RWG
Hi folks,
I have considdered trying the jack up adaptors on Ebay, they are for the rear suspension loop, has anyone got any experiance with theses fitted to the 851/888 bike. I assume they are to raise the rear in order to quicken the steering, is this so and does that have any negative effect on stabiity that may not justify it.
Any comments welcome, I am about to have new rear shock, loop complete with new bearings and bushes, so just thinking about jack up.
Regards
Paul
Pompey
There are a number of reasons for using a jack up for the rear. It will quicken the steering if you leave the front forks sitting stock. It does it by pulling the rake back for the front end. It can make the bike less stable at speed but the type of tyre profile can do the same... My own 888 is jacked back and front which keeps the same rake but allows greater ground clearance at the apex, hence a high rise platform boot on the side stand. If your suspension settings feel ok the be aware a very small adjustment in ride height can make an enormous difference in feel and a completely new setup... find the sweet spot and stick to it!
Hatman
Anybody source these from a U.S. vendor? McMaster Carr has them, but -- for whatever reason -- the female and the male parts have different threading pitch. Can't seem to find anyone else you has them in the needed 10mm size.
wgsvintage
I've been trying to figure out what "jack up adapters" consist of. Are they extensions for the rod ends at the bottom of the hoop? That sounds like a huge geometry change to me....
Mark - can you be more specific what you're looking for?
Bill
ducati dad
Mr.R
Yes those are the "Jack Up" kits in question.
The reason that they come with a different thread is because the cast rocker arms from the later 851/888 Strada's/Monsters had a suspension bow with a corser thread that allows less fine adjustment.
The early 851Strada's, 851/888 SP's and 851/888/926 Corse machines had the adjustable eccentric rocker arms with finer threads in the rocker arms for almost infinite adjustability.
Steve R
wgsvintage
Thanks d.d., am i missing something? Those strike me as being a solution waiting for a problem.
Mr.R
wgsvintage wroteThanks d.d., am i missing something? Those strike me as being a solution waiting for a problem.
Some riders love them Bill, personally I didn't like them so I sold on the couple of sets that I had.
On my '92 851/916 bike I've dropped the front end by 7mm and raised the back end by 10mm, then set the rear wheel back as far as I could, it's now pretty good to my way of riding.
It steers faster than standard but it's also very stable at warp speeds, as in speeds over 160 mph.
I'd like to try some yokes/triple clamps with less offset, (27mm in place of 30mm) but I'm not flush enough to get some made at this moment in time.
Steve R
wgsvintage
Thanks Steve, you're right, experimenting is all part of the fun of owning these things, and set up is a very personal thing. I have no doubt you arrived at your settings by doing just that, and they sound perfectly reasonable, but I got to ask - where the hell do you do 160 on that little island you live on? Don't you get your feet wet? In my garage, I have a steering range from a 748 w/160 series front, to a bevel SS, so I'm familiar with differences in steering. I just enjoy those differences. So Paul, why not give those things a try, and let us know what you think.
Bill
Hatman
Bill, I have the rod ends on my hoop extended as far as is prudent (10mm left threaded in), and my front end dropped as much as possible (front tire just kissing the V-piece under hard braking). I'm pretty comfortable with the bike, but would like to be able to continue to experiment with different settings this season. By giving me the option of raising the rear more, that gives me additional tuning latitude. I might end up raising the rear and the front. Who knows? But either way, I'd like to have the additional tunability.
Here's the McMaster-Carr page on the male-female standoffs:
http://www.mcmaster.com/#standard-th...ndoffs/=qfv9gt Right idea, but they only go up to M5. They have similar units in M10 sizes, but, as I noted in my original message, they're actually intended to attach different thread pitches, so the male and female side aren't the same.
The Louigi Moto units will certainly work, but they're pretty pricey at almost $60 USD shipped. Was just hoping to find something cheaper stateside, but a quick search didn't turn up much.
Pompey
Many riders jack up the rear on all makes of bikes. Some because they hear advise about improved handling and do it when they dont need to, I have even seen it done for fashion but I have a theory that many feel improvement due to a combination of wrong spring strength for weight and ride style. if you are feeling that you need to quicken your steering then what other symptoms are you feeling? This subject has been covered in length on a few threads so I wont go on too much but... best advice is find a reputable specialist and go and get it set up properly. They will look at your weight, ride style in order to ensure you have the correct springs front and rear in order to have the correct preload and sag in order to get a sensible damping range for conditions in order get the max out of your ride in order to be relaxed and confident in order to BE SAFE at speed!
Get a man who can and pay him. Its cheaper in the long run and better for your bike and you then chasing an error, getting frustrated, feeling wary and thus dangerous.
Gp riders have people to do it...
wgsvintage
Hi Mark,
My early hoop was tapped for M10x1.0. The rod ends were shot, and new M10x1.0 rod ends just were not available - anywhere. So, as was suggested here, I helicoiled the hoop for M10x1.5, which is what I believe the thread in your hoop is. I see what you mean about the McMaster adapters. If you helicoiled your hoop to a different M10 thread so you could use the McMaster adapters, you couldn't go back because M10 rod ends are really only available in the 1.5 pitch. What if you used a good female M10x1.5 rod end and a "long-ish" good quality M10x1.5 set screw to experiment with? That way you could go back if longer wasn't better.
Bill
Hatman
Hi Bill:
I'll probably just pop for the Louigi Moto units. A bit more expensive, but they're the easiest and cleanest way.
You gonna have that 851 out on the track this year?
wgsvintage
Mark, I've been dead in the water with that build for some time now, waiting for Bruce to finish the engine. Can't do any more on the chassis w/o the engine in place. Robbie and I are meeting Bruce for breakfast Friday, maybe the subject will come up.
Bill
RWG
Thank you to all, I am dropping the idea of jacking up, and have decided to stick with Maxton rear shock and tune front end in spring.
Cheers
Mr.R
I doubt very much that you'll be sorry with that decision.
I had Maxton overhaul, re-valve and re-spring the the OEM Ohlins forks and shock, to my weight (and the bikes) on my 851/916cc bike.
Then set the sag to suit myself, they've done a cracking job which I'd highly recommend to anyone.
Steve R
sagot
Mr.R wroteSome riders love them Bill, personally I didn't like them so I sold on the couple of sets that I had.
On my '92 851/916 bike I've dropped the front end by 7mm and raised the back end by 10mm, then set the rear wheel back as far as I could, it's now pretty good to my way of riding.
It steers faster than standard but it's also very stable at warp speeds, as in speeds over 160 mph.
I'd like to try some yokes/triple clamps with less offset, (27mm in place of 30mm) but I'm not flush enough to get some made at this moment in time.
Steve R
Just a note: If you reduce offset you will slow the steering...
Jasper
This mate help,from 916 but applies to ours:
The central issue with making changes to steering angle is stability. A motorcycle is designed to return to its straight-ahead condition after hitting an object or bump in the road that causes the front wheel to deflect slightly to the right or left. In other words, it has to remain stable for a variety of road conditions, and motorcycle stability is foremost a safety issue.
The way dynamic stability is assured is to design a bike with enough distance between the point where the front wheel touches the road and the intersection point between the steering axis and the road. This is called the trail dimension.
A longer trail dimension increases the motorcycle's stability on straights, but will also negatively affect the motorcycle's turning effort, i.e. more rider's strength is required in corners and transitions. However, the more trail, the greater the ability of the bike to self-correct it's steering. It's a longer trail dimension, for example, that allows you to easily ride with no hands on some bicycles, but not others.
A shorter trail dimension, on the other hand, produces a lower opposing force to steering inputs. It's kind of like power steering. So the steering requires less rider strength, but larger handlebar displacements from bumps in the road and corners are fed back to the rider. Said another way, the shorter the trail, the more rider input that is needed to hold a line and the more responsive the bike feels since it is more sensitive to steering inputs.
The two superbike steering angle positions, 23°30' and 24°30' produce trail dimensions of 91mm and 97mm respectively. The wheelbase, also an important factor in stability, remains unaffected when you change it. As a comparison, the Monster steering angle is fixed at 24° and the trail dimension is 94mm. Adjusting the trail dimension on most manufacturer's bikes is not an option.
As an aside, when you change to the steeper 23°30' position you loose a significant amount of steering lock making low speed U-turns more difficult. Also, the ignition steering head lock doesn't engage in the steeper position.
Now, here's Ducati's warning: "Trail should only be altered after all the other (geometry and suspension) changes have been made and you are comfortable on the bike. If the bike displays any instability problems they need to be sorted out first, as this steering head angle change will magnify these characteristics."
(One reason, for example, is that part of its effect mimics changing the rear ride height.)
The Haynes Service manual goes on to say "Warning: The steering head angle must be set to the road position (longer trail) whenever the bike is used on the road. If the steering angle is set to the race position (shorter trail) ... the handling of the machine could become unpredictable on uneven road surfaces."
So, shortening the trail is considered unwise for street riding (unlike tracks) where bumps in corners, potholes and other road hazards repeatedly challenge your bikes steering stability. Here's a case where inexperienced or uninformed riders who set-up their street bike chassis geometry as racebikes are just looking for trouble.
Trying to mimic factory race bike set-ups can get you into trouble. It's central to racing that race bikes often need to sacrifice high-speed stability to handling. Riders may initially run the steeper steering head angle, but often, as they get faster, they realize they want more stability, not less.
To get more stability there are two things that Ducati typically changes on their racebikes: the triple clamps and the swingarm. They use triple clamps with less offset, typically 27mm instead of the stock 36mm, and use a 25mm longer swingarm to increase the wheelbase. These changes to the triple clamps or the swingarm have the effect of moving the center of gravity forward which is the typical starting geometry of the Corsa race bikes. Remember, changing the steering head angle does not, by itself, change the wheelbase or alter the center of gravity.
According to an earlier post by Jeff Koch, for superbikes, for every 1mm decrease in fork offset:
Trail increases 1.1mm
Wheelbase decreases 0.9mm
Height of the bike's center of gravity increases 0.2mm
Percent of the bike's weight on the front wheel increases 0.05 percent
Some here have suggested that you can get the same effect (reduction in trail) with a finer adjustment by increasing the rear ride height instead. However, you'll need to raise rear ride height 16mm to get an equal amount of trail reduction, and in doing so you'll also end up increasing the height of the bike's CG by about 12mm that (among other things) will increase loading to the front tire, so when hard on the brakes, the rear tire gets/feels very loose.
Again quoting Jeff, for every 1mm increase in rear ride height:
Trail decreases 0.4mm
Wheelbase decreases 0.2mm
Height of the bike's center of gravity increases 0.8mm
Percent of the bike's weight on the front wheel increases 0.03 percent
One improvement with the steeper angle that's been observed is in trailbraking - The bike stands up less on the brakes, which can be a benefit on backroads where you never quite know what might be coming up around the next bend. Also, some feel that the steering is more neutral at large lean angles.
The area of major concern is tankslappers. Reducing trail by reducing the force that centers the front wheel will give you more headshake, especially when accelerating (less weight on the front wheel) out of bumpy corners.
You won't get a tankslapper out of most corners if you change to the steeper steering head angle, but you will make them more likely, and more violent when they do occur. Some will say to crank-up an adjustable steering damper to settle the steering, but dampers will only resist changes in steering direction and don't provide a restoring force to re-center the wheel like trail does. The higher damping rates also spoil your quick steering and cause weave instability problems when cranked-up too high.
One more thing.
Keep in mind that lowering the front ride height, or raising the rear ride height, are not equivalent adjustments. Lowering the front serves to lower the bike's center of gravity. With a higher front, raising the rear, raises the C.G.
Again, according to Jeff, for every 1mm that you raise the forks in the triple clamps (lowering the front end):
Trail decreases 0.2mm
Wheelbase decreases 0.5mm
Height of the bike's center of gravity decreases 0.4mm
Percent of the bike's weight on the front wheel increases 0.06 percent
Ducati Corse, in a 1996 memorandum that was posted on the old Ducati.com web site, recommended raising the front 10mm to increase "flickability" in turns. Yes, I said raise, not lower. Raising the front end raises C.G., and a higher C.G. makes the bike go to the tire edge quicker according to the memo. I was told that the same advice is given in the factory race bike setup manual.
Familiar with the Mille SP? It has the capability to raise the engine in the frame to increase C.G. to improve flickability. Same effect. Even the Mille R has the engine mounted higher in the frame to do the same thing.
From a chassis design point-of-view you generally you want the C.G. to be a distance equal to half the wheelbase above the line connecting the axles. Raising the C.G. above this point makes the bike easier to turn.
Once in a turn, a higher C.G. biases the weight more to the inside of the corner which helps the bike turn.
RWG
Jasper, a great read and very informative it also enlightens me more each time I read it.
Cheers
Paul
Jasper
According to my calculations, with my offset yokes and jack up kit I have reduced my trail by 6.4mm!
