Front Ends Over/Under: What does It Mean?

[AbyssmalTailgate]

I'm working on my first build, based on a Yamaha XS650, and I'd like to get a springer front end on there. As I'm looking at front ends, I'm trying to educate myself, and I keep running into descriptions of the fork being "5 over" or "2 under" and so on and so forth. Would someone please explain exactly what that means when the fork is referred to as over/under? Because I'm assuming this is going to affect the rake, trail, and overall stance of the bike.

[XFatTonyX]

It's the amount of inches over or under the length of the stock forks for whatever bike they are designed for.

More length makes more rake and will be more stable but less responsive. Shorter length makes less rake making it more responsive but also more twitchy.

[Saltytonk]

[docmel]

Read closeey, as it is correct: "Over or under the length of the stock forks for whatever bike they are designed for". In other words, you need to know what the stock front end lengh is for the frame that the front end was originally designed for

Typically the measurement is taken from the bottom of the bottom tree to the center of the axle (in case of springers, I believe the measurement is taken with weight of the bike on it

[Steinbvg]

I was always under impression that in modern terms the stock length for harleys implies modern softail as a reference point- total stock fork length about 31.5-32 inches.
An old bt springer fork[ total length 28"] used to be that 'zero over' reference point- but now is regarded as '-4 under'.
Stock dyna wide glide is i think '+2 over.
My chopper is 12 over, when i was ordering fork tubes from cycle engineering- they had the table[ total length of the assembled fork is 44'']

[Saltytonk]

Typically the listing for over and under is based off of 24.25" . which is why 28" is considered +4. The chart I posted above is the only one I could find with a Yamaha stock dims listed.


And that is the length of the fork tubes . assembled and installed with weight on they sit at about 31.5"

[Grantman]

Any idea of the years considered "early" and "late" for Sportsters on that chart?

[Tattooo]

I often wonder why a person ask a question and never gets back in the discussion???? Is it because they really didn't give a damn in the first place?????

[Saltytonk]

Any idea of the years considered "early" and "late" for Sportsters on that chart?

I don't know.. but I'm guessing early would be 33 mm and late would be 35mm ......I'm thinking 39mm isn't listed there.

[AbyssmalTailgate]

So I have a few more questions. Theoretically, if I swapped a set of forks from a Harley standard glide onto my Yamaha XS650, I would have a virtually identical rake and trail as before because the fork length is the same at 21 1/2"; however, between the two bikes they would have different rakes and trails with the same forks because of the frame geometry?

Additionally, it sounds like over/under is a term that exclusively pertains to swapping out forks, plus it changes with every bike (?). Like if 20" forks are 1" over for a bike that has a stock length of 19", they are 1 1/2" under for a bike that originally had 21 1/2" forks. Is this correct? What I'm getting at is knowing that a springer is 4" over for a sportster is essentially meaningless in practical terms for my Yamaha because that over/under value is going to be different? The only function that example would serve is calculating the length of the forks... if it isn't already listed?

Thanks for the help and replies so far!

[AsianDan]

It doesn't really change the rake if you add longer forks. Just tips up the bike. But that changes the way the steering & travel & handling for sure. I had stock length, then 2 under then 6 over then 5.75 over then 7. Lots of fork flop now.

[47str8leg]

what does anything mean ,Life is such a Question mark.

[Grantman]

I don't know.. but I'm guessing early would be 33 mm and late would be 35mm ......I'm thinking 39mm isn't listed there.

That sounds reasonable. Any idea what year that change took place? I knew I should have paid attention to stuff like this when I was younger.

[AbyssmalTailgate]

It doesn't really change the rake if you add longer forks. Just tips up the bike. But that changes the way the steering & travel & handling for sure. I had stock length, then 2 under then 6 over then 5.75 over then 7. Lots of fork flop now.

Ahh, I guess that does make sense that the rake doesn't change haha. Thanks for the input. Are you able to say, more or less, how a bike that is "tipped up" handles compared to one that isn't as tipped? Or do you know of any forum threads that discuss this?

[FredoSNR]

Pretty sure that chart is inaccurate, since the VL is slightly longer than the later springer. 1" longer from one source, 1/2" longer from another. Either way this depicts the VL as being actually shorter than the inline. Maybe this chart is from a catalog of repops?