rake and trail specs

In the zone above 12ish mph when mc wheel mass gryo resistance gets too much to easy over come its natural for pilots to counter steer > up to point that don't work any longer so must back off or lose control. Off roaders scooting along with spirit nicely beaten paths will be seen straight steering at hwy speeds pretty sharp turns. The DARPA robo cycles only use jerky straight steering at all speeds and I've seen a stunt video the fella never counter steered once for 10 min of crazy paths and antics. Only the weirder wilder conditions benefit by straight steering though so counter steering still best in more sweeper like conditions. Stunter stayed below hwy speeds but what fun is that? So can do steer either way effectively depending on need or desire, each way has its on best time and place. The ideal single direction sharp turn has bike at fouling lean angle hooking up so much power its simply wheelies sideways into the next aim line pulling blood from brain to dim tunnel vision till adrenaline hit pushed it back up. When a rider lets go of bars at hwy speed and uses body english &or aero drag to steer the bars will straight steer front pretty as ya please.

If the steering geometry makes the bike a bit too stable, it becomes difficult to tip into bends. If you reverse steer slightly the resultant gyroscopic precession causes the bike to lean in the opposite direction - the direction you want it to go. My bike is not like that. As you approach a corner under brakes the steering is not that stable that you have to reverse steer. When I originally raced the bike, I had Ducati 450 fork yokes fitted which had a lot of offset. It gave so much trail that under brakes the bike stood up and turned the wrong way under braking. It threw me off balance. I then fitted TZ350 fork yokes which greatly reduced the trail and moved the handling away from being very stable under brakes toward self-steering coming through the corner and out under power. It means you have to learn what the bike is going to do under power, and anticipate where it is going to go. The benefit is that you turn under the guys who have more powerful bikes and get on the gas earlier, so you have the run on them. If you read Tony Foale's stuff, he mentions the self-steering effect of reducing trail too much. When I first felt it happen, I thought I'd imagined it. If you had that sort of steering on a more powerful bike, you would certainly high-side it easily.

I wouldn't even comment on this stuff on this forum except that playing with rake and trail can catch you by surprise, and the situation which can occur with too much trail can be very difficult to recover from. When I experienced it, I knew I was gone and I turned the gas on to drive the bike onto the verge to crash. As I turned the gas on again the bike came out of it's swan dive - sheer luck, not skill. Most crashes are relatively harmless - however not all. That was the nastiest near miss I've ever had. I'd had some warning - my mate had raced the bike before me, and mentioned it almost grabbed him too. I thought it was bullshit, however at least I was a little bit prepared for something like that to happen. I don't think you will get into trouble swapping commando fork yokes between models, especially if you take care the first time you ride the bike, and recognise the symptoms. If the bike is really difficult to lean when approaching corners under brakes - look out when the speeds go up ! With too much trail, the harder you brake, the more strongly the bike turns the wrong way.

I've learnt my lesions hitting THE Gravel so absolutely taboo to me to ever brake while leaned unless so slow dragging feet would work as well. The trail braking I've sworn off of did not so much upset steering as just prenvented effective slowing in shortest time/distance though have had front or rear slip out in panic stops around blinds as front hit sand or mud or leaves or oil or blood smears to low side but reflexly stomped on rear that popped bike back up in line aimed across oncoming lane with car coming but gritted teeth squealing front as grill zoomed in till slow enough to completely let off brakes to snatch back in lane then hard on em to stop a bike length from impact. So have learned all I ever want to braking while turning and leave it to experts. Peel would not stoppie but maybe 1/2" rear lift with it wanting to swap ends but the weird tri links helped so much I was able to ride locked front many bike lenghts, long enough people would run up to me yelling I'd locked front , duh. Bike CoG above ground and front to back has a lot to do whether it tends to fall down or pop up on far over turns or harsh leaned braking. By time I see others brake lights going into turns I'm on traction testing acceleration on SuVee of Peel with a grin on the sneers wipped off before close apex and gone. Some cycles like the elite flat trackers have stem shims to change angle for the rider and tack conditons so maybe ain't no perfect geometery for speed and braking to discover the hard way. Peel has a mystery feature I've yet to try, she can change stance on the fly of suspension height and spring pre load and think I'll like her as a chopper look going into apexes and a chopped down fork dragster exiting and both ends hi for the rough stuff.

Hi " Acotrel" (Allan?), did you say that reducing the yokes offset decrease trail ?

Yes, in most cases. The result comes from a combination of both rake and trail. The trail is the difference between a line through the steering head to the ground and the vertical through the axle. When you measure the trail, you take a line down the fork leg and subtract the offset from the measurement on the ground. If you reduce the wheel size you usually increase the trail in doing so. I'm not very good at measuring it. I usually use 6mm rod, you probably need points turned on the ends of it to get accuracy and allow for the angularity. With about a 26 degree head angle a reduction in wheel size from 19 inch to 18 inch seems to need a reduction in rake of half a degree to keep the same handling characteristics. The problem is that a small error can have a big effect, so it is a dangerous thing to play with. Most of us never think to do it, however just accept that the bike is correct as received. I think most road bikes are set up more for stability than for race conditions. Most of the time it does not matter. These days moderns are usually set up for 'point and squirt' anyway and it's probably the safest way to go racing. The trouble is you cannot beat a more powerful bike by playing the same game. If you do 'point and squirt' with a commando against a modern bike you will find out what I mean. If you can turn under them and get the power on earlier - much better. With my own bike, I sometimes have the thought in corners that ' I should not be doing this'. It hasn't hi-sided me - YET.

"Hi " Acotrel" (Allan?), did you say that reducing the yokes offset decrease trail ?"
"Yes" Al
you lost me there Al

Taken individually:
Less yoke offset = more trail.
Smaller wheel dia. = less trail
Increase in head angle using a given size of both the above = more trail.

The problem is that a small error can have a big effect, so it is a dangerous thing to play with. Most of us never think to do it, however just accept that the bike is correct as received. I think most road bikes are set up more for stability than for race conditions. Most of the time it does not matter. These days moderns are usually set up for 'point and squirt' anyway and it's probably the safest way to go racing. The trouble is you cannot beat a more powerful bike by playing the same game. If you do 'point and squirt' with a commando against a modern bike you will find out what I mean. If you can turn under them and get the power on earlier - much better. With my own bike, I sometimes have the thought in corners that ' I should not be doing this'. It hasn't hi-sided me - YET.

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Boy Oh boy Alan do you know how is goes down around here. In a way racing clunker Commando power against half century advanced engines, frames and suspension is like a signle action revolver againtst machine guns. I knew nothing about the geometry details only that if I didn't blast into turns rather harsher than the moderns hard on brakes up to apex their pure power exiting would pass me wether on my SuVee or Peel. It just costs money to get power but takes experiments and risks to get superior handling. Ya can talk all the rakes and trails like professors till the cows come home but only experimenting with ride style in many conditions can reveal the actual facts. Elite race teams have data logging 'puters but its the pilots feedback from tweaking suggestions that actually guides them. I mainly want more Peel power because of constant fear in public blinds so I could point and shoot safer - knowing I should not be risking unknowns. All cycles will low side or hi side if pressed hard enough some can take more pressure before they do and when they do some are still in control but others are gonners. What i discovered maxing out a few types of cycles was some will fight ya trying to fling em down into a turn but would automaticaly pop back up to exit on power well while other would flip down on power fine but fight ya to pop back up in time. As the moderns i did this on had very similar geometry, I feel it was the difference in engines, V-twin or inline 4 CoG in those cases. Peel will have somewhat variable rake/trail on the fly and to me is totally neurtral on pitching over or popping up, same small effort either way, thank goodness.

Steve, when I was racing regularly years ago there were certain guys who, if they were on the start line beside you, made you feel like packing up and going home. Ron Toombs and Bill Horsman in particular, had loads of experience and more moves than any cat. These days in MotoGP there are a lot of young guys, and the teams rely on their feedback to help develop the bikes. You need to be young to race seriously, however I feel that their involvement at all levels encourages a certain riding style, and direction in which the bikes progress. There is an old saying 'if you have a victim's mindset, you will be a victim'. At the level most of us paupers race these days even a bike as silly as a commando can be made to go well. A lot depends on the race class - what you race against. A good 125cc two stroke would make the Seeley look very stupid.

Yeah man a drag race launch next to experts on elite cycles is adrenalized nerve tester. After the isolastic Commando there has only been incremental improvements & mostly better tires rather that the cycles to allow better handling. To leave counter steering limits takes a new state of mind and faith in cycle-tire behavior that's a blend of suicide injection drug during toe curling sex orgasm. I ain't backing off to come back down to Earth so will have to get it out my system as an old fart nothing to loose.

with the 850 frame fitted with the Atlas yokes and 14" rear shocks I bought , I calculate trail to be 4.12"

Ok brainyac what does a '72 750 trail clac with 15" shocks?

with your standard 72 yokes my calc is 3.24'

Thankyou maddass as I had no idea and may not like what it does on pavement flings so have air muscles to fudge somewhat more trail and a bit more with squaty profile 16" rear tire and 19" front. Peel would squat detectably as I nailed her into turns so may work out ok.

Cheesy said:

Rohan said:

Which year of 750 did you draw ?

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Not sure, I thought the 750s were the same other than the early frames that broke, its a combination of the steep steering angle and measurements from my own 71

top picture here.....

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Are all of these drawn with the steering stems parallel to the fork legs

they are drawn with the frame neck parallel with the fork legs as they should be, regardless of the angle.

then if the frames are drawn correct they are wrong

In the yokes and fork legs of the 850 (equals disc brake models), the fork legs do point backwards slightly. You can see this in the bare yokes themselves - the top yoke is slightly further out than the lower yoke.

splatt said:

then if the frames are drawn correct they are wrong

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What is drawn wrong?

"In the yokes and fork legs of the 850 (equals disc brake models), the fork legs do point backwards slightly. You can see this in the bare yokes themselves - the top yoke is slightly further out than the lower yokes"

thats correct Rohan as most of us know.
but I fail to see how the drawings are incorrect. I'm sure we will find out soon.