rake and trail specs

Stephen Hill said:

Then there is the seat of pants school of engineering. As in, IF you stick early commando yokes *Atlas style) on an 850 frame, does it work, or does it display bad habits?
According to a Roy Bacon book I was reading last night, you can put 750 yokes on an 850 frame. Course he doesn't specify which 750 yokes, and all he may be saying is that you can bolt them on, which doesn't speak to how they will function.

Stephen Hill
Victoria, BC

Click to expand...

I agree with this approach. 'Suck it and see' is much better than prediction. When I had the severe handling problem with my Seeley, I spoke to Rod Tingate who used to work for Colin Seeley. All he said to me was that 'all Seeley frames have 27 degree head angles'. I started looking for someone to make a set of yokes, however could not imagine what offset to use. Then I remembered that I still had a TZ350 Yamaha frame lying in the back yard. I knew they had 26 degree head angles with 18 inch wheels and were quite short in the wheel base - I also knew that the setup coped with a severe top end motor . So I simply machined the spindle hole in the bottom yoke and everything fitted. When I tried the bike it stayed fairly neutral under brakes (instead of rising) and it self-steered when gassed coming out of corners cranked over(which is extremely useful). It was purely another case of 'arse beats class'. I suggest that you have to be conscious of the natural handling tendencies of your bike, and the isolastics might not help that. What is important is what actually works for you with your own tyres, wheel size, bike length and suspension setup. I'd always measure the rake and trail just for record keeping purposes, I would not calculate the trail. I would not use angled fork yokes, it is another variable and it is unnecessary to add that complication. I would not test ride the bike on public roads, on a race track you have repetition of situations and safety. Be very careful on high speed sweeping bends until you are satisfied the bike will not turn nasty, if it shakes it's head or feels twitchy, back off slowly. If it is bad under brakes you should pick that up quite quickly. Ducati 900s ran a lot of trail and were apparently great to ride on race circuits with high speed bumpy bends. A friend of mine has an 860 and has reduced the trail to make it useful on our local race circuit which has tight twisty sections. I don't think the Ducati 900 has vices, however the one I rode years ago felt like you could jump up and down on it while it was cranked over.

just going over some old ground here, regarding the MK3 angled yokes (1 degree) with trail of 4.36" using an online trail calculator , (4.36" Jan 74 Cycleworld test)
other trail measurements as calculated are
MK3 frame with 750 yokes 3.77"
750 frame with 750 yokes 3.51"
MK3 frame with Atlas yokes 4.4"

Its been said not to use the 750 yokes on a MK3 frame but as you can see there is only .26" difference from standard 750.
Anyway fitting Atlas yokes to a MK3 frame gives virtually the same trail as a standard MK3, whether thats a good thing or not.

Don, I believe the early Commando yokes are essentially the same as the pre- Commando models except for the very early ones with a narrower width. Having once put a later disc front end - including yokes - on an earlier Commando S-type frame with non g/box cradle centre stand, I found it was not a straight swap. The later frames have a different depth of bearing recess in the steering head and these two recesses are further apart on the earlier frames which used loose ball and race bearings. The centre steering stop tab on a later frame does not work with earlier yokes and neither does the steering lock plate at the top of the headstock.

Dave , I have some 850 frames and I manufacture alloy yokes for Atlas, its not a problem for me to make them to suit,
I have modified some previously for Commando frames for customers

madass140 said:

Thanks, obviously Norton has printed the specs as Bob has detailed the 850 spec.
I thought these measurements would be readily available and known by the experts here on this forum, apparently not.
If I had the 3 bikes in question then I would be taking my own measurements,

Click to expand...

There are 4 sets of triple trees.
Atlas, commando: atlas style for halo head light, 71+750 and 73+850

I have very good precise #'s for 3 of the 4. I don't have the commando early halo ones. Atlas and early commando are definitely different...they have different thread pitch on the stem...and I highly doubt they would use the same offsets for featherbed and commando frames.
But... the late commando 750 and 850 are different but close to each other, but the atlas one is close to 1/2" off(shorter).

All the work was done back in 97 as I was designing in steering neck tapered roller bearings, custom triple trees, Betor forks and the Suzuki 4 leading shoe brake front end for my 66 atlas hot rod. Fun project still not finished sitting 8 feet behind me in the front office. Aw darn no toy to ride... but now I own a 3 bay garage with machine shop and and office with a filing cabinet with more norton data than you can shake a stick at. :mrgreen:
I'd like to get 3 of my norton projects done in time for the North Carolina INOA rally in july.
shop:
http://atlanticgreen.com/images/shop957.jpg

I'm positive the early Commando and Atlas have the same 2-1/4" offset, the different part numbers only pertain to the different fittings and possibly steering stops.

dynodave said:

There are 4 sets of triple trees.
Atlas, etc

Click to expand...

Bear in mind there are 2 type of Atlas yokes.

The earlier 7" centres of the early 1960s.

and then all the 7 & 3/8" centre versions...

yep, we are only concentrating on the post 64's

I read somewhere that the steering on the first commandos was too quick and a few inexperienced riders got chucked up the road after riding over 'cat's eyes'. Apparently on the next model the geometry was fixed. I suggest you need to measure both rake and trail and wheelbase and compare with something you know handles well. I any case when you first test ride the bike be ready for it to grab you. If it happens open your hands and don't fight it too hard.

or chuck some baggage on the pillion .

madass140 said:

I'm positive the early Commando and Atlas have the same 2-1/4" offset, the different part numbers only pertain to the different fittings and possibly steering stops.

Click to expand...

Atlas 2.25"
early commando w halo mounts ?no data
71+ command 2.78"

Early Commandos with Atlas style yokes have the same offset (2.25") as Atlas. They are also parallel. With a 90/90/19" Roadrider front tyre, they give approx. 3.9" of trail on the 27 degree frame, & 4.4" on the 28 deg. frame using a 3.25/19 Roadrider

Martyn.

Matchless said:

Early Commandos with Atlas style yokes have the same offset (2.25") as Atlas. They are also parallel. With a 90/90/19" Roadrider front tyre, they give approx. 3.9" of trail on the 27 degree frame, & 4.4" on the 28 deg. frame using a 3.25/19 Roadrider

Martyn.

Click to expand...

Is the handling of the early commando 'quick' or 'slow' - i.e. does it feel stiff under brakes when turning into a corner or self-steer slightly when gassed when cranked over coming out of corners ? With that spec. I'd expect it to be fairly neutral.

I haven't ridden it for a few years now. Lets just say that compared to the Queen Mary like stability of the 850, it feels like a Riva speedboat. The biggest problem with my 750 is the nervousness at higher speed, which I have still to cure.

"4.4" on the 28 deg. frame using a 3.25/19 Roadrider" Martyn
and calculating using the same tire but with the Atlas yokes on the MK3 frame , its the same 4.4" trail

The MK 3 frame is the same as the other 850 frames @ 28deg. My Mk2a has K75S BMW yokes (Showa) which from memory have 57mm offset (2.244") & parallel bores. It is stable at all speeds.

Stability is not necessarily the main issue, even tyres can affect that. I suggest it is more about whether the bike feels stiff as you turn into corners under brakes and whether the bike tightens it's line as you come out of corners with power applied. The stability issue is about whether a slight bump can upset the bike and cause a tank slapper, or cause it to suddenly turn the wrong way and throw you off-balance as you brake for a corner. Whatever handling you end up with you will adjust to, it is only the unexpected which is dangerous. Worst case scenario is probably when the road surface has grooves or ridges in it which run in the direction in which you are travelling, and you have very quick steering.

Nothing tests Commando straight line stability than Daytona track racers and Salt land speeders and every one of those I've quizzed said they never found limit to any Commando straight line stability. I've hopped on 850's and 750's in same day both in Arkansas twisties and Iowa long bee lines. The 750 geometry invites me to do donuts on pasture grass before hot dogging zig zags onto THE Gravel while the 850 felt too heavy steering to do anything but old man style smooth caution to avoid fight back steering effort to steer *very* sharp or fast. I've followed a well ridden 850 and they can zing around Mt twistes dam fine and fast but not as easy confident as 750's though for 750's to show up better handling to beat the 850 definitely takes scary effort and power applied on leans in public.

My 2000 SV650 has stem rake of 25°. Trail, 100 mm (3.9 in), 102 mm (4.0 in) and is light handling competent as any good fat tire sports bike but almost crashes just easy going trying to get to pavement as the sharp steering to compensate for fat tire drag would induce surprise instants of tank slap going in/out of counter/straight steering at 20-30 mph speeds so have to go a bit slower rougher than I like or a bit faster more self steering stable speed than I like not to get constant hints of slap downs. Same thing happens on tarmac when laying SV low accelerating turns the front will twitch into straight steer for surprise hi side flip up or rear skip out or sliding front tire I have to both let off quick and use athlete strength to stabilize forks in time to settle down/recover control, almost off outside road edge by then - so don't ride it on beaten paths very fast nor on hwy *very* fun to avoid its innate corner cripple-ness. To try to over come this innate fault I fitted Scott damper set at almost lowest damping and liked it for a time till a little reverse fork twitch minding my manners on THE Gravel prevented the fast fork correction swings > SLAT about 30 mph *going straight ahead*. That event plus the added drag to just cruise commute timidly-legally have been w/o steering damper for last 25K miles on SuVee.

I'd think the best Commando would be between 27-26' rake and between 3.5-4" trail. Rear squat matters too pressing limits far over accelerating - which increases rake/trail some - which tends to resist the rear swing out and down over sharpening turn - but if not enough rake-trail to begin with - this slight increase ain't enough to matter - and rear will force the front into inside road following - for two possibilities > you were too chicken on throttle so jerks hi side, or you were too brave on throttle and low sides first. i'd sure try to mimic acertel's bad ass 850 Seeley that he loves describing the self steering sharper as desired on increasing power turns and I love reading-feeling it each time he does. Counter steering tends to put more down force on the front relieving rear of traction planting while straight steering presses mass-inertia down at rear up at front. This happens because of the double hinge of fork stem pivot and bike lean angle on the axle plane to gravity. D/t THE Gravel harsh lesions then Ms Peel's rather harsher pavement lesions I may be the only one knowing the key factor is front axle angle to gravity with rear thrust trying to fold up hinge angle or open it up.

To get some hobot sense of this, stand next to bike, twist forks R/L to sense the reactive paddle front-rudder force on rear, which is easy to see move on isolastics compared to rigid mount, then lean with counter steer to see how it tends to increase fork/axle hinging fold up angle downward further easier, then on same side of bike lean with straight steer and push forward a little to see the fork/axle hinge angle tends to make bike harder to fold up downward into a lean. As long as not on enough power to lift front out of more traction effect than rear counter steering is the best natural way around BUT if on increasing throttle enough to lift front out of much traction counter steering can suddenly magnify the snatch down or the flip up, all depending on hold easy or hard bike is to fold up more or resist folding up on stem angle vs axle plane in line of thrust with gravity vector.

All the racers in the world [but me on Peel] all innately stick their foot out forward downward on balls to the wall counter steering or put a knee out down to prevent too far over and to push bike back up when falling over on its own. I tend to do this too--- reflexly/annoying on my SuVee and other sports bikes but quickly got pass that tripod corner cripple crutch style on slightly flexible w/o rebound linked isolastic with extended Roadholder forks on 750 geometry. If ya watch slo-mo of elites bikes self recovering spills you'll see it straight steering instants to pop back up, often dragging pilot behind till able to climb back on sheepishly but mostly spectacular spills poor dangerous things.

Once rest of the world catches on with capable cycle design they too will see counter steering same way I do - like slow parking lot straight steering transitioning suddenly into counter steering on its own - if brave enough to go a lot faster than parking lot speed to let it change styles for more G's glee. Hope above makes no sense as implies how corner crippled rest of the world is so far. I can not get sports bikes to do this w/o becoming wild air borne jelly wigglers loosing acceleration with hi sides or pushing front to slide so unloads rear to spin out loosing acceleration grip into low side swing outs. Corner school let me learn to over come sport bike frame/fork slapper upsets by letting/making them hi side to twist in air long far enough to let the jiggles settle down before landing already line up right to nail it more out of there. Sort of like a flat tracker crossed up drift above surface. But to launch like this one must have predictable tire patch stability but fat tires distort enough near edge they squirm around on it a few silly mm's which is same as moving forks a few sill mm's at hi speed, all hell can break loose. Best geometry is sensed by least fork effort no matter what ya trying to do and related to the common advice not to grip bars too stiffly when having a blast. I know why does anyone else yet?

The Seeley still feels very slightly stiff under brakes, however there is certainly no need to counter-steer it. I think if I did that it would turn itself inside out.

acotrel said:

The Seeley still feels very slightly stiff under brakes, however there is certainly no need to counter-steer it. I think if I did that it would turn itself inside out.

Click to expand...

?? Am I missing something here, Alan? Everyone counter steers to initiate turns at anything above parking lot speeds. There used to be some argument about that back in the '60s or so, from riders who didn't realize they were doing it, but it's been an established fact about motorcycle dynamics for decades now. Does the term "counter steering" have a different meaning down under? Not trying to yank your chain, I'm honestly curious what you meant.

Ken