Raising fork legs in triple clamps of 2nd Gen

[RogueV4]

Has anyone determined how high to raise the fork legs in the triple clamps of an '86-'87 VFR to get optimal turn-in without a noticeable loss of stability at speed?  

 

The factory stock position provides 27.5 degrees of rake and 4.3" of trail - which is alot by today's sport-bike and sport-touring standards.  Honda decreased the VFR's rake to 26.0 degrees on the 3rd and 4th Gens and then to 25.5 degrees on the 5th and 6th Gens - all of which had 3.9" of trail.  

 

Even back in its day, racers like Fred Merkel, Bubba Shobert and Wayne Rainey needed extensive steering head modifications to get the 2nd Gen to turn properly.

 

I've been running with my forks raised 7 mm for many years with no ill effects on stability...and I still have the stock 16" front wheel.  A 17 incher would increase rake and trail.  I've been thinking about raising the forks some more but thought I might try to learn from the experiences of others first.

[squirrelman]

10mm is about the limit.

 

[BusyLittleShop]

Correct... replacing the 16 with a 17 would increase rake and trail... so does raising the forks in the triples... but lowering the forks would decrease rake and trail to a small degree... if you wish to adjust rake and trial to a large degree add ride height under upper shock mount... name your tire sizes and I'll see if my Tony Foale rake and trail calculator will spit out some numbers...

[RogueV4]

Thanks Larry.  I was wondering how much I'd have to raise the forks (lower the front end) to decrease the rake to 26.0 degrees...and if that would provide at least 3.9" of trail.  However, I'm not really sure those parameters are even appropriate for the 2nd Gen, given its skinny tires (120/80-16, 140/70-18) and different CoG.  I bet someone's figured out the optimum 2nd Gen steering geometry setup by now.  Someone who knows what they're doing.  Maybe they browse this forum from time to time.

 

Some magazines are providing suggestions for suspension settings based on their test rides. Tuning can make a noticeable improvement but most riders don't really have the knowlegde, experience and confidence to mess with it so they just leave it alone.  A test track with bumps and such would be of help in finding the right settings...but there's none around.  And, surprisingly, nobody seems to be offering this service - though it would be a good job for an ex-racer with the proper creds.  I'd pay good money for it.  I had Ed Sorbo of Lindemann Engineering rebuild/revalve my forks and make a custom Penske shock for me a few years ago when the Fox twin-clicker gave out.  Though I adjusted the damper a bit, it still wasn't nearly as well composed as the stock BMW S1000RR I rode at the time.  

 

 

[ducnut]

On 7/17/2018 at 5:14 PM, BusyLittleShop said:

Correct... replacing the 16 with a 17 would increase rake and trail... so does raising the forks in the triples... but lowering the forks would decrease rake and trail to a small degree... 

The last 2/3 of your statement are inverse. Raising the fork tubes in the clamps DECREASES rake and trail and lowering the fork tubes INCREASES rake and trail. 

[ducnut]

I don’t know if you’ll ever find an optimal number. If you do it in measured, increments of 2mm, I don’t think you’ll get into trouble. Just make sure you have vertical clearance between the fender and upper fairing (contact will leave marks) and horizontal clearance between the tire and lower fairing. You’ll know when you start reaching the limit, because there will be almost no turn-in resistance at the bars and the bike will start weaving when trying to ride a straight line, with light shimmies over bumps. If you had a steering damper, your concerns would be minimized. 

[VARIABLE9]

1.  Rent a track

2.  Start at OEM setting

3.  Start turning laps, recording settings / weather / lap times

4.  Make changes in increments and turn more laps, recording all the data

5.  Analyze

[Captain 80s]

I have the forks slid up in the clamps 20mm+ on one of my 86 VFRs right now.  But I have a 140/70-18 on the rear and some 1.0 springs in the front that I was compensating for and wanted to quicken things up a little as well.  Zero ill effects ridden hard through some very bumpy country twisties.