Steer Clear - understanding steering for twinshocks

[cruxi]

That is good advice. I know something more age-appropriate might be better, but I'm never going to win anything any time soon, so hopefully no one will mind too much!
A GPz305 was a nasty mid eighties Kawaski road bike. I just happen to have a pair of those forks knocking around, so I thought I might give them a whirl! The only front wheel I have to hand is a YZ490 one, with a twin -leading shoe brakeplate. That might raise some eyebrows too...

I was just intrigued by the comments in this thread, as it looks as though some careful work on geometry could net some good results, and would be achievable in my humble worskshop!

[JC1]

Yes, careful thought given to geometry will indeed net good results/improvements.

Lastly, we look at the humble TL125, which suffers from slow steering & considerable flop-in.

You may recall that flop-in can be caused by too much rake, & exacerbated by more weight on the front end, more offset of the steering mass C of G, or higher C of G of bike & rider, so it can be improved by reducing any/all of those factors.

The humble TL125 is poor on all those parameters & it understandably suffers considerable flop-in. (The high C of G of the 4stroke engine contributes too & with so much rake [28.5deg] the head-drop factor is also considerable which effectively adds to the flop-in factor.) If we reduce all of those parameters, eg slide the forks up thro yokes (say 25-30mm?), get some ‘live’ weight off the front end (move foot-pegs back) and lower the C of G of the rider (move foot-pegs down), each of those will reduce flop-in to some degree. If we could reduce the offset of the steering mass C of G that would help too but that would require different triple clamps & fork sliders with offset axle.

While the offset of the TL’s steering mass C of G is not healthy since it’s not minimized by sliders with offset axle or angled triple clamps, the total axle offset is not excessive [70mm - comparable to TY175/250]. The TL has excessive trail [104mm] so this is one frame/bike we could de-rake with the same triples & forks.

Thus another option would be to de-rake the frame, say by 2deg, push the standard forks 12mm up through the existing triples (to compensate for the raised height of the front end from the de-raked forks), & lower & rear-set the foot-pegs. Wheelbase would thereby also be reduced by about 30mm which wouldn’t be out of place considering the engine’s power (or lack there-of).

Both options will reduce flop-in & also quicken/sharpen the steering & the turning; the second option more so on each count.

[JC1]

Interesting when you pull everthing to bits like this.
With the forks that kick out perhaps to trail staying more stable aids in the stability with a dive in the front end. Makes you wonder all those years ago whether they just experimented with geometry till they got it to feel good or did some smarty pants engineer work it all out on paper first.

Been reading Morley's book, Classic British Trials Bikes & it seems the answer is that the Comp Shop spam &/or riders just experimented; suck it & see style. Initially it seems all they did it for was its effect on wheelbase with little/no thought to trail or anything else.

Circa ’46-‘47ff it seems cutting & shutting the yokes was regarded as one of the easiest ways to change wheelbase.

Jim Alves at Triumph cut ½” out of his bottom yoke to reduce wheelbase (by about 2”) on the pre-unit Trophy.

At Norton the McCandless bros (of featherbed frame fame) also reworked the bottom yoke on their works trials bikes to reduce wheelbase by a massive 3” (56” to 53”) & the yoke mod was said to be “vital for good trials handling”.

At AMC, Hugh Viney experimented with both shorter & longer top yokes (ie more/less offset): “A shorter top yoke, pushing the front wheel spindle out, suited both fast, rocky going & long muddy slots, whereas a longer fork top pulled the spindle in to suit the more nadgery sand sections”. (That makes some sense considering the wheelbase changes, but it makes no sense considering the steering geometry changes.)

At Royal Enfield, they simply ditched the RE sliders with forward-offset axle on the trials Bullets & used BSA ones instead (which had no offset) to reduce wheelbase. The RE yokes remained standard with no run-out. Right up to '58 the works riders mostly used the BSA sliders with no offset (which they preferred) till they had to use the RE sliders with considerable offset (when they were told to use standard RE prod'n parts). (Later they developed RE sliders with a little offset which then came standard on the later prod'n bikes, eg Crusader, Clipper, Meteor Minor).

At BSA, they took a different tack. Bill Nicholson adjusted the rake by extending the backbone to achieve 4 ½” trail on the pre-unit rigids for trials use. Why that figure is not entirely clear. BSA were never too worried about wheelbase, always putting a premium on straight line steering. And believing their racers “steered like no other”, they appear to have set out to retain that asset on their trials bikes with similar steering geometry. It appears 29deg rake gave them their desired 4 ½” trail & that became common to their entire range in the early 50s.

However later when the C15 appeared, it seems they lost the plot. A 21” front wheel hit the down-tube on full compression of the forks. Initially they fitted a smaller 20" wheel on the production bikes to give clearance, but this only applied to a small number in the first batch. They then changed tack & fitted yokes with run-out (angular offset) to give clearance for a 21" wheel, however they kept fitting the 20” front wheel. It reads like someone tho’t of the 20” wheel & someone else tho’t of the yokes to solve the clearance problem & the ordering got stuffed up in the process.

Apparently they intended to fit a 21” wheel with those run-out yokes, but they'd over-ordered on the 20” items & never did fit the 21”. Of course, the combination of those run-out yokes & 20” wheel stuffed the handling (as it would) & demand never reached expectations. Another BSA bungle that cost them dearly.

The works bikes limited the fork travel & stuck with 21” wheel & parallel yokes to maintain the good steering & sufficient clearance, but they didn't want to limit travel on the production bike so stuffed the steering instead. Go figure! (Probably it was the accountants overruling the Engineering Dept & Comp Shop.)


Of course in due course they all (or nearly all) realized that playing with the yokes affected other parameters & traits as well so there’s more to this story yet.

[JC1]

Morley's book provides a fascinating history of the whole development of competition trials bikes up till the late 60s, not just the evolution of steering geometry that we’re focussing on here. Well worth a read. Some very interesting stories.

eg Triumph’s turning to their Tiger Cub for trials competition in ’54 was the result of their guru Jim Alves having a serious car accident & sustaining “horrendous” leg injuries so he could no longer ride the heavyweight (Trophy twin) used at the time.

His riding career was already fading a little but the enforced switch to the lightweight (first the Terrier then the Cub) brought him renewed success & the Cub almost instant success... “with far reaching but not always popular results”. Triumph never looked back & BSA followed suit with the C15 then RE with the Crusader etc.

By 1955 "all and sundry were already copying Alves' example and building their own four stroke lightweights" and by '58 Cub's had scored so many victories that it had "become the people's trials bike".

So what turned out to be a massive factor in the move away from the big heavyweights, which eventually culminated in the Spanish invasion, was caused by a car accident!

It was also significant in the acceptance of rear suspension in trials. RE had been using swingarm rear suspension in trials since ’47 but nobody followed suit. It was "an age when the star riders, factories and public alike, firmly believed that only a rigid machine could keep a grip in muddy conditions... So convinced were the trials riders of the day that a rigid rear end was an essential ingredient for success, that far from being praised, Royal Enfield found themselves in an acute crisis which virtually negated their efforts."

In fact the RE riders were mocked for seven long years, partly because initially their rear suspension had little damping & appeared to ride like a pogo-stick, but mostly because of that resistance to change & the belief that rigid is best for trials. Douglas used springers for a while quite successfully (circa '47) but went back to rigids, partly to use up existing frames, but probably feeling the pressure too.

Now Triumph wanted all their riders to ride the newly modified Terriers/Cubs 'plungers' but John Giles didn’t want to cos he loved the big Trophy twin, so the factory hung onto his twin. But rather than ride the Terrier/Cub he rode his swingarm scrambler twin instead in the next trial & very nearly won it despite the scrambler being too long & low.

He commented afterwards to Bob Manns (of AMC), “Johnny Britain must be having the time of his life on that Enfield springer because I have never been so impressed with anything as my scrambler today – I have never ridden up rocks so easily; it just flashed up & the suspension took care of everything.”

Manns went home and fitted his AMC trials engine into a springer scrambler/roadster frame, tested it in secret and a week or two later the entire AMC team turned up at the ’54 West of England trial with springers & “left the rigid machines for dead” with Manns famously winning outright on the first outing. AMC also won the manufacturer's award.

RE springers had already won the Scottish & Brit Experts trials in ’52 but nobody took any notice. Yet after Mann’s win, BSA’s Brian Martin dashed home & converted his Gold Star scrambler springer to trials trim & everyone else did likewise. The trials world then converted to springers almost overnight, despite the press being horrified!

But we digress. Back to steering geometry (shortly).

[JC1]

When Jim Alves took ½” out of his bottom yoke to reduce the wheelbase on the Trophy twin, as mentioned above, Allan Jefferies bent his Trophy’s backbone & changed the rake to achieve the same. Alves had also been playing with reduced rake by shortening the bottom rear sub-frame tubes on the rigid Trophies but it seems to have been mostly to reduce wheelbase & raise the rear for more ground-clearance.

At AMC Hugh Viney had a unique style, riding slower than anybody else – less than walking pace - on a mere sniff of throttle to find grip. Being the devolpment guru as well as the gun rider he developed the AMC bikes to suit his style, which didn’t always suit his team-mates’ style, like “fast man Gordon Jackson”.

Morley notes that the AMC machines always appeared to have steeper rake than any others to suit Viney’s “trickle tactics”, reportedly giving a mere 1 ½” trail – “unheard of on almost any form of road or competition machine”. Morley (who rode most of the bikes of the day) says: “on paper all wrong, yet it worked surprisingly well”... except he admits that it deflected easily in rocks, tucked under on steep drops, & you needed to ‘muscle it’ with strong arms . “Castor or self-centring is minimal... merely requiring strength and determination from the rider.” While the rake doesn’t look all that steep it’s easy to spot plenty of offset & run-out in the yokes.

Jackson had to make do for a while with a slightly higher top shock mount giving a smidgeon more rake & trail (since the back of the bike then sat lower). But the geometry of the later AMC machines (early ‘60s) looks much more conventional to me, tho no specs are given. Viney's influence seemed to diminsh after the change from rigid to springer since he was a 'rigid man' & had semi-retired from riding by '53. And Jackson increasingly became AMC's premier rider.

Back at Triumph, the early rigid Trophy twins had a little more trail (than the AMC machines) at 2 ½” – 2 ¾”. It’s easy to spot in pics of these bikes that they have very steep rake (about 25deg) & considerable off set & run-out (a few degrees) in the yokes. Alves was heading in the right direction cutting some out of his bottom yoke, while Jefferies seems to have taken the wrong tack reducing rake & hence trail further.

At the end of the ’51season Morley says “the works bikes had at last been fully sorted” and the production bike got new yokes & the “steering head angle modified for 1952 for more trail”, no doubt following the works bikes. Apparently the bike didn’t steer too well before that. Morley notes, “These later components improved the early Trophy’s vague handling and were the production result of Alves’ & Jefferies’ various cut and shut exercises”. Works rider John Giles indicated they settled on 3 ½” trail.

At Ariel, there’s little/no development history given; just the specs for the production HT350/500 (‘55ff) stated as 27deg rake & 4 ¼” trail. There appears to be no run-out in the yokes.

Meanwhile BSA settled on 4 ½” trail & 29deg rake for all the Goldie swingarm models (‘55ff), but “they suffered on the tighter sections with their 4 ½” trail”. However, Brian Martin “dramatically shortened & pushed down” the rear frame loop to lift the rear & reduce rake & trail somewhat below 29deg & 4 ½”.

Both Norton & RE trials models were said to handle well but no steering specs are given unfortunately. The RE Bullets look to have steep-ish rake while the Norton 500T looks mid-range.

So rake ranged from about 25 to 29 deg & trail from 1 ½” to 4 ½” but the evidence suggests the general consensus was moving towards the middle by the mid-late 50s.

The use of yokes with run-out was common, but why? There's several known effects, as above in the thread, but what was the main reason for using them? I have long wondered that.

[logan]

Interesting reading,

I love a bit of a history lesson,

But here is some food for thought!

For every poor turning trials bike I have ever ridden there is only one true remedy,
(It wont be found in too many books and not too many people will tell you!)

Pivot turns or floating turns or what ever else you want to call them!
Its been my experience that if the front is shit ,take it out of the equation.
I.E steer it on the back wheel.

Cheers
Goughy

[JC1]

Having returned to Morley's book to re-read/finish it, it seems that what came to be the main reason for using yokes/triiples with built-in run-out is disclosed.

He quotes Triumph's John Giles saying: "We used to cut the fork yokes to get the stanchions right back to the steering head stock, so as not to leave all that weight flopping about. Steering improved and... we opted for 3 1/2" trail which made a big difference."

Miller did likewise much later, when modifying his own personal Ariel.

Funny thing is that nobody seemed to realize using sliders with offset axle or "long lead" such as those on Velocettes and Enfield's achieved much the same thing without the need for cutting and shutting. Morley actually decries the "long lead" RE sliders, and the factory riders almost exclusively replaced them with BSA or Norton sliders.

It took Bultaco to combine triples with run-out and sliders with offset axle to gain the best of both worlds.

[Stu]

if you have to modify a twin shock or a classic bike so much to make it better, buy a new bike! Ride the older bikes as they came from the factory! this means no frame/steering head mods, new suspension inside forks or swingarm mods.. Simple.

[Guy53]

I have 2 running TY ( 1975-1978 ) plus a lot of parts, one them is '' stock '' and the other one is modified to Majesty 1st gen specs with a lot of help from top member of this forum. I recently bought a 2003 Beta Rev so one of the TY will have to go ( I have limited storing facility ). Witch one do you think I will sell ? The stock one ! The modified one , for me , is a much more enjoyable bike to ride. True, it's a lot of pain in the ..... neck to modify, it is a lot of time to search for the right info, to plan the work and I am sure very annoying for the people you have to badger for there knowledge. It is not an ego or show off trip , 99.9% of the people that look at my bike don't see any difference between the modified one and the stock one. Everyone that have ridden a TY before a test ride on mine do the same thing when they come back, they get off the bike hold it strait with one hand and look at it to try to find what is different from the other TY . I'm not saying I made it running like a modern bike or a Jumbo SWM, I'm just saying that it is worth the trouble to read post like this one get your hand dirty and enjoy the result of your work. Than again I totally can understand anyone that don't want to jump from the modified cliff.

Guy

[JC1]

I was going to post this, commenting on the steering geometry of a modified RL, in another thread but it's more appropriate resurrecting this thread.

RL modified, 52in w'base.jpg (106.36 KiB) Viewed 3888 times

I have no desire to cast aspersions on the workmanship of the fabricator (who does excellent work), and there's much to be admired in what he's done on the rest of the bike, but it shows evidence of not understanding steering geometry when modifying the rake on twinshocks.

Standard Suz specs for rake and trail for the RL are 27deg rake and 76mm trail, which for trail is the lowest for twinshock trials bikes of the day. He indicated that he de-raked it 3 or 4 deg depending on where the forktubes are in the triples, and you can see how much he's done so if you look at the bend in the downtube (a couple of inches above the bottom of the gusset). It's very noticeable, whereas production RLs had straight downtube.

If still using the RL triples (as it appears) that reduces trail considerably to a mere 50mm which is extremely short for a twinshock trials bike which would make the steering and handling rather 'nervous' to say the least.

Now look where the steering has settled at with the bike on the side-stand. Note that it's on a polished wooden floor with a well inflated tyre, ie low friction surface/contact, allowing it to settle fairly freely to its equilibrium point. The steering angle at which it settles is the point of zero trail, beyond which the trail becomes negative - a very unstable situation. That is the steering angle where you no longer have any self-centring effect and tucking under starts even on a flat surface as you steer. And that's before you consider the worsening effect of encountering a bump, ledge, rock, log or deep sand. It's a very 'shallow' steering angle on that bike. ie You don't have to steer far before that condition arises.

It's an illuminating example of the potential hazards of adjusting the rake but not the offset on these old twinshocks.

The problem could be overcome by fitting different triples with less offset, for example from the 80s model DR250S, which, with the RL sliders would reduce total offset of axle to 65mm which would bring trail back to a more workable 72mm. Even tho he lengthened the swingarm he said his mods reduced wheelbase to around 52" so a change to the DR triples would reduce it a little further but still be a bit longer than the TY250 wheelbase.