Fork Springs & Shock Absorbers

[David]

Considering changing the fork springs & shock absorbers on Bob's Bike.

Any recommendations or help would be welcome, thanks.

[David]

Emailed several suppliers/manufacturers, Bike Revival www.bikerevival.co.uk/ have replied, offering either UK made Hagon www.hagon-shocks.co.uk/, or US made Progressive www.progressivesuspension.com/.

[David]

Performance Parts Limited www.performanceparts-ltd.com/ can offer Netherlands made Hyperpro springs hyperpro.com/springs/

Hyperpro declined to give any spring rates, won't be buying theirs.

[David]

Waiting for a reply from IKON (UK) ikonshocks.co.uk/, wrt spring ratings before making a decision on what to buy. (The IKON springs are made in Australia.)

Still waiting for IKON to reply, have kicked them in to touch.

IKON have come back with a spring rate of 0.54 to 0.70 kg/mm & a free length of 545mm which is too long, OEM are 540.5mm.

Got the spring rating of the OEM fork springs from Yamaha UK, 0.421 to 0.582 kg/mm

The Hagon (UK) springs are 0.510 to 0.680 kg/mm.

The Progressive (USA) springs are 0.630 to 0.890 kg/mm.

I'm "leaning" towards the Hagon springs, they also make some nice looking shock absorbers with springs rated to the rider's weight.

[David]

Fork spring manufacturers are now quoting "air gap" figures for oil quantity, rather than a volume of oil like Yamaha did in 1980.

Assembly Setup, www.hagon-shocks.co.uk/common/pagedetail.aspx?PageCode=setup#nitro

Got the makings of a DIY fork oil tool.

[David]

Bob's Bike has "old school" damping rod forks, modern bikes have cartridge forks.



Fitting an emulator to damping rod forks produces some of the characteristics of cartridge forks.



John at Alpha Suspension www.alphasuspensionset-up.co.uk/, provided the following info.

"The age of your forks certainly takes them into the bracket of old technology, however, it is not impossible to improve their performance. The question is budget.

My information tells me they are really under sprung and so new springs would bring better support for you and the bike. I don't recall seeing springs that long before so it maybe that bespoke springs are needed. If springs were available off the shelf at the correct rate then they would cost £100.00 / pr, if bespoke springs were required then obviously they are more expensive at £190.00 / pr plus fitting. An alternative may be off the shelf springs and a bespoke set of spacers manufactured, although bespoke springs would probably be better value.

Your forks are the "Damper Rod" type of fork, where damping is changed by changing the oil viscosity. What this mean is that once a known oil viscosity is used the action of the forks can be felt and from that an opinion formed as to whether all is OK or a different grade of oil is required. This is typically focused on the rebound action which means that you then have to take the compression damping as it comes.

For those who want to make their Damper Rod forks more sophisticated it is possible to fit an emulator which then gives a more controlled compression damping action. I haven't checked 2017 prices but I guess that emulators for your bike will be around £165.00 plus fitting."

"Front SAG should be 35 - 43mm for road riding."

[David]

Using the Race Tech spring rate calculator, racetech.com/ProductSearch/2/Yamaha/XJ650R%20Seca/1980-86 it confirms that the stock fork spring rate is 0.42 kg/mm.

Taking in to consideration the rider weight plus bike weight, the Race Tech calculator comes up with a spring rate of 0.77 kg/mm.

John at Alpha Suspension has confirmed that these figures seem reasonable.

This would make the Progressive (USA) springs worth considering.

The Progressive (USA) springs are not the correct length, that rules them out.

[iconic944ss]

John is Mint and has rebuilt the forks on both my bikes now, well impressed.

Any chance you can use uprated forks from a later series bike? If you are not too bothered about total originality???

A buddy fitted Hagon shoks to his race classic and was pleased with the kit and level of service.

[David]

Feb 4, 2017 12:46:23 GMT iconic944ss said:

John is Mint and has rebuilt the forks on both my bikes now, well impressed.

John at Alpha Suspension has been very helpful with his technical advice.
I'm not after having the forks rebuilt, this was done in 2013, with new stanchions, oil & dust seals fitted.

Feb 4, 2017 12:46:23 GMT iconic944ss said:

Any chance you can use uprated forks from a later series bike? If you are not too bothered about total originality?

I'd like to keep the bike looking original in it's appearance, therefore, just looking at internal changes.

Feb 4, 2017 12:46:23 GMT iconic944ss said:

A buddy fitted Hagon shocks to his race classic and was pleased with the kit and level of service.

Hagon are a UK based manufacture with years of experience, that might just tip the balance for me.

[David]

Feb 4, 2017 6:10:16 GMT David said:

Got the makings of a DIY fork oil tool.

Tool manufactured using a syringe, length of plastic tubing, length of copper-nickel brake pipe, a couple of washers & two cord fasteners to set the "air gap" of 160mm.

[David]

Sonic Springs - Spring Rate Calculator, sonicsprings.com/catalog/calculate_spring_rate.php

Using this calculator comes up with 0.80 kg/mm, not far off the Race Tech figure of 0.77 kg/mm.

Setting Sag on Street Bikes, sonicsprings.com/catalog/setting_sag_street_tech_article.php, 30 - 35mm.

[David]

Might have a look at the rear shock absorbers SAG measurement, 30 - 38mm.

[David]

Preload - What does it do? sonicsprings.com/catalog/preload_tech_article.php

"Let's assume that we have a pair of fork springs, each with a rate of 50 lbs/inch. The pair then have a combined rate of 100 lbs/inch."

If this is true, Bob's Bike OEM springs are 0.421 kg/mm x 2 = 0.842 kg/mm.

[David]

Checked/adjusted front & rear rider SAG today, decided to aim for 30 - 35mm F&R.

The front was well out at a minimum of 40mm, no adjustment is available without fitting spacers.
Have fitted spacers to preload the springs in the past without success.
Used the cable tie around the fork stanchion method of measurement.
Feel a set of new springs coming on.



The rear shock absorbers have a 5 position preload adjustment.
In the low position, the SAG was 38mm, adjusted to the mid position, giving a SAG of 30mm.
Leaving further adjustment for luggage and/or a pillion.
Used a length of 2" x 1" timber between the swinging arm & the solid seat trim, plus an assistant, then a tape measure.

Setting the ride height will hopefully allow the suspension to work more efficiently.

Saving myself £30 to check/adjust & £5 to record the results on a piece of paper = £35.
That's over 40% towards the cost of replacement springs.

[David]

Ordered a pair of Hagon fork springs plus oil from Bike Revival, they were a couple of quid cheaper than direct from Hagon. Was quoted 3 weeks delivery.

[David]

Hagon offer both slimline & wideline rear shocks, with 50mm or 60mm OD springs respectively.

The damper rod is 12mm & damper body 28mm in both cases.

The springs can be matched to the rider's weight & overall length adjusted as required. Yamaha OEM are 314mm, Hagon standard are 330mm.

The Yamaha OEM springs are indeed around 317mm long, the OD of the springs is 60mm, the damper rod is 10mm & the damper body is 36mm.

[David]

YSS Suspension www.yssshocks.co.uk/, offer these Bravo twin shocks. They look very much like the Yamaha OEM ones with the 5 step preload adjuster. www.yssusa.com/series-bravo.html

[David]

New Hagon fork springs fitted together with 15w fork oil, the air gap was 160mm.

Rider SAG has been reduced from 40-45mm to 20-25mm.

Just need some new Hagon rear shockers & that's the job jobbed.

Suspension will then be tuned to my personal requirements.

[David]

Spoke to Martin Hagon and ordered a pair of rear shockers.

They are going to be bespoke for my personal requirements.
The standard length is 330mm, Bob Bike needs 320mm.
The Yamaha OEM springs are rated from 17 - 21 kg/cm depending on the 5 step preload setting.
The Hagon standard spring rate is 20 kg/cm based on a rider weight of 75 - 100 kg.
As I'm a bit of a light weight, the shockers are being made with an 18 kg/cm spring rate, 3 position preload adjustment is available.
The springs will be wide 60mm dia., not the slim 50mm dia. ones.

[David]

Fantastic next day delivery from Hagon using UPS.

Hagon shocks fitted and rider SAG set to 30mm with spring preload in the mid position, (was 38mm without any preload, same as Yamaha OEM shocks).