shock dissipates the energy in heat.
That is why shocks have a higher rebound rate than a jounce rate,to improve ride quality.
If the rebound rate is to high the shock will actually jack the car down over a bunch of simultaneous bumps.

That's why NASCAR has spec shock packages.At the superspeedways teams were setting the rebound rates so high that the cars were riding on the bump stops to gain an aero advantage.Probably ruining the drivers backs.

15. I should have pointed out that my comments were in reference to race cars. Re sprung to unsprung weight, I was part of a Citroen agency in the late 50s. They had inboard disk brakes in front and a very light suspension system. People credited the nitrogen/oil suspension for the smooth ride but it really was needed because the very soft suspension reacted so much to load that there had to be a compensating mechanism. It pumped in oil to raise the car back up when someone put a heavy load in it. Demonstration rides were great fun. We knew where all the speed bumps were and would drive over them at exceeding 30 MPH while the passenger cringed. All we felt was two tiny bumps. Another of my better memories is going around a cloverleaf in an ID 19 station wagon in good form watching a 4 cylinder Austin Healey trying to keep up with his rear end hanging out and rubber smoke filling the air. I have never driven anything close since but Citroen stopped importing them, probably because of the crash test requirement. Their progressive crumple monocoque frames no doubt saved a lot of lives but they started to crumple in a minor collision.

Bill

16. Aluminum
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I've never owned a Japanese bike that was sprung too stiff - they're usually sprung for a 130lb Japanese guy. Harshness is usually due to poor compression damping which may or may not be adjustable on the OEM shock. As others have mentioned, springs in about any rate and/or length are available in the neighborhood of \$100 from places like Racetech or GP Suspension, but I doubt it will solve your problem. Check your static sag first. I wouldn't change the spring unless the static sag couldn't be set within proper range with the shock at it's least preload.

If the OEM shock doesn't have damping adjustment a cheap route that might actually solve the problem is to find an OEM shock of the right length that does have adjustment. Sportbikes tend to have shocks with compression and rebound adjustment and they sell used on Ebay for \$50-\$100. There are also services that rework OEM shocks to improve the damping and install the correct spring.

17. Titanium
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The carrying capacity of a CTX is 320 lbs so I assume the suspension is designed to not beat to death the spring/shock if two people weighing a combined 320 are riding it. Far more than I alone weigh.

Preload on this bike is adjustable and there is some preload already adjusted onto the shock mount and reducing that is the first thing to try. Some riders report they consider that to be enough "improvement", others not. Hoping to try this out in the very near future.

Steve

18. Titanium
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I've been working up the suspension on my R6, it presented a very harsh ride. The owner had a put in a fancy penske rear and gold valves on the OEM front which had been tuned by the suspension shop. He had obviously been monkeying around in there- front and rear preloads were high and compression damping front and rear were also at the high end of the adjustments. The suspension wasn't fully locked but it didn't move much so anything over 40 was a harsh ride- and the R6 <really> wants to be going 70+

Putting the front back to the shop's setup adjustments and moving the rear damping back to midrange and reducing it preload by 30% or so got the suspension working well, rider sag is reasonable.

If the shock in question has damping adjustments I'd go for them first, likewise preload- before talking about new/modified springs. I put a gixxer rear on my old Bandit which did need respringing, the Bandit's linkage and weight required a stiffer spring. Suspension shop did the new spring and shock overhaul for a couple hundred \$, a night and day improvement over the OEM unit.

All that said, the suspension shop I go to has a side business of modernizing old motorcycle shocks- obsolete unobtainium units with no adjustments etc. He goes in a fixes, revalves, resprings as negotiated with the owner to provide a more modern ride for old bikes. Since some of these shocks are unobtanium, fixing the dead OEM units is the only way to get the bike on the road. Most of his customers are BMW airheads- I've never been an old bike guy but people sure do love them.

19. Diamond
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Ride faster, the suspension will seem softer then...