BTinNYC 

Bike: 84 Centurion Prestige, bought the frame on ebay.
Issue: On some upstate NY roads with regular waves or bumps, this bike has a front end wobble on decending that is not acceptable. My other bikes don't get this resonance, bumps yes, but not scary wobbles.
Two Pronged Solution (hope and pray 😀 ): I suspect this Tange race-bike fork could be part of the problem. The DOs are spaced at 96 mm, which isn't right, it's bent some how. I lack the skills/tools to really measure this fork, also I don't like it, at all.

I have 2 replacements try out; a Time carbon fork with the exact same geometry, and a heavy generic steel fork with 9mm more rake.

I'm trying the carbon fork first, with testing in a couple weeks.

Predictions, comments, howls of outrage?

seypat

Well first of all, that's not an 84 Centurion Prestige. They were only made in 85, 86 and 89. It's not an 85 or 86 either. It could be an 89 with a repaint. That being said, if you haven't done it already, I'd make sure the headset is adjusted/working properly. Also try it with a couple of different wheelsets to see if that changes anything. If it needs a fork swap, Tange made/makes a chrome fork out of Prestige tubing. You could get one of those.

Here's what the various years of Centurion Prestiges looked like.

Centurion Prestige

seypat

Here's some pics with one each of Centurion's top models from 83-85. The silver with red accents one is an 85 Centurion Cinelli Equipe. One year production only and some prototypes. The Iris one had the fork swapped out for one of the chrome Prestige tubing forks when I got it.

seypat

Seems like I remember a thread about a gold "Prestige" with a busted rear DO that got replaced. Is that this bike by chance?

BTinNYC 

Yes, and it's an 89, you're correct.

I did check the headset.

tricky 

Yeah, throw that carbon fork on there. Hopefully it fixes it. Should be lighter too. Sounds like you don't mind the aesthetics and might even like them better than this unicrown fork. I tend to agree.

If that doesn't work I'd have a framebuilder check frame alignment on a frame table. @Doug Fattic has made me a believer that a lot of frames are out of alignment. Could be a contributing factor here.

repechage

Some bikes are just misaligned.
from day one

i read that the bike has an unknown history.
maybe a replacement or misaligned fork.

frame and fork deserve an alignment.
plenty of ways it can be out and cause a wobble/shimmy

as one descends on a bike the geometry is tested- the weight transfer to consider.

a fork with more rake will change the trail a bit- probably reduce it.

the carbon fork has a decent chance of being straight. That would be a cheap evaluation to assist on what to do.

Bianchigirll 

An old school shop might have the tools and the knowledge to check the fork’s alignment.

It would be helpful if someone knew the geometry of the frame

Mr. 66

It does look to have a steep headtube, that could make steering twitchy. I also think your saddle is back farther than most ride, weight to far back could feel squirrelly with a short wheel base.

seypat

That info could be on the site I linked in post #2. It might have catalog scans.

Charles Wahl

Do whatever you have to to get rid of a unicrown fork.

madpogue 

What event broke the dropout? Could it have also done something to the overall frame geo, or the fork?

gugie 

I don't think that frame misalignmment has anything to do with high speed shimmy. Many years ago a friend of mine and I were flying down Alexander Avenue, which is the route from the Golden Gate Bridge into Sausalito and he developed a "death wobble." He slammed on his rear brake and was able to keep the rubber side down. This was on a fairly new Ron Cooper, which came from Bicycle Odyssey, and I know that Tony Tom (owner, RIP) always checked frame alignment on high end frames before they were built up. Since we were in the neighborhood, we stopped to discuss this with Tony. He said he really didn't know what caused it.

As a mechanical engineer I learned about spring-mass-damper systems and how this can happen. The problem with solving this analytically is that there are too many unknowns, one of which is the physical attributes of the rider. The solution is probably just to change something to get away from the harmonic frequency of the system.

Things I've read that can cause this:

• Thin top tubes
• Low trail (although I did have a bike with high speed wobble which went away when I increased the rake in the fork)
• Weight distribution front to rear

There's probably more. Maybe frame alignment has something to do with it, but I've ridden plenty of mis-aligned frames that didn't shimmy. YMMV

jethin

I’m going to guess that it isn’t specifically the fork, but some type of resonance in the whole bike. Maybe check the tires/wheels installation?

repechage

Not the First Ron Cooper to experience it.

I have a straight descent near my house- I do a no hands test. It is interesting what wobbles and what does not.
every bike that does wobble has something slightly askew - one easy thing to check, well not that easy- is to reference are the two wheels on the same plane and not offset.

a super cheap way is to set the two wheels aligned with a linoleum tile floor. Set the bike up parallel and while only holding the front- walk the bike backwards, only control the front and have it as straight up as you can.
rear wheel remains on the line, good, drifts one way or the other? Try again.
very very often a bike with true wheels and all else happy will not diverge.

if it does, one has some checking to do definitely. Out of plane wheels are pretty common. Dish, twist in the structure as seen in the assembly of the bike...
pretty darn common.
bicycles are pretty good at working even when things are askew.

RiddleOfSteel

@Mr. 66 took a look at two of my recent Swhinn frames ('85 Voyageur SP and my '73 Paramount) and "we" (he, mostly) worked both over. The Voyageur got some rear spacing increase, but was mostly fine, which made sense given its condition. The Paramount needed a lot more work and thus time. Nearly 50 years old, a huge 66x62cm size, and double butted 531 with a number of dents and haggard paint means yeah, checking alignment is a really good idea. Rides great now. I am a believer, too.

You (tricky) and everyone else have all contributed great ideas and possibilities, and I especially appreciate @gugie 's response. Taller bikes seem to be more susceptible to this, and unless it's a Cannondale, anything 65cm and taller with decent (read: thin/thinner) tubing seems to get a no-hands speed wobble above 12-15 mph with me.

Prestige (at least in 1985 and 1986) frames ran parallel 74.5° HT and ST. That's a real good time in the front end department. Stock fork rake was 40mm which produces a trail number of 53mm, which is my no-man's-land for whatever reason (a few bikes were funny at lower speeds with that figure, but fine on either side). This unibrow fork looks to be that or a little more in rake, as an educated guess from looking.

Your setup is a little rear biased with regard to saddle position, stem length, handlebar height, and (more importantly) brake/shifter height. I don't think it's "out of the norm" for a bike setup, so it's like others have said: it's something in the total system of the frame and setup that is giving it a proclivity towards this. And some bikes are more sensitive to this than others. Yours probably is one of those, but I think it's workable. Swapping to a carbon (or just different) fork should give you something to work from. Hope you're able to cure it easily!

BTinNYC 

Wow.
Great insights friends, thank you. I feel the decades of experience weighing in.

Along with the "parts change" I'll do rep's overall alignment test.
Then give it a ride before moving the saddle and bars forward. One change at a time.
Maybe this is a naturally twitchy bike, but I'll try a few things because it's a sweet ride on the flats and uphill. My last trick will be the fork with more rake.

Thanks again, and I'll let you know how it goes.

old's'cool

Try a longer stem.