Decisions Terra/Strada 650 vs Duke 690

Now that's interesting. However, I haven't been able to follow all your numbers. Where does the 1,79/1,70 inch travel come from? As the ABS ring has a fixed size, the travel between slots remains the same on all wheel sizes, just the passing time varies at a given speed.

Well, here's my logic, maybe I got something wrong. Hopefully my technically limited english will be sufficient to explain; haven't found everything I wanted in my preferred dictionaries:

The stability of a wheel and therefore the motorcycle is determined by its rotational forces which are directly related to the mass of the wheel and moreover its circumferential speed. A larger wheel therefore provides more stability at low speeds - the main reason why it is preferred by off road people?

For the ABS to react the time between the slots of the ring passing the photo sensor is the determing factor - as long as the tacho indicated a minimum speed before potential locking occurs. So at same revolutions of the ABS ring, the circumferential speed of the larger wheel is naturally higher due to the greater angular speed - it has more stability. In other words, when the threshold, at which the ABS releases the brake, is undercut (the slots passing by too slow), the tyres of the larger wheel naturally still have more stretch (=more outer speed) than its smaller counterparts.

In accordance, the 19 inch wheel will be travelling slower (on its relevant outside, the rims) and therefore be closer to the instablility realm as soon as the ABS starts its service. To put it different: The ABS reacts later. For sure the ABS has been set up to deal with this

But if you go down further to 17", the tyres/circumferential speed and therefore rotational forces MAY already be too slow before the ABS finally reacts, to provide the stability needed to keep the bike up. Not beeing not a sport bike and being from BMW, the ABS is most likely setup pretty much on the secure rather than hardcore side of things. However, more so for the 21 inch wheel than for its 19 inch fellow.

And, on the other hand, an even wilder GUESS to debate, the Terra ABS surely has not been set up for super sticky street tyres. So when you decelerate hard, albeit controlled, you may run into a unwanted releasing of the brakes, because a knobbie or touring tyre is already expected to be close to move over to sliding friction and therefore the ABS initiates a release. So from this point of view it still could release too early as opposed to my previous statement. But given the different friction of the various surfaces we travel on and that can't be determined, I really doubt this could be part of the ABS programming.

Lest we forget, an ABS is not a black/white loop that releases wheels after locking, but acts before the wheel potentially locks and therefore interprets the curves created by the passing of the slots and comparing those to its preprogrammed values.

Of course, I'll happily stand corrected, obvioulsy I am no expert at all. I just would be carefull with going too small. But it remains fact: I do not know.

Edit: Of course the instability of the wheel during slow rotation speeds is not an issue for the short time of its existence while braking and not the reason for the ABS, of course the locking is.
But the instable realm just comes before locking and is the part where I suspect the ABS to take up its work. So "instable" refers more to slow (or none) rotation of the wheel which is detected by the ABS. Could easily be misinterpreted in above post

Ok. Some facts. the front and rear ring on the Husky has 48 slots.
The STD Strada rolling diameters are 659mm for the front and 656mm for the rear unloaded. That is almost exactly the same loaded diameter, though under braking with the front loaded the front will be smaller ie. rotating quicker.
Therefore the slot speed in slots per second for any road speed would be almost exactly the same under normal conditions, certainly within any tolerance.
If I fitted my preferred tyres the front would be 586mm (110/70-17) and the rear 624mm (160/60-17). That's a real difference. The rear is 6% slower rotation than the front. Is that enough to trigger the ABS? Can the std ABS simply re-normalize the rotational/real speed as part of it's initialization at rolling start ( it definitely doesn't start working until you have started moving)? Is 6% within the std tolerence for differential ? Does it not matter at all as the ABS measures the rate of deceleration of the wheels individually and couldn't care what the other wheel is doing? These are the questions that I need answering.

Note: the circumferential speed for a given road speed must be exactly the same . It is attached to the ground. The angular momentum will however be different. At 60mph the circumferential speed will be 60mph.... The larger motorcycle rim diameter has the effect of moving the mass furthur from centre and thus increases the gyroscopic effect, EVEN for the same outer diameter, as the rim and tyre are the heaviest part of the assembly. Plus dual and off road and higher profile tyres tend to be much heavier. How much heavier? From my racing days a 120/80-17 road tyre was 5.1kg and a 120/60 of the same brand and type was 3.1kg.... Knobbies move even more mass furthur out.

Just buy the Terra and forget about the ABS.

Heh. Not much for antilock braking to do on a stationary vehicle. I don't think it's not working until you start moving, it just hasn't completed it's start up system check procedures, and I guess the last test would be that the sensor is detecting wheel rotation when the wheels start rotating, and the warning light on the dash will extinguish when it has completed it's tests.

@Greg: The wheels are controlled independently - to a certain degree. If you do a long wheelie and applying the front brake, thus just the real wheel rotating, the ABS would switch of after some time because it detects a faulty/non normal condition - front wheel not moving, but real wheel does. Lots of people who drive motorcycles with a not switchable ABS put their bikes on the center stand and let the rear wheel spin to switch it off, if so desired. Very common f.e. with the ABS version of the Yamaha Tenere

However, while I wouldn't hold my breath your 6% should not matter at all. I would expect way more tolerance.

So the wheels are unlocked (if needed) independently, but the ABS does a few sanity checks with both wheels. And one of them (the only?) is to check, wether front and real wheel are turning within a sane range. I do not know the time frame or spinning difference, until the ABS shuts of, but maybe we do have ABS equiped wheelie experts here who want to find out?

It is also true, that the ABS is switched of after a certain speed has been undercut (4kph?). Otherwise, when applying the brakes while standing in front of a red traffic light they would be released. Most likely not what you want. This is close to what it is doing at the beginning, when the light stops blinking. After ingnition I guess it just does a few more checks.

Of course circumferential speed is the same for a given speed. That means, that the 17" wheel will turn faster and therefore the slots passing by faster causing the ABS not to react, maybe even if it should, because it has been programmed for larger diameters. Just the opposte way of looking onto it.

We just should not forget, we are not talking about constant speeds, but about a rapid deceleration of the slot speed. So when you panic brake, how much time is there on various surfaces until the wheel would lock up? Full turn? Half turn? Quarter turn? I do not know, but that's the time the ABS has to react. How many variables can be changed so it still works on different surfaces? Suspension, wheelsize, tyres (grip)?
I still guess, a 17" inch front wheel would work, I'd just be careful. I just do not know about whats really going on inside the ABS computer.

And on very slippery surfaces chances are, it won't work anyway. Classical, wet european cobble stone pavement or wet foliage can become veritable ABS killers. Panic braking there is kinda like playing lotto. As would be going real fast.

So it's not that the ABS would stop working, unless the 17" front wheel exceeds the spinning tolerance talked about above, it may just work less reliable under extreme conditions. Which after all is the reason, we've opted for it.

And, depending on the country you live in, the insurance may or may not refuse to pay with a non standard size front wheel in the unlikely event of loss of cabin pressure...

Again, it's mostly just guessing and shall act as a motivation for further investigation.

If the ABS worked by measuring the differential between the front and rear wheels, the actual speeds don't matter much. There are other reasons to measure actual rotational speed such how MUCH pressure to vary, but WHETHER to actuate at all only needs to measure the difference in wheels road speed. The limits on difference is speed related too long an interval ( slow) and error is too high, too short (fast) and accuracy suffers.
Re: the effectiveness on slippery surfaces. The main reason I even care about whether the ABS is working is because this unit is the same as on my BMWF800R and I know it works on ridiculously slippery surfaces, primarily because I am still alive. I think I related this elsewhere in the forum...Scene: BMWF800R. Metzler Roadtecs. North Queensland monsoon rain ( maybe 300mm/hr). Rain so heavy I could just see the back of the semi about 80m in front. prob 10mm of water on the road and it had been raining for months so the roads were covered in slime. Highway speeds. Semi jams on his brakes and starts sliding in front of me. Thinks..."I'm dead"...while grabing the brakes. Bike brakes to a stop is a ridiculously short distance way short of the semi... I love this ABS....love, love, love.
Ergo sum: I may want to have 17" wheels for better turn-in ( like the KTM), I may want much better road tyres ( which I can fit to a KTM) , but I really want this ABS to be working .
I have a drz400sm (17") with good tyres now as well so I might just do an experiment with the front wheel when I get a chance. Only need to set it up to try it. brake is on the same side so I will have a look.
Still maybe I should have bought the KTM Duke or maybe I will give the new 390 duke a try.
Insurance isn't a problem.

I have seen a 690R at the dealer I bought my Strada from on the day I picked it up. I love the Duke, but not for $2500 more than what I paid. So far, I think I made the correct choice. I hope the support for the TR is good given the issues with the sale of Husky.

Motosportz, that's interesting. I wouldn't have guessed it was that bad until it got really loose or at really high speeds. I have a BMW F800R which has , I think, the same ABS unit though the firmware/software or variables may be different. I have had it on dirt roads a few times ( not lots) and it seemed OK. Have you ridden the BMW dirt 880 and 1200 twins . I heard their ABS was pretty good in the dirt, though had to be turned off when you really started to push it. Seems strange that BM would stuff it up that bad on the smaller dirt bike. Was it the usual lack of braking feel once the front gets loaded under brakes or something else. Sally (GF) was thinking of a BM650 single caus unlike the Husky she can reach the ground but with this info may require a rethink.

" Motosportz, that's interesting. I wouldn't have guessed it was that bad until it got really loose or at really high speeds. I have a BMW F800R which has , I think, the same ABS unit though the firmware/software or variables may be different. I have had it on dirt roads a few times ( not lots) and it seemed OK. Have you ridden the BMW dirt 880 and 1200 twins . I heard their ABS was pretty good in the dirt, though had to be turned off when you really started to push it. Seems strange that BM would stuff it up that bad on the smaller dirt bike. Was it the usual lack of braking feel once the front gets loaded under brakes or something else. Sally (GF) was thinking of a BM650 single caus unlike the Husky she can reach the ground but with this info may require a rethink."

My BMW F650GS had a switch right on top of the steering stem to turn off the ABS. I liked it on for gravel roads generally. Gave the feeling you weren't going to tuck the front from too much front brake. But it HAD to be turned off in the really technical stuff. Especially steep downhills. You couldn't stop or even slow down with the ABS activated.

ABS is a WONDERFUL unit if used correctly. Saved my ass 2-3 times on pavement. Offroad I just switch it off.

How did you like the BMW engine compared to your Terra?

The Strada rear gives up really quickly, even on pavement. I have to be very light with it. Off pavement with any sort of loose stuff and downhill it's very easy to end up not being able to slow down. Yes, it's switchable, but you have to remember to do it every single time you start the bike. I found myself forgetting a bunch. On VStroms there are after market switches/homemade that kept it off regardless, that would be a good option for Strada off road riders it it would work.

FYI, I found that the ABS can be switched on/off while stopped in Neutral and the bike still running.

I love the ABS on the pavement which is where I do almost all my riding. But we were considering doing some dirt road exploring if Sally bought the BMW 650 ( now 700) which she could actually reach the ground on. With the info above I think she just might scratch the BM from her purchase list. She would never remember to switch it on or off anyway. and she lacks confidence as it is. Imagine what she would be like after an ABS induced anxiety attack on the dirt going downhill...

Turning off the ABS isn't so much of a pain in the butt that it should get in the way of riding or influence a decision to buy or not to buy a bike which is probably otherwise ideal. My wife is also short 5'4 and lacks confidence, and she bought the F650GS (800cc twin -factory lowered version) when they were first released in '08. She still has that bike and is pushing towards 50000km on it (mostly on sealed roads) and has great confidence on the dirt roads (except for the very slow stuff). When we get to a gravel road, she has to come to a complete stop and hold the ABS button to deactivate it. It's only a few times a day, if that. I'm used to it so slow down and give her a chance to catch up because whenever we reach the end of the sealed road I know she'll be stopping. The end of the sealed road is the reminder for her that she needs to stop and deactivate the ABS. She doesn't forget. We're not fast riders on dirt. Neither of us have any prior experience growing up riding dirt bikes or that stuff like lots of other riders.
I don't even think ABS on or off makes much difference for her in reality. I doubt she ever hits a corner hard enough to lock up the wheels except under the most extreme unexpected condition. She's almost always riding behind me, so she always has that few seconds advantage of watching me stuff up the corner so she doesn't have to.
That BMW 800cc engine is so smooth and responsive.

Having now done three off-road weekends with my mostly transformed Strada into a Terra I personally like the ABS off-road. While I grew up on dirt bikes and think I'm an above average off-road rider--I can't say that I can do better than ABS overall. Yeah, I can descend a rutted steep goat path, mash on the rear brakes, and not much happens in reducing forward velocity. But if I also add front to the equation the Husky does very well. And I guess I can see the *idea* of a person concentrating carefully to extract both front and rear traction without ABS...kinda like a ballerina dancing down back-and-forth-sideways.

But, i'm not a ballerina. I'm a rider who bought the Husky to ride on-road to get to off-road..and then off-road to enjoy the scenery until I have to go back on-road to get home and ride the rest of the week commuting on-road.

But, the biggest appeal to me of leaving ABS on is that if I'm going down that slope, a 4x4 comes from around a corner and fills my world....when I grab a handful of front and rear brake...the motorcycle is probably going to do a better job than at slowing things down than I will in that panic situation. Heck, without ABS I *might* even grab too much front and land on my face! But, with ABS I'm positive the things stays totally upright and pulses front and rear until I come to an unceremonious and digitally assisted stop.

If I didn't like the ABS I could either turn it off for the session, pull the fuse, or change the fuse to a switch if I wanted to without too much effort.

Back to the OP. I haven't ridden the 690..can't comment. However, I'm totally glad I went with a Strada! About $1200 into wheels and fender to make it a Terra, and love it. Plus it was $100 cheaper than a Terra when I bought it.