Aftermarket shock lengths and geometry questions

[28hodge]

I'm looking at getting a new rear shock, but need some help with the lengths and geometry of the various shocks.

First my bike is currently as follows.

Aprilia rs250 rear shock (non ride height version measures 310mm hole to hole)
17" rear wheel
Nc35 dog bone

The shock I am looking at is a wilbers 641, mainly as I can get one out of Germany for a little over £400 thanks to the exchange rates. Plus I have heard good things about them, mct also rate them and they know lots more about suspension than I ever will, so they must be good enough.

The std nc30 wilbers shock is 305mm I can get it with a ride height adjuster that will gain me back the 5mm but it's another £60.

So my questions are as follows -

What's the standard nc30 shock length (is it 295mm)
What is the effect on the ride height for losing this 5mm on the shock length
Do I need to get the ride height to recover the lost 5mm


I'm trying to understand the effects of changing the ride heights and playing with the geometry of the bike, anybody got any good reading or books they would suggest

[lukemillar]

This book:

http://www.amazon.com/Techs-Motorcycle- ... 0760331405

Is one of the best I have read regarding suspension. A lot of talk about different types of shocks/forks with detail drawings and easy to follow explanations including the effects various things have. The how-to's are a little centred around Racetech products but are still pretty useful.

[gavins]

The standard shock length is 305mm. If you run the standard linkage you want a shock length of about 315mm for the track, or if you run the NC35 linkage or a shortened NC30 link then 305-310 is fine. Any longer and i think you get clearance issues with the Swingarm and possible stability issues.

Rick did do an excellent explanation about shock lenght in a thread somewhere but i can't seem to find it now.

[28hodge]

Cheers Gavin,

As I remember from when I did the rs250 shock change on mine, the 5mm extra length on the shock gained me back the ride height I'd lost from swapping the 18" rear to the 17" rear. Then the nc35 dog bone gained me the additional ride height that is desired.

So if I now revert back to a 305mm shock with the wilbers I am losing ride height. We all know the nc30'sits low at the rear as standard.

Looking like I may need to get the additional ride height adjuster added to the shock to keep me as I currently am.

Luke, cheers for that book, downloaded it so will get myself reading through it.

[gavins]

Yea, if you're comfortable how it is now then you probably wan't too keep it the same. If you do want to play around with it i do have an old Fox Twin Clicker shock with adjustable ride hide that was modded to fit the NC30 (i think it was orignally for a cbr600 but not sure) that i used last season before i got my maxton. You're welcome to borrow it for a while, it'll be in my box of bits if you're at brands this weekend.

I managed to find something of what Rick wrote on the subject:

Laying a few misconceptions to rest...

The standard shock length on an NC30/35 is 305mm, not 315mm.

315mm is the length we commonly specify for an aftermarket shock. The 10mm increase when multiplied up by the shock linkage ratio raises rear ride height by around 35mm - this improves cornering clearance and sharpens the steering to just nicely on the quick side of neutral.

If more is good, lots more is not better! A 330mm shock will give a ride height increase of around 75mm and far too much oversteer for comfort - it will make your bike very twitchy, prone to unsettling tankslappers and likely to tuck the front end and dump you in the ditch on tight bends with marginal grip (i.e. almost any `A` road roundabout in the wet...)

Rick

[28hodge]

Cheers Gav,

Appreciate the offer but really this is a winter mod i will have a look at after this season. The only thing that got me looking into rear shocks was when i had a bounce on a few bikes at Oulton, i was amazed how firm the rear of the bikes were. Mine is really supple and soft in comparison.

I'd always kinda thought my suspension was good and pretty well set up. The Sag works out well and i get really nice tyre wear.

I emailed Maxton and they sent me a data sheet back saying that the shock they sell raises the rear ride height by 35mm which ties in with what Rick says. Ideally i'd by a Maxton but at £715 new it just not going to happen.

[gavins]

I can heartily recommend the maxton, which is 315mm and i run with a stock link, but i was lucky enough to pick mine up just serviced from a guy in ireland for a good price due the weak euro.

The wilbers unit is a good shock though from what i hear, i think Colin Martin runs one and hes no slouch!

[28hodge]

yeah its the weak Euro which is making me look at the wilbers as I can get one for sub £500 out of Germany brand new set to my weight/riding style etc and with a 5 year warranty.

with the Maxton can you run the NC35 link or does this cause the shock to foul the swingarm?

[gavins]

I'm not 100% certain. I ran my friends maxton, marked for an NC35, for a round last year with the NC35 link and it was fine but i think a little of the paint got rubbed off the spring. I didn't measure it to see how long it was.

The one i use at the moment is marked for an NC30 so i'm not sure if theres a difference or not. I don't think the seat height was any different between the two but i'm a shorty anyway, so it's heard to say for sure.

The spec sheet for the NC35 does say 'The unit does not have a ride height adjuster as you can not raise the ride height anymore than 15mm. If you try to raise it anymore, the Maxton unit fouls the swing arm.' I don't have the '30 sheet to see if it says the same or not.

[28hodge]

Just checked the data sheet they sent through to me i asked specifically about the nc30.

They said their shock raises the rear by 35mm as anymore isnt possible as it fouls the swingarm. But i would imagine they assume that you are using a NC30 link with this.

I might ask them the direct question though, I am pretty sure 35 and 30 aftermarket shocks are interchangeable.

If possible Gav, can i have a measure up of your swingarm pivot point and a bounce on your suspension at Brands GP?