tweeking the steering head

This thread is about frame straightening. If you get it wrong it can kill you.

Everything you post about here seems to be able to kill you....

Oddly, it seems to be you that crashes a lot, from your reports.... !
What does that tell us ?

acotrel said:

This thread is about frame straightening. If you get it wrong it can kill you.

Click to expand...

Its actually about tweaking the steering head and of course if you are not careful you could damage the tube itself.
The Commando counterbores for the steering head bearings are not that strong and the bottom one can flog out so pulling on a rod inside the tube would need some caution to avoid compromising the tube itself.

All of the opinions have been interesting though,people have used come alongs to fairy dust it seems. :lol:
The first thing to do is positively identify the actual problem area and remove any doubt regarding the head itself being plumb (ish) but off the centre line.
I can't wait until tomorrows exciting installment.

Most people here seem to be bending the frame with it clamped to the floor by the bottom frame rails. I would consider sinking a tube (close to/same as the inside diameter of the steering head tube) into the concrete at about 27 degrees from the vertical, then dropping the frame onto it inverted. A couple of stops on the floor near the rear shock mounts would allow you to pull the frame rails to where you want. I would also consider cutting the cross tube between rails to allow more accurate bending of each side before re-welding. This is where you find out how thick your concrete is in your garage floor :lol:

gripper said:

Most people here seem to be bending the frame with it clamped to the floor by the bottom frame rails. I would consider sinking a tube (close to/same as the inside diameter of the steering head tube) into the concrete at about 27 degrees from the vertical, then dropping the frame onto it inverted. A couple of stops on the floor near the rear shock mounts would allow you to pull the frame rails to where you want. I would also consider cutting the cross tube between rails to allow more accurate bending of each side before re-welding. This is where you find out how thick your concrete is in your garage floor :lol:

Click to expand...

Most of the professional frame straighten jigs have the whole engine installed when straighting the frame , like the motoliner jig.
http://www.bing.com/images/search?q=mot ... ORM=IQFRBA

Unless you are racing like Alan in public a bent bike will not kill you any more likely than just leaving home. It may take more effort to steer one way or the other and some compensation to stay straight but not as much as merely low tire air. I doubt you can even tell its bent by riding unless going so harsh/fast you'd be arrested on 1st sight. Best to be straight but its not a life death dramatic danger at all - just chain tire and pilot wearing down faster and may certainly limit your confidence to kick up heels, but the warning signs will be apparent as a low tire to compensate for by going a bit slower and less fun taking turns.

Alan my SV650 balloon tire Asian Ducati knock off with 25' rake will every now and then about twitch bars out my hands and front out from under going pretty dang slow on Gravel and Pavement with NO Intention but get some where intact and save super soft race tires - its exactly what you experience with bike suddenly wanting to leap up-off front tire [or push it to scrub along if going fast and powerful enough]. The balloon tire bikes need sharper rake to take some load off arms to steer but makes more prone to hi side when bike in a condition it tends to lean over on its own and front suddenly reverts to straight steer road following instead of counter steer road scrubbing. The twitch that goes through frame is what sets off wobble/weave up to tank slap down. I have to be more carefull-force full on my SV650 to get to pavement than isolastic C'dos and SV650 was more easy calm to get to pavement with a Scott's steering damper set on light level but when damper prevented the fast fork action required to save a surprise Gravel slip out, I removed it for safety and rather less steering effort on hwy legal cruising.

I've explored this low side hi side stuff to find bike CoG comes into play weather bike rake wants to flip off front trying to hold it down low in leans or when trying to fling back up in time. D/t my half*ass fame locally on m/c's people bring bikes to me to diagnose steering handling issues, so have tasted bent frame, binding slider action and restricted bearing freedom and bad tires to get a firm opinion of what's doing what to uspet pilot and cycle. Being straight was the least bothersome and hardest to make show up. Too loose or too tight sure did at surprising low speed power and lean, like just staying in lane going straight to get back ok, ugh.

I've had 2 broken rear axles, 1st on tr-linked Peel that made tire rub swing arm raw by twisting in travel direction and tipped side ways to cut into thread on on fender edge while standing on pegs full up right 90 mph over broken heaved seamed cement sections with just tips of fingers on bars enough to keep on hi throttle and didn't even know it till I was pulled ove d/t clouds of smoke and then tried to get going again, horrific wobble like a blow out tire - till about 5-6 mph then sweet true easy as ever, as long as I didn't flip forces one side to the other fast like a racer in chicanes. Second was on Trixie working on parking lot stunts cut short by sense of flat tire, then rode home 65-70 mph just fine - after getting past the 5-6 mph flip flop slack fork loading zone.

C'do have match book paper thin tubes so may have to protect stem tube by both bearings left inside and exterior clamps or risk foil like crumpling at weakest area or prezeling the whole thing making it hard to assemble for lined up good looking close examination.

If you get the steering head angularity wrong in the lateral direction, the bike can become stiff when you lean it. If you get the rake wrong two things can happen. If you increase the rake the bike becomes more stable when you ride it accelerating or holding constant speed, under brakes or as you back off on a bend it can stand up and turn and throw you off balance. If you decrease the rake (decrease the trail) the bike becomes less stable and more prone to self-steering as you gas it when cranked over. The margin for error is quite small. You have adjustment sometimes if your fork stanchions pass through the top yoke, and you can raise or lower the front. Also if you can jack up the rear end by adjusting the springs, that also changes the steering head angle. Most standard street bikes are set up to be stable and neutral. If you get it wrong when doing a repair, it is possible for the bike to chuck you off in some circumstances. On a race circuit you can crash safely - in traffic, not so easy. It is essential that after doing a mod to the steering that you sneak up on the bike and detect whether the handling has altered. To do that you have to know how it handled before the change . I'm not being alarmist in saying it can kill you. If you get if wrong, the bike can feel OK if you are a bit insensitive, however the vice can be revealed at a critical moment.
I don't have a problem with a bike self-steering under acceleration, I can use that. If it misbehaves under brakes, that is very nasty. Bikes are always set up to be stable under brakes, so you find that if the front brake drags in a corner the bike can run you off the road. If the bike stands up and turns the wrong way throwing you off-balance - what do you do with that ? When it happened to me I knew I was on the deck, so I tried to drive the bike to the grass verge to find a soft spot to land. As I re-applied the power, the bike came out of it's stupidity, and I recovered. It all happened in about half a second while I was still going very fast. ('Arse beats class' )
You don't need that experience.
The way to detect the vice is to ride the bike up towards a corner and brake. If you feel the bike tends to rise in the opposite direction to the lean - you have it - (too much trail).

You are absolutely correct in my experience too Alan, but you are forgetting or on purpose ignoring that even with the screwed up geometry you did not really encounter it in pits or just racing the easy sections, only on some excessive competition driven harsh turning efforts, so I remain firm that you can not detect bad rake or even twisted stem until mis behaving taking chances anyway or face some emergency going normal speed in public. When everything angled and aligned and nailed down from slackness spring back then bike will invite you into more and more loads and any upsets are usually recoverable to learn to avoid or compensate for to carry on wild as can be. Look at IOM video to see how cycles walk around on wide tires and forks deflect front tire way out of line with rear. Anywho best wishes on getting c'do frame to look right on close examination and not crumple doing it.

100mm of trail has been noted as a good all rounder for a long time on near any era motorcycle including contemporary.
I wonder what the trail is on a Commando.

My Seeley is 40mm shorter than a commando. I have a note that the trail is 72mm with the TZ350 fork yokes, but I don't know how reliable that is - I could have been half asleep when I measured it. I think some modern MotoGP bikes use 24 degree rake and 92mm of trail, however the tyre sizes etc are radically different. I use the old TZ350 Yamaha as a benchmark. It had 18 inch wheels, 26 degree head angle, about 30mm yoke offset. It had neutral handling , however the usual technique of riding it was 'point and squirt'. If you gassed it too early you got the hi-side. I figured that with the 27 degree rake of my Seeley, the TZ yokes would be OK. The bike tightens it's line in corners , and at times I gas it really early and hard. I always feel t hat I should not be doing that, however it has never stepped out. I do everything as smoothly as possible.
Somebody should measure and record the steering geometry of the standard 750 and 850 Commandos for reference purposes. I think 100mm of trail was common on 70s road bikes. From what I've read, it appears that reducing wheel size by one inch, is usually compensated for by a reduction in rake of half a degree. - needs to be confirmed by 'suck it and see'.

I have a hunch if you have 30 mm offset triple clamps and a tyre around a 100/90/18 sizing you have well over 100mm of trail.
The RC212V has specs around 27 degree / 94 mm trail.

It is going to be hard for me to remember all of this when I'm 1000 miles from home, on a worn rear tire, loaded up with 50 pounds of camping gear, and trying to get by a truck coming up Yolo Canyon. Add in the changing fuel load... Oh Dear!

Greg

I'll measure it again sometime. When you do the calculation, do you subtract the yoke offset ? From memory I drop a vertical through the axle, and a line straight down the side of the forks. That gives the forward offset, then you subtract the yoke offset, so you get the difference of a line through the steering head form the vertical through the axle - that is the trail ? There is a slight error due to angularity doing it that way.

Steve, if your commando already has too much trail and is very stable, loading the rear end might make it steer better by reducing the trail, however the C of G might give a problem.

splatt said:

gjr said:

Hi Steve,

I am heartened by your success with getting things more or less in line and having the bike ride well. I'm going for somewhere between anal perfection and the hobot mash it into submission.

concours,

I am fairly handy at fixing things. I put the motor in this bike together about 50,000 miles ago and have just put in 0.020" oversize pistons a few years ago. Our local bike shop handled the machine work. I have several Volvos ('67, '87, '04) that I maintain. The '04 is demanding better diagnostic tools to sort out the ABS light ( Volvo went their own way on DTC codes). I have been all through a '40 Farmall H (motor, transmission, hydraulics, etc.), various other tractors, combines, and assorted vegetable equipment. I can measure, cut, and fit to a few thousandths. I have a lathe, but have been using carbide tools since my tool grinding skills are still not 100%. My welding skills are so-so, but two neighbors are very good. I have calipers, mics, dial indicators, straightedges, a 12 T press, a decent set of hand tools, a 9000# lift in my garage...

Greg

Click to expand...

No all totally inadequate, when you log in your name needs to come up in blue like all the other knowledgable Norton experts, most of there bikes run like a dream, because that's all they are

Click to expand...

Classy. Don't be the guy who shows up at a party empty handed and bitches about the beer selection.

acotrel said:

I'll measure it again sometime. When you do the calculation, do you subtract the yoke offset ? From memory I drop a vertical through the axle, and a line straight down the side of the forks. That gives the forward offset, then you subtract the yoke offset, so you get the difference of a line through the steering head form the vertical through the axle - that is the trail ? There is a slight error due to angularity doing it that way.

Click to expand...

Yes the offset is calculated to get the vertical difference based on 27 degree's that is deducted from axle to ground measurement to get the base triangle,the adjacent being trail.
Trail
Vertical axis line at the axle.
Axis line through the steering stem.
Logarithms once then a piping pro calculator, now phone apps.

#
If you load the rear with luggage trail is increased depending on rear sag.
There are a lot of variables and how bikes behave loaded not to mention popping out into open space passing large vehicles.
Rear shock absorber condition,spring poundage and sag,even the forks come into play,maybe even a Norton with a Interstate fuel tank.
Luggage shape can even play a role.

gjr said:

It is going to be hard for me to remember all of this when I'm 1000 miles from home, on a worn rear tire, loaded up with 50 pounds of camping gear, and trying to get by a truck coming up Yolo Canyon. Add in the changing fuel load... Oh Dear!

Greg

Click to expand...

JUst take the bus!

olChris said:

gjr said:

It is going to be hard for me to remember all of this when I'm 1000 miles from home, on a worn rear tire, loaded up with 50 pounds of camping gear, and trying to get by a truck coming up Yolo Canyon. Add in the changing fuel load... Oh Dear!

Greg

Click to expand...

JUst take the bus!

Click to expand...

The main thing is that the smoke does not escape and the wheels keep turning . The rest is purely cosmetic.

Don't forget, when calculating the trail, that some Commandos actually point the fork tubes slightly 'backwards' in the yokes, to increase the trail, slightly.
This was only in the 850 models though.
Or all disk braked models ??