Well, there I was hoping to slope off for a break, doing a bit of boating, and the Forum thread on Modified Steering Stem went mad. 74 postings between July 29 and August 12. Gentlemen
!. However, I did have chance to think about some of the discrepancies between Greg’s findings which are that 45 lbs/inch springs with soft damping suite him on a Comet, while people over here find that 45s are too strong for a heavily loaded twin, two up, and prefer the ride with 36 lbs/inch. As a counterpoint, Chris Launders finds 45 s suites him on his twin with his weight, about 300 lbs. So, even allowing for personal preferences what is going on here? First a caveat. I am an astronomer, not a suspension pundit, so there is only amateur experience behind the following comments, not years of tuning race or touring bike suspension. Also it should be noted that I designed my springs etc. for more or less upright use on the road, not knee scraping round a race track.
Let us consider a twin first with a bike weight of 450 lbs. Let us also assume that this weight is distributed 50:50 between the front and rear wheels giving 225 lbs at each of the front and rear. I will assume/guess that the un-sprung weight at the front is 70 lbs. This is composed of the front wheel complete with drum brakes etc., the fork legs, mudguard and stays, head lamp and stays, Shadow clock and any other bits and pieces I might have forgotten. Removing the 70 lbs from the 225 lbs leaves 155 lbs as the total weight to be sprung at the front end. Now let us sit a 200 lb rider on the bike and again assume that his weight is distributed 50:50 front and rear. This adds a further 100 lbs to the 155 lbs to give 255 lbs to be sprung. For a 300 lb rider this would be 305 lbs and for a 100 lb rider the figure would be 205 lbs. Let us consider the intermediate value first, 255 lbs.
When installed and with no rider the two 36 lbs/inch springs are compressed 3” and they exert a total force of 216 lbs. That figure is for the bike on its wheels and no rider. When the rider sits on the bike the springs are compressed a further amount. If that was half an inch then the upwards force becomes 36 lbs more giving a total of 252 lbs which is near enough the 255 lbs that were guessed at earlier. Considering our 300 lb rider then the force to be resisted is 305 lbs. Two 45 lbs/inch springs compressed two inches, as installed with no rider, give total upwards force of 180 lbs but a further one inch of compression when the rider sits on the bike gives a total upwards force of 270 lbs, which is approaching the figure of 305 lbs above. A total deflection of about 1.4” matches the value of 305 lbs.
Now consider both situations when an additional 3” of compression of the springs,, beyond the installed height, which is what the intended limit of travel is intended to be. The 36 lbs/inch springs will give a total upwards force of 432 lbs (2 X 36 x 6) and the 45 lbs/inch springs will give 450 lbs (2 x 45 x 5). Near enough the same for these rough estimations. If we convert these figures into g forces for both riders and spring sets then for the lighter ride the maximum compression is equivalent to a force of 1.7 g and for the heavier rider a force of 1.5 g.
I am assuming that a pillion passenger on the rear contributes little to the static loading but might well exert additional forces when braking hard.
Now consider a Comet. Total weight about 400 lbs giving 200 lbs front and rear. Remove the un-sprung weight, guessed at 70 lbs, which gives a front sprung weight of 130 lbs. Add the 200 lb rider and that goes up to 230 lbs. If one really wants to use 45 lbs/inch springs then to offset the unloaded Comet front end one would need 1.4” of compression and with the 200 lb rider seated one would need 2.6” i.e. an additional compression of the springs of 1.2”. A further 3” of movement, which is what the design was intended to allow, would give a total upwards force of 600 lbs. If the initial load was 230 lbs then that can resist 2.6 g.
Suppose now that we used 33 lbs/inch spring with 3” of preload. They would give a static upwards force of 198 lbs. A further compression of half an inch would give a total upwards force of 231 lbs, near enough the guessed at weight of 230 lbs above. A further 3” of compression gives another 198 lbs of force to give a total of 429 lbs, able to resist 1.9 g. If 30 lbs/inch springs were used, instead of 33 lbs/inch then these figures become 180 lbs static without rider, a further 0.8” of movement to compensate for the intermediate weight rider and when the full 3” of travel has occurred then 360 lbs of force will be being produced, able to compensate for 1.6g.
Last weekend I was at the Bucolic Hever rally and on the Saturday afternoon fell into conversation with a fellow Vincent owner. His name is Neil Spalding and he is clearly somewhat a pundit on bike suspension. Most of his interest is in tuning modern bike chassis’ for racing but he was good enough to return on the Sunday with a reference to a book on chassis and suspension tuning and with two tables copied out of spring strengths used on modern bikes for various purposes. The spring rates are not what I am used to, Newtons/mm rather than lbs/inch, but I have done some of the conversions. The first thing which stands out is that Ohlins specify different springs for different weights of riders, four different spring rates over a range of rider weight of 40 to 100 kgs with three of them over a range of 60 to 100 kg rider weight. Many of the bikes listed are race bikes so presumably relatively light but it is interesting to look over the lists and compare spring rates with those discussed above. Spring rates range over about 36 to 45 lbs/inch, in units used above, with pre-loads in the range of 6 to 10 mm. One example with some relevance to our own bikes is a Harley Sportster 48 with a kerb weight of 545 lbs. This uses 46 lbs/inch springs while a Honda CBR 1000R, kerb weight 431 lbs, uses springs in the range of 57 to 63 lbs/inch. Clearly modern bikes with upside down forks etc. only have limited relevance to our own bikes but we all seem to be playing with similar specifications.
I will leave it as an exercise for the reader to do the calculations, as above, for the rider weights in the range 100 to 150 lbs.
Enjoy.
