Calculating Spring weights for rear suspension for first gen and sidecar

[greg_in_london]

I've been expermenting periodically with suspension settings for the past few years, especially since the air assistance began to leak.

Initially I added a 1cm spacer which helped a little, but was not enough without the air pressure. Then I went to a 650lb spring, before deciding to upgrade. before the new spring arrived I fitted the spacer to the 650lb spring, but the thicker spring became coil bound on bumps in Irish roads and broke the spring seat. I fitted the upgraded shock after we'd limped home.

 

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[Problem with pasting table, please see attached excel worksheet]

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In case it's not clear, the columns are: spring rate, the free length of the spring, the fitted length, the amount of precompression (assuming no sag) and then the force on the suspension.

When I had the new suspension made I asked for it to be a 1/2" longer, so the next column gives the load after that 1/2" of travel - at would be fully extended on the standard shock.

I made a few assumptions (like about seven square inches internal area) to guess the force generated by the air pressure which I estimated as 500lb or 225kg, so the next column gives the combined spring and air pressure at rest (assuming no sag).

 

The last four columns are the equivalent figures at fully compressed, which is important as you need to be sure that springs not only don't become coil bound, but have some movement left so they do not weaken through over-compression.

To work out the air assisted max compression load I have used the same pressure figure. This could be grossly wrong, however, as the pressure may triple on full compression. I don't know for sure, though.

 

At present I have an 850lb/in spring fitted and it sits way too low - with little ground clearance and not much suspension travel left. I'm not sure how much preload I wound on on the coilover Gaz shocks, but it's not enough. I'm guessing that I preloaded it 1/2" and then it compressed another 1 1/2", leaving 15mm, just over 1/2" of movement

(equating to nearly 2" movement at the back wheel) - meaning that static, without a rider, by bike needs about 700kg force, 15-1700lb just to support its own weight. Without a sidecar, or with a lighter one you'd need less, with a trailer you'd need more.

 

In the next couple of days a 1000lb/in spring will arrive and I'll preload it at least 1/2" and prefereably closer to 1".

 

If you're thinking about upgrading your suspension, maybe the above will help - some car suspension manufacturers will find it hard to believe that spring weights as high as this are needed. Coilover springs are much safer to fit than the standard type of shock.

 

WARNING

If you fit a heavier spring to your standard shock, DON'T RELY ON SPRING COMPRESSORS (you won't compress them enough anyway) - make up a jig, preferably with 12 or 14mm studs (4 off) to wind down the spring. If the spring were to break free when fully compressed and if you were too close it could kill you. :240:That's because you have to compress the spring enough to fit the circlip into the groove and with the 650lb spring that's practically a tonne of force on the spring.

Edited July 27, 2008 by greg_in_london
problems pasting table

[greg_in_london]

Sorry - having problems pasting the table - it's fine in the edit window, but goes haywire when I try and preview or save the edit.

 

I'll keep trying.

[greg_in_london]

[sorry - I've given up trying to paste the table - it may be that there's a size limit on tables, perhaps on the number of columns ?]

 

The spring came yesterday - I'm going to try and fit it today.

 

It's a fair size - dia 15mm springs - I took a couple of phots, but couldn't find a ruler to put next to it so I used a mug of tea.

 

The overall length is 8" - 20cm. There are eight coils and each is 15mm, which means I have approximately 120mm of spring and 80mm of gap (just over 3").

 

If you look carefully at the pictures, though, you will see that the flattened off ends of the spring overlap each other top and bottom, reducing the amount of spring movement that I have there. Measuring the gaps between coils gives me figures from 78mm to 90mm depending on where I measure it (the opposite side has more gaps/less spring, if that makes sense).

 

Either way, that means I have about 3" of movement. I need a good 2" for suspension travel (I had my shock made 1/2" overlength, plus 40mm standard travel) and want to leave 10% movement unused to make sure the spring is not damaged/weakened, so the maximum preload I can use will be about 3/4".

 

I'll take a couple more shots as I do it.

[greg_in_london]

Call me a bloody fool.

 

I went to pack a couple of photos and realised I'd left the memory card in the laptop at my girlfriend's when I uploaded the pictures above.

 

I got the shock out and started dismantling it. I went to take a picture with my phone, but the battery was flat.

 

I took the old 850lb spring out and it was a 1000lb spring !!! When did I upgrade that ??? Rang DFaulkener and I'll have to drive down to swap it for a 1200lb/in spring.

 

That seems one heeluva upgrade to me ~404lb/in ~> 1200lb/in, but it matches the change on the front fork springs: 0.4kg/mm (K1) ~> 1.2kg/mm recommended by Sonic Springs and the maths works out. Because the 1200lb spring has 7 coils as opposed to 8 on the 1000lb I'll have more clearance too.

 

Fingers crossed it will work out.

[greg_in_london]

Am I boring everybody here ???

 

I picked up the 1200lb spring today and it has more coil clearance. Hopefully tomorrow ...

[greg_in_london]

Fitted the spring and I have ground clearance again - on a quick test run it feels better, but I'll be happier when the front fork springs from Sonic arrive as well. I'll post a couple of pictures later.

[greg_in_london]

Posting from the Czech Republic - 2,500 miles on.

 

The Sonic Springs just arrived in time and I now have effective front suspension again. Sits much higher with about an inch or so or comprssion before I sit on it and uses three to four inches of movement as I roll over the bumps.

 

The back suspension is riding much better with a much more reasonable amount of ground clearance.

 

The suspension has still struggled with Polish Roads and Czech motorways in particular, but that´s probably down to the weight of the trailer stopping it from working properly.

 

Overall much better.

[DRAGIN TAIL]

Greg,

Well I got my sidecar frame mounted to the bike. I am going to need more suspension help in the rear. What size spring do you recommend? What is the ID of the spring and overall length? Have you thought of removing the current single shock and replacing with two side by side coil over Heavy Duty Shocks. I don't know if there is room for this or not. Just thinking out loud. Thanks, Ronnie

[greg_in_london]

It depends a bit on how happy you are with the standard shock, whether you will have a pillion and how heavy the sidecar is. If you're keeping the standard shock with air assistance I'd try a 6" 650lb, 700lb or 750lb per inch spring.

 

I had another shock made and the 1200lb/inch spring is right for me, but my outfit carries a lot of weight.

[DRAGIN TAIL]

Thanks for the information Greg. I'm making this a winter project. Ron

[GigaWhiskey]

Am I boring everybody here ???

 

 

Not me, you are too deep for me to comment and this thread is not as big as the other you had going! The only thing I could say on that other one was the amount of posts and views, lol.