I'm playing about with the spring on my bike. I did have a 700 lbs spring on there. often thought it was riding a bit deep in the stroke. Had a chat to a few people and there was a logical suggestion that I try a 750 lbs spring.

Anyway, I would a bit of preload onto the 700 spring and to get the sag right needed 4mm of preload. 4mm of a 700 lbs spring equals (4/25.3 X 700) 110 lbs. Therefore, I bought the 800 lbs spring which seems to give the bike a better poise. However, I'm not really getting full travel even when hitting things as hard as I do.

I then started to wonder if I should have got the 750 spring. However, that would need 2mm of preload to give the right sag (59 lbs).

This is where I want to bounce ideas off someone.

800 lbs spring with 0mm preload, 45 mm into the stroke takes 1417 lbs
750 lbs spring with 0mm preload, 45 mm into the travel takes 1329 lbs
750 lbs spring with 2mm preload, 45 mm into the travel takes 1388 lbs

Percentage wise, not a lot between any of them. Would swapping my 800 for a 750 make much difference? Would I notice a 6% softer spring/6% more travel?

Really? No one's got anything?

I'm not completely sure what you're trying to demonstrate with the numbers here other than question how much difference a 750lb spring might yield versus your 800lb one. I can see your logic in the sag calculations, but I wonder whether there is not a lot more to how a spring performs across the whole stroke of the shock. The force that the spring 'sees' is determined by the leverage and obviously that can change quite a lot throughout the travel of the bike so how it sags might be only part of the story.

What I have been told by the chaps at TF and Mojo in the past, is that springs tend to cover quite a relatively broad range of weights. For example I was 110kg and lost 10kg and was advised that the spring I had was still good. If that's the case then if you're on an 800lb and should be on a 750lb, it would suggest that there will be quite a difference.

of course there is really only one way to find out. Steel springs aren't particularly expensive and you should be able to get a credit against your existing one.

So,

700lb spring was too soft
800lb spring was too firm
your not sure if a 750lb spring will be just right.

You're not goldilocks are you?

Jam that's a bit harsh but terribly funny 😆

The calculations seem right I'd use a conversion factor of 25.4 for mm to inches ( 😕 )
The rate of the springs may not be that accurate so your 800 could be an 815lbs/in. I'd try a 750. The other thing if your having to run such a high spring rate to get the 25-30% sag is that your probably operating the shock outside of the range the damping was designed for, which will make it blow through the travel, unless you've had it tuned, taking in to account the way the linkage works through out the stroke.

tricky questions posted at midnight might not get a quick response as most of the clever people are in bed resting their massive brains.

springs can be a little un-intuitive; your 700lb/inch spring with 4mm of preload is still a 700lb/inch spring.

all you've done with the 4mm preload is put a 110lb 'starting force' on the spring (this is bad as it affects small bump sensitivity – have you wound off the compression damping to compensate?)

it would be helpful to ask what you weighed, what bike you were riding, what shock it had, and the shock stroke, but we don't need to do that; 700lb wasn't enough, 800 too much, i think we can answer the question without too much maths…

(i started, i had graphs and everything. but it's just easier to suggest trying a 750)

I guess I set myself up for some of that. What I was really asking was whehther anyone could see a flaw in my numbers. They suggest that trying the 750 is only going to be 6% softer which doesn't sound like a lot.

I have had the shock tuned for me/the bike/my weight/my riding. The reason I tried the 800 was because I wanted to run minimum preload so I could keep the small bump sensitivity. However, A softer spring with preload has a lower end rate than a stiffer spring without preload. So, while I might lose some sensitivity, there is a suggestion that it would yield more travel.

My next thought is about the bottom out bumper. As I'm not really reaching it. Just wondering if it's worth removing it. I could always use the boost valve to protect the bottom out if needed.

Numbers are good, I get the same answers as you.

I guess any way you run it though… it sounds like you need to try a 750!

I wouldn't take the bottom-out bumper off personally.

Yeah, I guess at £33, it's got to be worth a shot.

As for the bottom out bumper, it kicks in just before my linkage goes from a slight rising rate to a slight falling rate (linkage passes through 90 degrees to the shock).

I was wondering about cutting the bumper down. It's listed a s a two inch shock but even without a spring on it, I can't get more than 45mm travel out of it. I had toyed with the idea of replacing the bumper with a large cross section O-ring for a little mechanical protection (although my issue is that I just can't get full travel).

Was just about to write something intelligent and analytical sounding that I then realised might well be wrong.

I'd try the 750lb if I were you.

Warning not a suspension expert, but I'm good on springs

Preload is only an issueif the load to the point where the spring is fully extended. Less preload would only give you more small bump sensitivity if the you were running the suspension with no sag.

Once your unsagged the maths is simple, each extra lb of load deflects the spring the same amount for the same spring. End of story. So the best bet is the 700 lb spring as that will always defflect more for the same increase in load

Put it another wayall 3 springs have the same force in them when the bike is static with you on it at the same sag. Surely a hit will always deflect the softest spring the most……

Another obvious, if your not getting full travel you need a less stiff spring (or use the least stiff spring you have)

Yeah, I guess at £33, it's got to be worth a shot.

Don't TF do a £10 exchange? so even if 750 is wrong and you go back to 700/800 you're still £13 better off – easy maths 🙂

I don't recommend taking the bottom out stop off, it protects the shock in the event of it bottoming, and some pretty expensive damage.
What are the details of the tune and your air/boost pressure setting and bottom out position e.t.c

It's simples really… have a look at Hooke's Law which states:-

F = -kx

where

F = the force provided by the spring
k = spring rate
x = offset from rest position

This means that the force that a spring pushes back is directly proportional to it's spring rate and the amount it has been squashed.

So, while a 700lb spring with 100lb of preload might give the same force at a single point as an 800lb spring, the sag point in this case, it will require less force for the 700lb spring to move a further x through it's travel than the 800lb spring.

If you look on ebay you might be able to swap it for the cost of the ebay/paypal fees?

I'm running the boost valve collar all the way out (maximum chamber volume) and only 80- psi in the chamber. The problem with the 700 lbs spring was that the bike sat too far into it's travel. Made it ride a bit like a chopper.

I didn't measure the total travel I was getting with the 700, I just took it off because it felt wrong.

Are you saying you couldn't get the correct sag with the 700 lb spring? I was under the impression that you'd got the correct sag with 4mm of preload. Or did it still feel like a shopper even with the correct preload.

If thats the case you don't like the way your bike feels when its using its full travel (or more of its travel)?

Pressure in the boost seems a bit low to me, fully open yes but I'd try more like 150psi which will 'prop' the shock up a bit. So maybe you could use the 700lbs/ in spring.
The problem is not having all the facts. Rider weight, bike, what the tune is/who it's done by and what kind of riding your doing.

P.S I'd never trust anyone that calls themselves a guru or a maverick for that matter 😉

That's the boost valve setting with the 800 lbs spring. On the 700 I had it wound all the way in and maxed out the pressure and it still felt too soggy.