I had radials fitted last week - the roadholding is SO much better than bias..can really drop into bends now (after 100 miles of wear of course)
However - if I touch 100mph, I feel the bike is prone to wobble with the slightest movement. So I had the balancing checked (ok .25g added), slight improvement - but just not "planted" at a ton as before. I set the pressures at 36psi front, 42psi rear, the bikes a 1989 2 valve LT.

Incidently, I had progressive springs fitted 3 years ago, these were getting "bouncy", so I had the fork oil changed, and spacers fitted. this reduced the springyness, but not as good as new. I didnt get the 100mph wobble with the bias tyres.

Has anyone else experienced this issue fitting radials? - I dont want to revert to bias as otherwise all is much better - I wondered if anyone uses different pressures at high speed.

Ps - steering head bearings changed last year

Thanks for help anyone??
Colin

Welcome to the wonderful world of radials, I think! An improvement all around was and is my experience when fitting radials to my K bike.

What size tyres did you have fitted, which brand/model, and are they on Y spoke wheels or the three spoker version?

110/80/18 Bridgestone BT023 front
Avon Azaro ST AV46 - 140/80 ZR17 
Y spoke wheels not 3 spoke

That's a proven combination of brands, although I'm not the biggest fan of mixing brands, and those sizes are just what the doctor ordered for your wheels. Duck, here in the Seattle, Washington area, uses them, as well as many others. The first thing I would look at, and we assume you've had a competent workshop do the work, is to be certain the tyres have the direction of travel arrows (look at the sidewalls) facing the proper direction. I use 38+ psi in the front and 41 or 42 psi out back, but 36 psi is pushing just barely sufficient. With speed (and weight) comes heat and too low of an initial pressure will raise the tyre's pressure very quickly.

The other things is, the rate of speed. 100 mph will turn all manner of irregularities in road surface, camber and slight misadjustment to bearings, shock and front fork issues into much bigger issues very quickly. It's fun to do the ton, however. I can't accurately answer the why regarding your old bias ply tyres in comparison to the radials. Perhaps they're simply exacerbating a hidden problem because they're so well-planted at 'normal' speeds, otherwise.

I run the BT45 front at 38psi. Feels very planted but I got a new rear on last week, also BT45, was not too impressed initially, did 150 mile round trip and a few 22 mile round trip commutes, then rechecked pressure and went to 42psi. Vast improvement. I cant say how it compares with what you have put on other than to say the K seems very sensitive to tyre pressure and to tyres.

22:37:04
Thanks for that very helpful advice, I'll check & feedback. So pleased otherwise

I know this doesn't answer your issue, but I used to run BT45's with great results on my old K100RT (36/42). As said before, you're using a proven combination, so I'd look elsewhere. Enough oil in both fork legs? Opened them up/cleaned them out?

Good choice of tires, and welcome to the club man.  :afro: 

1) Before switching to radials I put the bike through its paces on the old bias plys (I have more than one set of rims for this purpose). The reason being I wanted to check the suspension and make sure everything was set-up properly since it may otherwise be hard to tell any difference between suspension problems and tire problems (if you know what I mean).

2) One thing I noticed is that radial tires are a bit "swarmy" at high speed until they are completely and totally broken-in. Consider running through at least two tanks of gas before attempting any high speed riding.

3) I've not gone back and looked at tire pressure, but probably will in a short time. Right now I'm running the standard 35psi front and back, and the ride is just fine.

So what are you using? That might be handy knowledge after such an endorsement.

have to say ...against all advice  the Pirelli sport demon (crossply )  on the front of a big sucker rear redial, feels far more planted at speed and everything else, than the matching radial (110 x 80x18) for the front  and  (160 x60 x 18) Pirelli angel st on the rear .
as aj says the whole thing felt always squirmy  down the highway...and even after being well worn in  ...for me at least ...tings like being sucked into , and pushed away from trucks traveling the same direction  just typifies he stability challenge that I had noticed .

yes that mismatched offsets on the rear and y spoke on the front ...but what I found that works well for me ...at well over 8000 (you know the rest )

 I guess you have to find a good match for the typical handling characteristics of the machine and your riding style ...or maybe have to make some changes to suit .

perhaps later forks with that bridge across the front might tighten it all up ....or move to k1100 stuff up front ...I have a feeling the shorter angle (rake ) of the forks than standard makes for a different feel to radials on the front ....of corse there were models that came with factory fitted radials .....for many back in those times it was quite an amazing change to feel and grip

but with the new tyre technologies that we have now ...almost all good "current" brands out perform the "as supplied" originals from the distant past (thank hevens !)

I had originally Dunlop arrowmax tyres fitted as at purchase ...what a load of shite those things were ...but I found here and the "good suggestions " things are very different from then ..

just my thoughts