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Discussion Starter · #1 ·
I've been fascinated by bicycle geometry for a couple of years and recently have delved into the geometry of vintage MTB's.

I know that the earliest bikes (Breezer, Ritchey, etc..) basically used the geometry of the old Excelsior type bikes they had been using with 70/68 seat/head angles being common. I know the head angles tightened up a few years later with 70 seeming to be common and I know many are now 71 or 71.5 degrees. I just bought Bicycling Magazines 1985 special on ATB's (All Terrain Bicycles) and several builders, including Breeze, Ritchey and Cunningham) mention the geometry they started with and at least one mentions how the angles evolved.

My question concerns fork rake. Were they maintaining a constant amount of fork rake thru all these head angle changes? Finding catalogs with that information from that far back has been difficult. From the 90's to now, a 40-45mm rake seems very common but I don't know if that's what they were using back then.

Does anyone have any insight on what fork rake measurements were doing as the MTB head angles increased?
 

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I've been fascinated by bicycle geometry for a couple of years and recently have delved into the geometry of vintage MTB's.

I know that the earliest bikes (Breezer, Ritchey, etc..) basically used the geometry of the old Excelsior type bikes they had been using with 70/68 seat/head angles being common. I know the head angles tightened up a few years later with 70 seeming to be common and I know many are now 71 or 71.5 degrees. I just bought Bicycling Magazines 1985 special on ATB's (All Terrain Bicycles) and several builders, including Breeze, Ritchey and Cunningham) mention the geometry they started with and at least one mentions how the angles evolved.

My question concerns fork rake. Were they maintaining a constant amount of fork rake thru all these head angle changes? Finding catalogs with that information from that far back has been difficult. From the 90's to now, a 40-45mm rake seems very common but I don't know if that's what they were using back then.

Does anyone have any insight on what fork rake measurements were doing as the MTB head angles increased?
Some of the earlier bikes ran 2.125 or 2.2" offset if I recall and that tightened up to 1.75 or 1.8" as the angles increased. Yeti ran a 1.25" which was different than most all the others. Bontragers ran less (than standard) offset as well.
 

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Another data point - The Fat City Cycles Fat Chance ran 69/72 angles with 2" rake and their Wicked Fat Chance had 71/72 angles with 1.75" rake.
 

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Discussion Starter · #4 ·
Interesting numbers. I'm looking at trail figures for those early MTB's.

Most of them come up with a trail measurement in the 72-77mm range (assuming a 68 degree head tube evolving to a 70 degree head tube), which seems consistent with modern MTB's. However, the two Fat Chances are real outliers at 55mm and 62mm of trail. The 1.25" measurement I didn't even calculate but that one would have to have a whole lot of trail unless it was a very steep head angle, which none of these seemed to have.

I'm currently riding an early 80's MTB that appears to conform roughly to the old 70/68 angles but I have no idea how much rake the fork has or how much trail it has. I know it has a TON of wheel flop, which was disconcerting at first but I quickly got used to it.

I'm trying to figure out if it's the high trail measurement I like or some other aspect like the wider handlebars or the longer chainstays.
 

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Interesting numbers. I'm looking at trail figures for those early MTB's.

Most of them come up with a trail measurement in the 72-77mm range (assuming a 68 degree head tube evolving to a 70 degree head tube), which seems consistent with modern MTB's. However, the two Fat Chances are real outliers at 55mm and 62mm of trail. The 1.25" measurement I didn't even calculate but that one would have to have a whole lot of trail unless it was a very steep head angle, which none of these seemed to have.

I'm currently riding an early 80's MTB that appears to conform roughly to the old 70/68 angles but I have no idea how much rake the fork has or how much trail it has. I know it has a TON of wheel flop, which was disconcerting at first but I quickly got used to it.

I'm trying to figure out if it's the high trail measurement I like or some other aspect like the wider handlebars or the longer chainstays.
The bikes with the 1.25" of offset (Yeti and Bontrager) had 71 head angles.

Another popular fork was the Koski in the late 80s with the constant radius blades and it had a 1.6" offset if I recall correctly. The straight blade version could be made to have any offset desired.
 

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Discussion Starter · #6 ·
The bikes with the 1.25" of offset (Yeti and Bontrager) had 71 head angles.

Another popular fork was the Koski in the late 80s with the constant radius blades and it had a 1.6" offset if I recall correctly. The straight blade version could be made to have any offset desired.
The 1.25" comes out to about 81mm of trail (assuming a 559 wheel and 50mm tire on all of these measurements).

I'm looking for a good, fat-tire bike and under consideration is one (production) that is a 700c bike that will take 60mm tires but the trail is 56mm and the other is a custom 26" bike on which the builder feels 60mm of trail is "right". The bike I'm riding now has 61mm of trail with 700c wheels and 40mm tires and it feels downright twitchy to me. I've been wondering about the later 80's MTB's that had the steeper head angles but it seems the fork rake decreased to maintain similar trail measurements with the earlier MTB's.

The dilemna continues.
 
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