Need advice: Geometry for custom rando frame and fork

[maslow]

I’m looking for some advice on suitable geometry for a 700c Rando frame and fork. I might have the opportunity to attend a frame building course next year and this is the sort of frame I’m thinking of putting together.

I’m keen on the rando frames built by Fern in Berlin so to give an idea of what I’m after please see the link below:

http://www.fern-fahrraeder.de/arbeiten/crossroad/

I know my ideal top tube length of around 55cm to 55.5cm with a 100-110mm stem. Saddle tip to bar centre of 54cm. I ride a seat tube of 58cm C-C and saddle set back of 8.5cm to 9cm. I’ve included a shot of my geometry sheet for a 700c road frame.

I plan to use 700c wheels with 40-43mm panaracer tyres with real room for fenders (I was thinking at least 30mm between top of tyre and underside of fork crown according to one of Jan Heine’s blog posts) instead of 650b. I’ve got several wheel sets that I can use and I’m happier with the tyre selection available in 700c.

I wondered if someone could suggest an axle to crown measurement, fork rake and an ideal trail number? I’m guessing low to mid trail as the bike will mainly be ridden with a front rack and ostrich 12 litre bag (I might even include extra braze on’s for a low rider).

For simplicity I plan on using v-brakes with suitable drop levers (or Sheldon v brake problem solvers). Gear shifters will be down tube friction levers.

Can anyone suggest some tweaks I’d need to make to this geo drawing?

Hope you can help. Thanks.

[owly]

Can't offer any advice though those Fern guys make a nice bike. I like the racks.

Looks like you're all legs?

[David Tollefson]

I'd have to do some layouts to get you an A-C (would depend on crown type as well), but my initial suggestion would be to increase the BB drop and stretch out the back end to accommodate clearance between the rear tire and seat tube. The tires you're planning would already bring the BB up. No need to make it higher (unless you're planning to run 190mm cranks).

[unterhausen]

one thing to consider with a front bag is making the top tube longer and the stem correspondingly shorter.

[palincss]

Originally Posted by maslow

I’m looking for some advice on suitable geometry for a 700c Rando frame and fork. I might have the opportunity to attend a frame building course next year and this is the sort of frame I’m thinking of putting together. [snip]

I plan to use 700c wheels with 40-43mm panaracer tyres with real room for fenders (I was thinking at least 30mm between top of tyre and underside of fork crown according to one of Jan Heine’s blog posts) instead of 650b. I’ve got several wheel sets that I can use and I’m happier with the tyre selection available in 700c.

I wondered if someone could suggest an axle to crown measurement, fork rake and an ideal trail number? I’m guessing low to mid trail as the bike will mainly be ridden with a front rack and ostrich 12 litre bag (I might even include extra braze on’s for a low rider).

For simplicity I plan on using v-brakes with suitable drop levers (or Sheldon v brake problem solvers). Gear shifters will be down tube friction levers.

Can anyone suggest some tweaks I’d need to make to this geo drawing?

Hope you can help. Thanks.

Jack Taylor build a large number of bikes with very suitable geometry for your need. 73 degree head angle, 2 1/2" [63.5 mm] fork rake was their standard geometry. They were mostly for 27 x 1 1/4" but when you substitute 700x32C, as I've done with my 1963 Sports model, you end up with 40mm trail, definitely low trail, which works superbly with my Berthoud GB-28 bag.

[maslow]

Originally Posted by David Tollefson

I'd have to do some layouts to get you an A-C (would depend on crown type as well), but my initial suggestion would be to increase the BB drop and stretch out the back end to accommodate clearance between the rear tire and seat tube. The tires you're planning would already bring the BB up. No need to make it higher (unless you're planning to run 190mm cranks).

I’d planned to use the pacenti Paris Brest fork crown

https://framebuildersupply.com/produ...h-modest-tangs

With regards to increasing chain stays, what sort of length would you recommend?

Thanks for the speedy response

[maslow]

Originally Posted by unterhausen

one thing to consider with a front bag is making the top tube longer and the stem correspondingly shorter.

Would that be to help reduce the “twitchiness” of the bike ?

[palincss]

Originally Posted by maslow

Would that be to help reduce the “twitchiness” of the bike ?

No, to bring the bag closer to the head tube and to better line a the stem-mounted decaleur with the top of the bag.

[bicycletricycle]

I know you said no 650b, I would reconsider. That big of a tire with fenders and a 55cm top tube will yield quite a bit of toe clip overlap. You can lengthen the top tube and shorten the stem some to help, this will also make more room for the front bag. I would imagine your fork will be something like 400 millimeters long depending on the crown style. I prefer a 10.5-10.75" bb height which would probably need something like 85mm in bb drop with 700x42 tires. The V brake set up with drop levers works well with problem solvers travel agents in my experience. Not sure what the max tire w/ fender set up this will clear though. I run this set up with 700x38 tires and fenders plenty of times in the past but I cannot remember how much room there was under the cable/noodle link thing.

I have bikes of different trail now and while I do like my lower trail bikes I do not think they provide much benefit overall, especially with lighter loads, especially if you ride high trail bikes a lot as well.

[maslow]

I hadn't thought of that.

FWIW I've attached a shot of my Mercian King of Mercia touring rig. Old Reynolds 531 ST; it handles well when front loaded and the stem on this is 100mm with a 56cm top tube so yes your point makes sense. Thanks.

[David Tollefson]

Originally Posted by maslow

I’d planned to use the pacenti Paris Brest fork crown

https://framebuildersupply.com/produ...h-modest-tangs

With regards to increasing chain stays, what sort of length would you recommend?

Thanks for the speedy response

Here's how I sequence my builds when it comes to unknowns like this:

* Lay-out the design knowing that the head tube length will vary with the fork crown height.
* Build the fork to the clearance needed for the tire plus fender. THAT will determine your A-C.
* Adjust the main frame layout with that A-C to maintain contact points.

I second the notion of maximizing TT length. 70mm is my go-to for stems on my personal gravel builds.

As for rear end, I'd be looking towards 435mm+ stays. My current gravel build uses a 72 degree STA, 700x42mm tires that measure out to 40 on I19 rims, and 440mm stays. There's barely enough room for a fender. You can offset the seat tube forward on the BB (assuming a fillet-brazed build) to buy another couple mm's of clearance -- I've done this on many of my builds.

[maslow]

Originally Posted by bicycletricycle

I have bikes of different trail now and while I do like my lower trail bikes I do not think they provide much benefit overall, especially with lighter loads, especially if you ride high trail bikes a lot as well.

Thank you- that's some real food for thought.

I've just posted a pic of my Mercian touring bike. it only has room for 28mm Panaracer Ribmo tyres but seems to handle well with a front bag (although obviously the steering response is slower due to the increased weight).

But the Reynolds 531 ST tube set is "heavy". The frame was designed to be ridden loaded with bags/panniers attached (hence my desire to build a light weight rando... otherwise I'd use this)

Referring to the old Mercian spec sheet for the frame, dimensions state: 73º x 73º but it states "Fork rake 23 3/4mm front ??? BB drop is 10 5/8"

[false_Aest]

A-C = R + C + S

R = Radius of tire
C = Crown Height
S = Space for fender, mud, safety, etc.

Here's an example.
According to Cateye's chart 700x44 circumference = 2235mm

R = ~ 356mm
C = 32mm (please measure this in RL)
S = 15mm (uhm. 15 just sounded good to me)

A-C = 403mm. Round up to 406 for fun and tire variance. (A-C of a Lauff IIRC).

[KVN]

This is good advice.

Have you tried out a low trail bike before? Try to see if you can find one to borrow for a while to see if you like it. I bought an OAC Rambler, and although I could tell the bike was great, low trail just wasn’t for me.

Originally Posted by bicycletricycle

I know you said no 650b, I would reconsider. That big of a tire with fenders and a 55cm top tube will yield quite a bit of toe clip overlap. You can lengthen the top tube and shorten the stem some to help, this will also make more room for the front bag. I would imagine your fork will be something like 400 millimeters long depending on the crown style. I prefer a 10.5-10.75" bb height which would probably need something like 85mm in bb drop with 700x42 tires. The V brake set up with drop levers works well with problem solvers travel agents in my experience. Not sure what the max tire w/ fender set up this will clear though. I run this set up with 700x38 tires and fenders plenty of times in the past but I cannot remember how much room there was under the cable/noodle link thing.

I have bikes of different trail now and while I do like my lower trail bikes I do not think they provide much benefit overall, especially with lighter loads, especially if you ride high trail bikes a lot as well.

[Doug Fattic]

Maslow, what is your primary motivation for taking the class? Is it mostly just to make a custom rando frame to your own specifications or do you hope to make more in the future? I’ve been teaching frame building class longer than anyone (longer than most of my students have been alive) and know how important it is to keep frustration and disappoint to a minimum for maximum learning to take place. It is a harder task then some have realized. Your pre-class decisions can add or subtract to your challenges and difficulties. For example your choice of a 700C wheel means your are going to have a deeper BB drop than using 650B to have the same BB height. Most commonly available shells are designed for a 70mm (+-) drop. With fat 700s you might have a drop even greater than 85mm. It is not impossible to blacksmith a shell to your specifications but that takes time and skill that might not be available on a 1st build. Not only that but the changes can also make brazing more difficult too. My reason for asking about the future is that those with average ability and plans to make more will do better if they start with something that isn’t as difficult (to refine their basic skills) and do the complicated things in future builds. Those that want to make their dream bike in class can borrow on my or Herbie Helm’s ability to do the hardest parts.

It is a good idea to collect a lot of information before class and you’ve already been given some excellent answers. I recommend being open to changes during class. You’ll learn stuff that will impact some of your decisions. Designing the frame will be a balance of several competing factors and some compromises will probably have to be made.

Some observations. Lots of my students made a randonneuring frame in class. This is harder to do than a standard road frame so be prepared for the required extra effort. I’m big on teaching how to create a design and crave blank lugs. While some are complex many are simple but unique.

Instead of a full scale drawing, I have fixtures which are in effect a full scale drawing. We place the chosen seat and stem on the fixture to their bike position and slide the pieces that represent the tubes to match. Don’t be surprised if your bicycle position and frame design require some adjustments. I’ll attach a picture.

If you haven’t chosen a class yet research throughly. There are big quality differences. I get quite a few students that have already taken a class somewhere else so I know what they have and haven’t learned.