17" Wheel Conversion Woes

[ajacstern]

Hello all,

Hope everyone is doing well. Over the winter I decided to convert my '87 FJ1200 to 17" front and rears. So far I have only converted the rear as I wanted to get a feel for the difference each individual change made. I used a 93-97 gsxr1100w rear wheel (I believe) and a 180/55/17 Dunlop Roadsmart 4. 38f/42r psi. For the brake I wanted to keep the original mounting point so I used a GSX750F Katana rear caliper and torque arm, had to hammer the torque arm a little for clearance but it bolted right up after that. Unfortunately, removal of the rear wheel now requires removal of the caliper as well, not sure if that is the same with the underslung, if not then I may switch to the underslung after all.

I went out for a test ride and at first it felt great. Tire heated up very fast, quite sticky, feedback and compliance felt good. On low speed turns the bike felt like it wanted to fall into them and you had to turn the bars the other way to keep the bike up, but it wasn't crazy bad given I don't do Motogymkhana. Once you got to 20-25mph the handling feels absolutely perfect. However, at high speeds (55mph+) the handling is incredibly stiff, takes your whole body weight to switch lanes quickly. This problem was present with the old wheels too and is the main reason I switched, though it feels a little bit worse now. I am hopeful that switching to a late model front wheel and raising the forks will fix this.

The biggest two new problems come in the braking department. I tried to trailbrake a little bit and the bike violently shot upright, much more than it has ever done before. It used to be fairly accepting of trail braking but now it is barely possible to turn and brake at the same time, this is using the front brake. I don't know if this is because of the wider rear, 0.3" lower rear ride height, or switch to radial tires. Has anyone had this and found a fix? It is bad enough that I would go back to the 16"s which I really don't want to do. The other issue is that this rear brake is barely a suggestion. It is functional, but even if I stand on it with my heel it can't lock up the rear and barely slows the bike down. If that is somewhat normal I will switch to sintered pads but don't want to throw money when the caliper is simply not up to the task...

Thank you!

[red]

Ajacstern,

Can't help with the problems, except to say that front tire inflation pressures may be a factor in your turning issues.  I would try adding pressure to the front tire, in 5 PSI increments, and see if things improve at any point.

A 17" front wheel is more important that a 17" rear wheel, IMHO.  There are lots of choices in a 17" front/16" rear combo.  There are very few 16" front tires available lately.  I would not be overly concerned about running a 16" rear wheel right now.  The rear wheel parts you have now will still help at some later time, if needed.

If I was going to switch to a 17" front wheel, I would grab an entire front end (used) from a suitable donor bike.  You can often find such deals on eBay.  With a bit of luck, you can be upgrading the front suspension, brake caliper, brake master cylinder, top steering crown, handlebars, and oh yeah, the 17" front rim, all without needing any special adapters, spacers, shims, or other hardware.  Chances are, this deal can cost less than just a few parts, bought separately.

[Millietant]

I will probably get shot down in flames here, but.......................and if I'm reading things correctly, you have swapped the 16" rear wheel for a GSXR 17" wheel, but are still running the original FJ 16" front wheel.

Into this mix, you have put a 180/55 Dunlop Roadsmart tyre on the back, but must still be running the original size/type of tyre on the front.

"Standing up" while trail braking always has been a factor with the FJ's 16"  front wheel/tyre set up - it was something i liked on my first 86 FJ, in that if I braked as I leaned over going into a corner, releasing the brake would make bike turn in more sharply and it made me feel like I was race hero....until a track school taught me otherwise 

Personally I think you're suffering from a mismatch of tyres, profiles, sizes and types, which is likely why your handling problems are exacerbated. As Red has said, you may be able to get some improvement by playing with tyre pressures, but you have to realise that the FJ wasn't originally designed to run with such odd tyre/profile mixes. I also think that if you do manage to swap the front wheel to a 17"x3.5" (or even a 17"x3" FJ wheel) and fit it with a Roadsmart 120/70x17 tyre, to match your rear, you will have a much better base from which to start getting your set-up perfect.

When I did my wheels swap, I deliberately waited until I had both wheels that I wanted, so that I could fit them at the same time. Most of the more modern big bikes have been running 120/70x17 and 180/55x17 matched tyres for around 25 years and those sizes are obviously known to work well as a matched pair - and all of the advice I have had regarding wheel/tyre swaps over the years has always been to match the combination to what we know works, rather than just doing one wheel at a time.

Sorry I can't be more positive, but I do hope you get some joy quickly so that you can go out and enjoy your FJ to the full.

[ajacstern]

I appreciate the replies, and you are likely right that getting the front switched over could fix a lot of problems. I do already have a front end from a late model FJ (thanks woodcreekpete!) and matching tire so that is not a problem, I wanted to do the conversion step by step so that I could feel what differences mods made for my own knowledge (plus the 16" front tire is still in perfect condition...not that I am a cheapskate or anything).

That being said I know there are some bikes that run a radial rear and bias ply front, and given that the late model front tire is the same section size (120) and final diameter, I was expecting mixing the tires to not cause too much of an issue. I could of course be totally wrong and the only handling problem is not having a radial in the front, just wanted to eliminate any other obvious possibilities first. The drop of 0.3" in the rear from the new tire causing too much trail, too wide of a rear tire having some really obscure effect on the front end, that kind of stuff.

I'll either raise the forks up a bit tonight and see how that works, or switch over to the 17" (and potentially raise the forks as well), not sure which yet.

Also on the brake side, does anyone know what diameter the FJ's rear master cylinder is, is it 1/2"? I wonder if there are any swaps that could give me more brake pressure, though maybe these organic pads just really suck.

[Bones]

Concerning brake calipers, I have the underslung but don't think it would make any difference with either of them the way the caliper sits inside of the rim, so taking the caliper off makes for easy removal and fitment of the wheel.

The braking power with the gsxr wheel is pitiful compared to the FJ, the smaller diameter rotor has a big effect on stopping power..

I ran mismatched tyres for years with no ill effects, that was front cross ply, with rear radial, 17" both ends, but I did notice an improvement when I first fitted the bigger rear wheel, seemed to corner more stable compared to the high walled 16".

Well sorted suspension makes a world of difference to handling as well, so no matter how good your tyres are saggy worn out springs and shock will have a profound effect on handling, that should be the first thing to sort out then go from there to fine tune it.

[Millietant]

Concerning brake calipers, I have the underslung but don't think it would make any difference with either of them the way the caliper sits inside of the rim, so taking the caliper off makes for easy removal and fitment of the wheel.

The braking power with the gsxr wheel is pitiful compared to the FJ, the smaller diameter rotor has a big effect on stopping power..

I ran mismatched tyres for years with no ill effects, that was front cross ply, with rear radial, 17" both ends, but I did notice an improvement when I first fitted the bigger rear wheel, seemed to corner more stable compared to the high walled 16".

Well sorted suspension makes a world of difference to handling as well, so no matter how good your tyres are saggy worn out springs and shock will have a profound effect on handling, that should be the first thing to sort out then go from there to fine tune it.

Although the 120 on the front has the same "section"/width as the 120 on the 17" wheel, I would hazard a guess that the cross section profile is quite a lot different. I''d also guess that the same could be said of the cross-sectional profile of the 180/55x17 tyre vs the 150/80x16 tyre. I believe it's this mismatching of the profiles rather than the radial/crossply mismatch, which is at the root of your steering and turning-in issues.

I do understand wanting to see the difference each individual change makes, but in terms of tyres, and especially cross sectional profiles/widths, I think you're going down the wrong path by not considering how the 2 tyres (front and rear) work together.

[ajacstern]

I started to work on assembling the front end for the conversion over to '89+ and began by mounting the 120/70/17 Dunlop on the front wheel. It looks quite odd, I was very convinced I didn't have the bead all the way seated but that is not the case. The tire measures 4.5" wide at it's widest point, where as the Avon I have on the 16" wheels measures 5.0". The Dunlop also has a ridiculously peaky profile with a scarily tiny contact patch in the center. With a ruler pressed against it the Dunlop measures 2cm of "center flatness" where as the Avon measures 3cm. It seems very odd to me that two tires of the same section could have a half inch of difference between them.

I suppose the 3" rim (which dunlop says is acceptable but recommends a 3.5") is likely pinching the tire producing a more extreme profile. They don't make a 110/70 in the same model as my rear, but there is a 110/70 Dunlop Q3+, it goes from a 58 to a 54 load index though which is quite a drop. Not sure whether this width is perfectly acceptable and to try running this tire or to return it and see if I can find a better fitting front tire. The slow speed handling already has too much fall away, I can't imagine this tiny tire would make it any easier and is so skinny that it looks unnerving for a bike of this weight.

You all are certainly right about the profile being much different haha.

[Millietant]

The 89 FJ came with a 120/70x17 front tyre as standard on its 3" rim and the general consensus was that this definitely improved the steering vs the 16" front tyre on the earlier models.

As you've found out, the ideal wheel size for the 120/70x17 is 3.5", especially with a radial tyre, but many people are running that tyre in radial form with a 17" rear wheel using either 180/55 or 170/60 tyre.

It's been my view for about 4 decades now that tyre width is of minimal importance on steering vs tyre cross section profile (especially on the front) and matching (but not necessarily the same) profiles are the key to predictable, steering and turning in. 

[Pat Conlon]

The 89 FJ came with a 120/70x17 front tyre as standard on its 3" rim and the general consensus was that this definitely improved the steering vs the 16" front tyre on the earlier models......

This is easy to understand when you consider the weight difference between the two....
The oem narrow 16" front rim with the 120/80 bias ply tire and those double faced vented brake rotors were considered pig heavy. The weight difference between the two wheel assemblies is measured in pounds, not ounces.

If the 3" wide stock FJ rim gives you heartburn, get a 3.5" FZR rim and call it a day.
I am not a fan of using a 110/70-17 front tire on a fully ladened 800lb + sport tourer capable of the speeds we carry.

[ajacstern]

Agreed on all points.

I checked eBay for a bit looking for FZR1000 front wheels and found many but none from '87-88 which appears to be the only ones that fit (well I found one '88 wheel for $1200 but that won't work...). I suppose if the only difference is the axle size I could get a 17mm to 15mm bushing made. It would all add up fairly quickly though I imagine, and that is assuming the rotors are interchangeable between the FJ and FZR wheels, would rather spend the money on a new rear shock if possible.

I certainly concur that a 110/70 seems way to small for a bike of this weight and power, but I am concerned that the contact patch of this 120 will actually be smaller than a 110. I measured a 100/90 tire on a friends bike and it came out to 105mm at the widest tread portion, measuring the 120/70 Dunlop on the FJ rim comes out to a mere 113mm, just 8mm wider than a 100 section with a smaller flat center section. How does that make sense?

Guess the only other thing to do is to try it, though I am hesitant to throw away $150 on a tire just to have to immediately replace it for safety. Is the torque spec for the 89+ front brake rotor bolts still 14 ft lbs like it is for the '87 rotors?

[Pat Conlon]

I recall folks drilling out their FJ fork legs for the 17mm axle with plenty of material on the fork legs for the larger holes.

Or....Get a set of wheel bearings for the 89/90 FZR rim that fit the 15mm FJ axle. 

I would go with the bigger axle.

[Bones]

I recall folks drilling out their FJ fork legs for the 17mm axle with plenty of material on the fork legs for the larger holes.

Or....Get a set of wheel bearings for the 89/90 FZR rim that fit the 15mm FJ axle. 

I would go with the bigger axle.

Concerning the bigger axle Pat, not sure about the FZR models but the axle on my 93 threads into the left side fork leg and a pinch bolt on the right side stops the axle from loosening when tight. The axle is not long enough to put a nut on the end so you'd have to either get a new thread cut for the bigger axle assuming it's the same length and style as the old one or you'd have to buy a longer axle and put a nut on it.

[Millietant]

To be honest, if you have  the 17" x 3" front wheel for your 89 forks, why don't you just put the 120/70 17 matching radial for the rear 180/55 17 radial and see how you like it. It will not be worse than what you have now and will likely be a big improvement.

If/when your get a 3.5" front wheel, just take the front tyre off your 3" wheel and put it on your 3.5"....unless of course you've done a lot of miles on it, at which point I'd say save the wheel swap until you need another new front tyre.

[Pat Conlon]

I recall folks drilling out their FJ fork legs for the 17mm axle with plenty of material on the fork legs for the larger holes.

Or....Get a set of wheel bearings for the 89/90 FZR rim that fit the 15mm FJ axle.  

I would go with the bigger axle.

Concerning the bigger axle Pat, not sure about the FZR models but the axle on my 93 threads into the left side fork leg and a pinch bolt on the right side stops the axle from loosening when tight. The axle is not long enough to put a nut on the end so you'd have to either get a new thread cut for the bigger axle assuming it's the same length and style as the old one or you'd have to buy a longer axle and put a nut on it.

Correct, you use a castle nut and a cotter pin on the 17mm axle.  
I don't recall the origin of the 17mm axle you use on the FJ forks.

Any help fellas?

[edit] see reply #9 and #12
https://fjowners.com/index.php?topic=18130.0

[ajacstern]

Well, the FZR wheel swap does look really simple. I think I will go the route of trying this tire on this wheel and then if not looking at either switching tires or switching to the FZR wheel.

When I compared the 1989 FJ1200 forks to the original '87 forks, I found that the 87s had a textured surface on the uppers where the triple clamps are where as the 89s were totally smooth. I am guessing either are fine, but curious if there is a torque difference on the pinch bolts? I also noticed the 89 forks did not have the three little washers under the bottoming cup on the damper rod, I am guessing those were antidive related and are thus supposed to be absent.