Suspension corrected or not? Telling frames apart ....

[letmetalktomark]

The frame shown below is currently using 420mm AtoC forks.

The TT is sloping towards the ST very slightly.

Can you tell from a photo whether a frame is suspension corrected and if so for what length fork?

I appreciate that suspension corrected will mean different things to different people but my understanding is.....

390mm rigid AtoC for - Non suspension corrected
400mm rigid AtoC for - Borderline suspension corrected
410mm rigid AtoC for - Suspension corrected for a 63-75mm'ish suspension fork with a 440mm AtoC
420mm rigid AtoC for - Suspension corrected for an 80mm fork with a 460mm'ish AtoC
440mm rigid AtoC for - Suspension corrected for an 100mm fork with a 470mm'ish AtoC

The frame above is now fully built and has been ridden.

Front end is low and it feels quick.

Just thinking for future tinkering ..... plus I know the whereabouts of a 80mm modern v brake SID fork .....

 

[velomaniac]

I reckon you could get a 80mm or less fork in that easy. I do tend to bodge suspension into frames but work on the principal that if it looks ok it'll probably be ok. If the chainstays slope upward toward the bottom bracket then generally you've gone too far

 

[Woz]

Angles looks about right to me. The best way is to go from the original spec.

 

[cce]

"suspension correction" was alteration of a frame design to achieve a chosen set of angles (typically 71/73) with a suspension fork of a given length sagged to a given degree....

 

[Anthony]

The frame shown below is currently using 420mm AtoC forks. The TT is sloping towards the ST very slightly. Can you tell from a photo whether a frame is suspension corrected and if so for what length fork?

I think it's very difficult, but you can usually judge from the original fork. I'm surprised if the orange fork there is as long as 42cm, it looks much shorter to me, but if you say it is then it must be. In which case, I would guess it's longer than the frame was designed for, but it looks as though the stem is shorter than the original, which may well compensate for the longer fork.

I appreciate that suspension corrected will mean different things to different people but my understanding is.....

390mm rigid AtoC for - Non suspension corrected
400mm rigid AtoC for - Borderline suspension corrected
410mm rigid AtoC for - Suspension corrected for a 63-75mm'ish suspension fork with a 440mm AtoC
420mm rigid AtoC for - Suspension corrected for an 80mm fork with a 460mm'ish AtoC
440mm rigid AtoC for - Suspension corrected for an 100mm fork with a 470mm'ish AtoC

First off, it also depends on what the head angle was with each length of fork. Not all makes went for 71, and even some makes who said they were 71 weren't really - if the designer thought it would work better at 70.5, but the marketing director said they'll only buy them if we call it 71, then marketing would usually prevail in the catalogue, but not in the actual bikes.

Each cm of fork length makes c0.6 of a degree difference to the head angle.

I personally would amend your table very slightly to:

390mm rigid AtoC for - Non suspension corrected
400mm rigid AtoC for - Borderline suspension corrected
410mm rigid AtoC for - Suspension corrected for a 60mm'ish suspension fork with a 430mmish AtoC
425mm rigid AtoC for - Suspension corrected for an 80mm fork with a 450mm'ish AtoC
440mm rigid AtoC for - Suspension corrected for an 100mm fork with a 470mm'ish AtoC

but as mentioned above, in all cases you can compensate for a slightly longer fork by fitting a slightly shorter stem than the original, as this will speed the steering back up again to a vaguely similar extent as the longer fork will have slowed it down.

 

[FluffyChicken]

it wasn't just the angles that where altered but tube length, especially the top tube made a bit longer, I assume to keep the reach you loose when the front end is raised and the tweak of the angles.
BUT really ride it. IF it feels good then it'll be fine.
Angles, tube lengt, bottom bracket height, stays etc, varied between manufacturers and with a manufacturer depending on the target audience. So you're really only trying to keep it's intended angles, if the angels slacken and the seating position isn't a problem. Then other than reach and if you like the bar steering/position all is good.
Of course you may have just turned a race bike targeted bike into a lower end comfort targeted bike but if it work for you why worry.

If you know from the specs what it was aimed at then I would tend to ride it how it was aimed, if not. Best guess.

That's at least a mid 90's frame plus, just from the drop out.
Year of bike is generally a good indicator.

 

[stuey]

LMTTM - frame looks 'tidy' - is that a bi-oval down tube? What is it?

 

[walleater]

Bottom bracket heights on older bikes tended to be around 11.5". If you put a longer fork on it and the BB height goes much higher than 12" then the bike will start to feel a bit giraffe like and not so good in the corners. My 1992 RMB Vertex should have a BB height of 11.5 but it's now jacked up to a tad over 12" and for me everything is an improvement (more suspension, slacker head angle but not raised too much). To give a modern example, a Chromag Aperture has a BB height height of 12.5" but has to deal with a 6" fork sagging. Seeing bikes with forks way too long for them (often MC San Andreas ) and the BB being launched into orbit makes me cringe.

 

[Woz]

Here's a long one which may be of interest.

I once had a frame where I couldn't get the original spec, and rather than go the route of trying to measure angles without
a reference point and the usual trial and error by swapping parts (which I did on a previous build and found frustrating), I
decided to set up a simple comparision jig with another frame which I had the full specs on and knew exactly
what fork length it required and position set-up.

The jig simply aligned the rear axels with a long rod, each frame standing up right on a large table top.

The first step was to compare the chainstay lengths; this gave me a rough idea if I would be sitting more or less
over the back wheel. In this case, they were as near as damit the same.

Then I checked if the seat-angles were different, which in this case they were not. Then I eye-balled the head-tubes
for angle comparision, and again in this case they were not. I then measured the distances from the bottom of each
head-tube to the table top surface, and the difference was 1.5cm. A calculation of 1.5cm - 30% gave me a
rough additional equivallent to how much more the fork axel to crown distance was required for a rigid fork
for the unknown frame. Then I compared the top-tube lengths, which helped indicate what length stem I needed
for a similar reach position to the reference frame. Finally I then compared the top of the head-tube heights, which
gave me an estimation of how many spacers / stem rise needed for a given steering column length to maintain
a similar saddle height vs handlebar height to the reference frame.

This is by no means foolproof, and still could depart from the manufacturers intended geometry, but at least I got a
reasonable idea of what was needed and how the position would work on the unknown frame. In this case though it
worked absolutely spot-on.

 

[Neil]

Have to say, I'm enjoying the discussion, and the info is likely useful, and for posterity.

That said, whilst taking nothing away from the understanding, surely the real test will be whether it feels "right" to ride. If purity matters, in it's own right, then fair enough, but plenty of people, BITD, banged on suspension forks as soon as they were available / they could afford them.

Not me, personally, mind - I've never been happy with boingy forks ongoing, 'cept for the odd ride. And I'm not saying this with the intent of stifling discussion, because the more details, the better, just to point out that the result for some will be pragmatic in terms of how the bike rides and feels to them, and BITD, people were rather blasé about it.