Classic MX Suspension Setup ?

Spent 30 minutes or so at lunch today looking at the suspension on 3 of the Husqvarna’s. Like a lot of riders, probably spend more time tuning engines, fuel and ignition than we do suspension and even then spent 80% of that time looking at the rear suspension. The front suspension on the 360 Automatic was looking a bit wrong at Abbeycwmhir on the weekend, but decided to do some more research, before doing an overhaul.

Most conversation about suspension is whether it is within the regulations

The measurements are as follows, all in mm.  Rear suspension travel was calculated using the ECMO online calculator, and front was measurement/estimate on the front forks.

[table width="500px"]
Bike, Suspension, Travel, Free,Load, Sag
Husqvarna CR250 1970,Rear,108,435,405,30(27%)
,Front,200,775,738,37(15%)
Husqvarna 250 Bolt-up 1966,Rear,115,440,403,37(30%)
,Front,200,750,720,30(27%)
Husqvarna Automatic 1976,Rear,140,420,380,40(26%)
,Front,250,820,775,45(18%)
[/table]

 

Looking at a modern MX suspension set up example, it says that rider sag should be 33% of the total suspension travel, but this is of course for modern bikes that might have 300mm+ suspension travel, where of course pre74 classic bikes are limited to 130mm in Europe and 120mm in the UK. Therefore, for your classic scrambler, rider sag of 35-40mm would be about the maximum using this formula.

One thing you also notice, well I did with the Automatic, is the impact of having an imbalance between the front and the rear if one set of suspension is either too hard or too soft.  On the 360 Automatic, I’d picked up some very used Fox rear shocks at Telford, but with them on the bike it’s clear they are worn out and too soft. On the other hand the front was a bit too stiff and so this made the rear work in a different way. They need to be in balance to work effectively and getting the rear wrong impacts on the handling of the front.

Based on the measurements above,  the front was too stiff and sticking so decided to strip it down.

The fork oil had been contaminated with something, probably water and needed a complete strip to clean
The fork oil had been contaminated with something, probably water and needed a complete strip to clean

One of the fork stanchions was also bent, as usual just below the bottom yoke and the took a bit of cleaning out. The springs seemed okay and put in 250ml of oil into each leg. One of the advantages of having an Automatic is that you have plenty of HVI26 oil, which is primarily for hydraulic applications, and same spec as Ohlins fork/shock oil.  Works out cheaper than fork oil and is around 10 weight, it seems.

 

The fork tube isn't quite straight
The fork tube isn’t quite straight

 

Some further adjustment needed as once back on the bike, they still seemed to be too stiff but this might be because of the rear being too soft and that is where all the give is in the suspension.

 

 

 

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