Weight bias

[tonyuk]

Hello all, looking to build a gravity bike. Based in South Wales.

Read all the previous posts and was very interested to see the link to the Tony Foale site.
A great site and for anyone "messing" about with two wheeled vehicles i would highly recommend his book.

Some questions, if i may?

Q1. What sort of weight bias is needed. Front? Rear?

Q2. I read that 65mm trail is a good start but what sort of steering head angle is this based upon?
65mm trail can be "created" at a multitude of steering head angles by playing about with fork offset.

Q3. Centre of gravity with rider onboard. Is it better to be as low as possible for speed but higher for the turns/curves/bends?

Q4. Why add weight to the frame if racing is downhill?
I understand maintaining momentum for any incline, encountered when racing, but purely downhill?

Q5. I read the post about vertical steering head. But what about too much rake or trail ie. wheel flop?
I`ve been involved with building or modifying BMX bikes into cycle-speedway bicycles. The large, track-style, frames use around 15 degree rake, no fork offset and 110mm-120mm trail to induce a severe wheel flop for quick banking into the turns. The rotation, gyro forces, of the 26" wheels assist with straight-line stability! BMX bikes with 20" wheels are best with 80-90mm trail, repositioned fork lugs for less offset.

Look forward to any replies. Regards Tony

[tonyuk]

Dug out two BMX frames and used a wooden jig to hold front wheel. One frame set up as normal while the other has the frame flipped. The flipped frame would need a "goose-neck" extension to lengthen wheelbase (+42") and a rake to pull forks back to around 15-18degrees.

Cycle speedway bars appear comfortable, when reversed, plus a screen could be mounted off the bolt-on crossbrace. Didn`t feel right with racer, reversed bmx and flat bars. Got a set of multi-adjustable, Laverda Jota, bars somewhere which would help fine tune the bar shape.

Due to previous motorcycle injuries can`t get my right knee to bend far enough for either frame, if foot position is on rear axle. Seat height has to raise to get any sort of fitting! Mmm, suits you sir! :-)

Has anyone run footpegs about 6" in front of rear axle/spindle? Just thinking that while it will still need a relatively high seat, to "scrunch" into the bike, at least i could get similar ergonomics to a motorcycle.

Found a post, by TRIKE, saying that a 53% rear weight bias works. Is this with both wheels level and on the floor? Reason i ask is that, imho, when travelling downhill weight would shift forward to more like a 60F / 40R?

Just my rambling thoughts. Feedback or comments appreciated.

Regards
Tony UK

[danv]

Trikes is a good source of knowledge. Knows what he is talking about (unlike me)

[Ian r]

Hi Tony,

am very jelous, you'll have some fab local runs, in south wales !

I've never weighed my bike but I guess (and 'aim for') approx 60% of the weight to be on the front end, which, as you say, will become exagerated as the hill becomes steeper.

Again, I've never measured the head angles on my bikes, but estimate them to be approx 70-73 degrees (reciprocal of 17-20) and 65mm sounds very familiar.

I have experimented with 2 different handle bar set-ups, primarily to give different Centre of Gravity set-ups. For fast, open, high speed (need as aerodynamic tuck as possible, willing to sacrifice slow speed handling / frnot end traction / feel) I use Racing bike handlebars.
For courses with a lower top speed, tighter corners and lots of confidence when in close proximity to other riders (gives a good 'head up' riding position) I use flat bars. This set-up allows me to move around alot more on the bike, to change the front/rear bias and put me upright, so my ears can give accurate info

In terms of pure physics, a lighter bike is preferable, right until you are travelling through Air. Which is dense. To achieve maximum 'push' through that dense air, weight has a major advantage. But this becomes a disadvanage when you're pushing off the line, trying to get that weight to change direction, or braking. So it's a compromise on the day (I run more weight on days with a head wind).

Back in 2003, me & dad built 'the beast' which had a head angle approaching 65 dgrees (or 25 degrees, depending on terminology) with a compensating trail in the region of 150mm. We wanted to find out once and for-all if the extreme's could be any use. It handles satisfactorilly above approx 30mph, but was a dog to turn and had no natural stability, in fact, quite the reverse below approx 25mph ! This is why I'm so vehnemently against 'flippin' the frame. True, turning the frame over lengthens the wheelbase and lowers the centre of gravity / rider position, but a significant cost......definately not reccommended to newbies, and in my opinion, anybody

Should be no problem to mount the rear pegs in front of the rear wheel axel, as long as they can be mounted securely (welded lugs?) Like you say, helps the rider's comfort / confidence.

Hope that helps......